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  • 6/9/98 2:09 PM 24379001.DOC

    INTEL CONFIDENTIAL(until publication date)




    Pentium® II Xeon™Processor/Intel® 450NXPCIset AGTL+ LayoutGuidelines

    Order Number: 243790-001

    June 1998

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    INTEL CONFIDENTIAL(until publication date)

    Information in this document is provided in connection with Intel products. No license, express or implied, by estoppel orotherwise, to any intellectual property rights is granted by this document. Except as provided in Intel’s Terms and Conditions ofSale for such products, Intel assumes no liability whatsoever, and Intel disclaims any express or implied warranty, relating tosale and/or use of Intel products including liability or warranties relating to fitness for a particular purpose, merchantability, orinfringement of any patent, copyright or other intellectual property right. Intel products are not intended for use in medical, lifesaving, or life sustaining applications.

    Intel may make changes to specifications and product descriptions at any time, without notice.

    Designers must not rely on the absence or characteristics of any features or instructions marked “reserved" or "undefined."Intel reserves these for future definition and shall have no responsibility whatsoever for conflicts or incompatibilities arising fromfuture changes to them.

    The Pentium® II Xeon™ processor and the Intel® 450NX PCIset may contain design defects or errors known as errata whichmay cause the product to deviate from published specifications. Current characterized errata are available on request.

    Contact your local Intel sales office or your distributor to obtain the latest specifications and before placing your product order.

    Copies of documents which have an ordering number and are referenced in this document, or other Intel literature, may beobtained by calling 1-800-548-4725 or by visiting Intel’s website at

    Copyright © Intel Corporation 1998.

    * Third-party brands and names are the property of their respective owners.

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    1.0. INTRODUCTION ...............................................5

    2.0. ABOUT THIS DOCUMENT...............................5

    2.1. Document Organization..................................5

    2.2. References .....................................................5

    2.3. Definition of Terms .........................................5

    3.0. AGTL+ DESIGN GUIDELINE ...........................7

    3.1. Determine Components..................................7

    3.2. Initial Timing Analysis .....................................8

    3.3. Determine General Topology, Layout, andRouting Desired............................................11

    3.4. Pre-Layout Simulation ..................................13

    3.4.1. METHODOLOGY..................................13

    3.4.2. SIMULATION CRITERIA ......................14

    3.5. Place and Route Board ................................14

    3.5.1. ESTIMATE COMPONENT TOCOMPONENT SPACING FOR AGTL+SIGNALS ..............................................14

    3.5.2. LAYOUT AND ROUTE BOARD ...........14

    3.6. Post-Layout Simulation.................................15


    3.6.2. CROSSTALK ANALYSIS......................15

    3.6.3. MONTE CARLO ANALYSIS.................15

    3.7. Validation ......................................................16

    3.7.1. MEASUREMENTS................................16

    3.7.2. FLIGHT TIME SIMULATION.................16

    3.7.3. FLIGHT TIME HARDWAREVALIDATION.........................................17

    4.0. THEORY ..........................................................17

    4.1. AGTL+ ..........................................................17

    4.2. Timing Requirements ...................................17

    4.3. Noise Margin ................................................17

    4.3.1. FALLING EDGE OR LOW LEVELNOISE MARGIN ...................................18

    4.3.2. RISING EDGE OR HIGH LEVEL NOISEMARGIN................................................19

    4.4. Crosstalk Theory ..........................................20

    4.4.1. CROSSTALK MANAGEMENT .............22


    5.0. MORE DETAILS AND INSIGHTS...................23

    5.1. Textbook Timing Equations ..........................23

    5.2. Effective Impedance and Tolerance/Variation .......................................................24

    5.3. Power/Reference Planes, PCB Stackup, andHigh Frequency Decoupling.........................24

    5.3.1. POWER DISTRIBUTION ......................24

    5.3.2. REFERENCE PLANES AND PCBSTACKUP .............................................24


    5.3.4. SLOT 2 CONNECTOR..........................26

    5.4. Clock Routing ...............................................27

    5.5. Conclusion....................................................27

    6.0. VREF GUARDBAND.......................................28

    7.0. OVERDRIVE REGION.....................................28

    8.0. FLIGHT TIME DEFINITION ANDMEASUREMENT.............................................29


    Figure 1. Example 6-load and 5-load Plus 6thStub Termination Network Topology...12

    Figure 2. Example 5-load Network TopologyOptimized for 100 MHz Bus................13

    Figure 3. Test Load vs. Actual System Load ......16

    Figure 4. Rising Edge Noise Margin....................19

    Figure 5. Propagation on Aggressor Network .....20

    Figure 6. Aggressor and Victim Networks...........21

    Figure 7. Transmission Line Geometry: (A)Microstrip (B) Stripline.........................21

    Figure 8. One Signal Layer and One ReferencePlane ...................................................25

    Figure 9. Layer Switch with One ReferencePlane ...................................................25

    Figure 10. Layer Switch with Multiple ReferencePlanes (same type) .............................25

    Figure 11. Layer Switch with Multiple ReferencePlanes .................................................26

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    Figure 12. One Layer with Multiple ReferencePlanes................................................. 26

    Figure 13. Overdrive Region and VREFGuardband.......................................... 29

    Figure 14. Rising Edge Flight Time Definition..... 29


    Table 1. Pentium® II Xeon™ Processor andMIOC AGTL+ Parameters..................... 9

    Table 2. Example TFLT_MAX Calculations for100 MHz Bus ....................................... 10

    Table 3. Example TFLT_MAX Calculations for 90MHz Bus (Cluster Controller Design) .. 11

    Table 4. Example TFLT_MIN Calculations(Frequency Independent) .................... 11

    Table 5. Example Backward Crosstalk CouplingFactors with εr = 4.5, VOH_MAX =1.5 V, and Z0 = 65 Ω ........................... 22

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    The Pentium® II Xeon™ processor is a follow-on to thePentium Pro and Pentium II family of microprocessorsand is the first 100 MHz Slot 2 processor. The design ofthe external Pentium II Xeon processor bus enables thePentium II Xeon processor to be “multiprocessorready.” To relax timing constraints on a bus thatsupports up to six loads, the Pentium II Xeon processorimplements a synchronous, latched bus protocol thatallows a full clock cycle for signal transmission and afull clock cycle for signal interpretation and generation.This protocol simplifies interconnect timingrequirements and supports 100 MHz system designsusing conventional interconnect technology. ThePentium II Xeon processor bus uses low-voltage-swingAGTL+ I/O buffers, making high frequency signalcommunication between many loads easier.

    The goal of this layout guideline is to provide thesystem designer with the information needed for thePentium II Xeon processor and Intel® 450NX PCIsetAGTL+ bus portion of PCB layout. This documentprovides guidelines and methodologies that are to beused with good engineering practices. See the Pentium®II Xeon™ Processor at 400 MHz and Intel® 450NXPCIset for component specific electrical details. Intelstrongly recommends running analog simulations usingthe available I/O buffer models together with layoutinformation extracted from your specific design.


    2.1. Document Organization

    This section defines terms used in the document.Section 3.0. discusses specific system guidelines. Thisis a step-by-step methodology that Intel hassuccessfully used to design Pentium II Xeon processorsystems using the Intel 450NX PCIset components.Section 4.0. introduces the theories that are applicableto this layout guideline. Section 5.0. contains moredetails and insights. The items in Section 5.0. expand onsome of the rationale for the recommendations in thestep-by-step methodology. This section also includesequations that may be used for reference.

    The actual guidelines start on Section 3.0., “AGTL+Design Guideline.”

    2.2. References

    • Pentium® II Xeon™ Processor at 400 MHz

    • Intel® 450NX PCIset

    • Pentium® II Xeon™ Processor Power DistributionGuidelines

    • Slot 2 Processor Bus Terminator Design Guidelines

    • Pentium® II Processor Developer’s Manual (OrderNumber 243341)

    • VRM 8.2/8.3 DC-DC Converter Specification

    2.3. Definition of Terms

    Aggressor - a network that transmits a coupled signal toanother network is called the aggressor network.

    AGTL+ - The Pentium II Xeon processor system bususes a new bus technology called AGTL+, or AssistedGunning Transceiver Logic. AGTL+ buffers are open-drain and require pull-up resistors for providing thehigh logic level and termination. The Pentium II Xeonprocessor AGTL+ output buffers differ from GTL+buffers with the addition of an active pMOS pull-uptransistor to “assist” the pull-up resistors during the firstclock of a low-to-high voltage transition. Additionally,the Pentium II Xeon processor Single Edge Connector(S.E.C.) cartridge contains internal 150 Ω pull-upresistors to provide termination at each bus load.

    Bus Agent - a component or group of components that,when combined, represent a single load on the AGTL+bus.

    Corner - describes how a component performs when allparameters that could impact performance are adjustedto have the same impact on performance. Examples ofthese parameters include variations in manufacturingprocess, operating temperature, and operating voltage.The results in performance of an electronic componentthat may change as a result of this include (but are notlimited to): clock to output time, output driver edge rate,output drive current, and input drive current. Discussionof the “slow” corner would mean having a componentoperating at its slowest, weakest drive strengthperformance. Similar discussion of the “fast” cornerwould mean having a component operating at its fastest,strongest drive strength performance. Operation orsimulation of a component at its slow corner and fastcorner is expected to bound the extremes betweenslowest, weakest performance and fastest, strongestperformance.

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    Crosstalk - the reception on a victim network of asignal imposed by aggressor network(s) throughinductive and capacitive coupling between thenetworks.

    • Backward Crosstalk - coupling which creates asignal in a victim network that travels in theopposite direction as the aggressor’s signal.

    • Forward Crosstalk - coupling which creates asignal in a victim network that travels in the samedirection as the aggressor’s signal.

    • Even Mode Crosstalk - coupling from multipleaggressors when all the aggressors switch in thesame direction that the victim is switching.

    • Odd Mode Crosstalk - coupling from multipleaggressors when all the aggressors switch in theopposite direction that the victim is switching.

    Edge Finger - The cartridge electrical contact whichinterfaces to the Slot 2 connector.

    Flight Time - is a term in the timing equation thatincludes the signal propagation delay, any effects thesystem has on the TCO of the driver, plus anyadjustments to the signal at the receiver needed toguarantee the setup time of the receiver.

    More precisely, flight time is defined to be:

    • The time difference between a signal at the inputpin of a receiving agent crossing VREF (adjusted tomeet the receiver manufacturer’s conditionsrequired for AC timing specifications; i.e., ringback,etc..), and the output pin of the driving agentcrossing VREF if the driver was driving the TestLoad used to specify the driver’s AC timings.

    See Section 3.7.2. for details regarding flight timesimulation and validation.

    Figure 13 in Appendix A shows the VREF Guardbandboundaries where maximum and minimum flight timemeasurements are taken. The VREF Guardband takesinto account sources of noise that may affect the way anAGTL+ signal becomes valid at the receiver. See thedefinition of the VREF Guardband.

    • Maximum and Minimum Flight Time - Flight timevariations can be caused by many differentparameters. The more obvious causes includevariation of the board dielectric constant, changes inload condition, crosstalk, VTT noise, VREF noise,variation in termination resistance and differences in

    I/O buffer performance as a function of temperature,voltage and manufacturing process. Some lessobvious causes include effects of SimultaneousSwitching Output (SSO) and packaging affects.

    • The Maximum Flight Time is the largest flight timea network will experience under all variations ofconditions. Maximum flight time is measured at theappropriate VREF Guardband boundary.

    • The Minimum Flight Time is the smallest flighttime a network will experience under all variationsof conditions. Minimum flight time is measured atthe appropriate VREF Guardband boundary.

    For more information on flight time and the VREFGuardband, see Appendix A of this guideline and thePentium® II Processor Developer’s Manual.

    GTL+ is the bus technology used by the Pentium Proprocessor. This is an incident wave switching, opendrain bus with pull-up resistors which provide both thehigh logic level and termination. It is an enhancement tothe GTL (Gunning Transceiver Logic) technology. Seethe Pentium® II Processor Developer’s Manual formore details of GTL+.

    Network - the trace of a Printed Circuit Board (PCB)that completes an electrical connection between two ormore components.

    Network Length - the distance between extreme busagents on the network and does not include the distanceconnecting the end bus agents to the terminationresistors.

    Overdrive Region - is the voltage range, at a receiver,located above and below VREF for signal integrityanalysis. See the Pentium® II Processor Developer’sManual for more details.

    Overshoot - Maximum voltage allowed for a signal atthe processor core pad. See the Pentium® II Xeon™Processor at 400 MHz and Intel® 450NX PCIset forovershoot specifications.

    Pad - a feature of a semiconductor die contained withinan internal logic package on the cartridge substrate usedto connect the die to the package bond wires. A pad isonly observable in simulation.

    Pin - a feature of a logic package contained within theS.E.C. cartridge used to connect the package to aninternal substrate trace.

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    Ringback - is the voltage that a signal rings back toafter achieving its maximum absolute value. Ringbackmay be due to reflections, driver oscillations, etc. Seethe Pentium® II Xeon™ Processor at 400 MHz andIntel® 450NX PCIset Electrical Mechanical andThermal Specification for ringback specifications.

    Settling Limit - defines the maximum amount ofringing at the receiving pin that a signal must reachbefore its next transition. See the Pentium® II Xeon™Processor at 400 MHz and Intel® 450NX PCIset forsettling limit specifications.

    Setup Window - is the time between the beginning ofSetup to Clock (TSU_MIN) and the arrival of a validclock edge. This window may be different for each typeof bus agent in the system.

    Simultaneous Switching Output (SSO) Effects - refersto the difference in electrical timing parameters anddegradation in signal quality caused by multiple signaloutputs simultaneously switching voltage levels (e.g.,high-to-low) in the opposite direction from a singlesignal (e.g., low-to-high) or in the same direction (e.g.,high-to-low). These are respectively called odd-modeswitching and even-mode switching. This simultaneousswitching of multiple outputs creates higher currentswings that may cause additional propagation delay (or“pushout”), or a decrease in propagation delay (or“pull-in”). These SSO effects may impact the setupand/or hold times and are not always taken into accountby simulations. System timing budgets should includemargin for SSO effects.

    Stub - the branch from the trunk terminating at the padof an agent.

    Test Load - Intel uses a 25 Ω test load for specifying itscomponents.

    Trunk - the main connection, excluding interconnectbranches, terminating at agent pads.

    Undershoot - Maximum voltage allowed for a signal toextend below VSS at the processor core pad. See thePentium® II Xeon™ Processor at 400 MHz and Intel®450NX PCIset for undershoot specifications.

    Victim - a network that receives a coupled crosstalksignal from another network is called the victimnetwork.

    VREF Guardband - A guardband (∆VREF) definedabove and below VREF to provide a more realisticmodel accounting for noise such as crosstalk, VTTnoise, and VREF noise.


    The following step-by-step guideline was developed forsystems based on four Pentium II Xeon processor loads,one MIOC load, and one optional cluster controller.

    The guideline recommended in this section is based onexperience developed at Intel while developing manydifferent Pentium Pro processor family and Pentium IIXeon processor based systems. Begin with componentselection, an initial timing analysis, and topologydefinition. Perform pre-layout analog simulations for adetailed picture of a working “solution space” for thedesign. These pre-layout simulations help definerouting rules prior to placement and routing. Afterrouting, extract the interconnect database and performpost-layout simulations to refine the timing and signalintegrity analysis. Validate the analog simulations whenactual systems become available. The validation sectiondescribes a method for determining the flight time in theactual system.

    Guideline Methodology:

    • Determine Components

    • Initial Timing Analysis

    • Determine General Topology, Layout, and Routing

    • Pre-Layout Simulation (Sensitivity sweep)

    • Place and Route Board

    Estimate Component to Component Spacing forAGTL+ Signals

    Layout and Route Board

    • Post-Layout Simulation

    Interconnect Extraction

    Intersymbol Interference (ISI), Crosstalk, andMonte Carlo Analysis

    • Validation


    Determining Flight Time

    3.1. Determine Components

    Determine whether a cluster controller will be used andwhether it will reside directly on the PCB or occupy afifth Single Edge Contact (SEC) connector slot.

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    3.2. Initial Timing Analysis

    Perform an initial timing analysis of the system usingEquation 1 and Equation 2 shown below. Theseequations are the basis for the timing analysis. Tocomplete the initial timing analysis, values for clockskew and clock jitter are needed, along with thecomponent specifications. These equations contain amulti-bit adjustment factor, MADJ, to account for multi-bit switching effects such as SSO pushout or pull-in thatare often hard to simulate. These equations do not takeinto consideration all signal integrity factors that affecttiming. Additional timing margin should be budgeted toallow for these sources of noise.

    Equation 1. Setup Time


    TFLT_MAX + MADJ ≤ Clock Period

    Equation 2. Hold Time


    Symbols used in Equation 1 and Equation 2:

    • TCO_MAX is the maximum clock to outputspecification1.

    • TSU_MIN is the minimum required time specified tosetup before the clock1.

    • CLKJITTER is the maximum clock edge-to-edgevariation.

    • CLKSKEW is the maximum variation betweencomponents receiving the same clock edge.

    • TFLT_MAX is the maximum flight time as defined inSection 2.3.

    • TFLT_MIN is the minimum flight time as defined inSection 2.3.

    • MADJ is the multi-bit adjustment factor to accountfor SSO pushout or pull-in.

    • TCO_MIN is the minimum clock to outputspecification1.

    • THOLD is the minimum specified input hold time.


    1. The Clock to Output (TCO) and Setup to Clock (TSU)timings are both measured from the signals lastcrossing of VREF, with the requirement that thesignal does not violate the ringback or edge ratelimits. See the Pentium® II Xeon™ Processor at 400MHz and the Pentium® II Processor Developer’sManual for more details.

    Solving these equation for TFLT results in the followingequations:

    Equation 3. Maximum Flight Time


    Equation 4. Minimum Flight Time


    There are multiple cases to consider. Note that while thesame trace connects two components, component A andcomponent B, the minimum and maximum flight timerequirements for component A driving component B aswell as component B driving component A must bemet. The cases to be considered are:

    • Pentium® II Xeon™ processor driving Pentium IIXeon processor

    • Pentium II Xeon processor driving MIOC

    • MIOC driving a Pentium II Xeon processor

    • Pentium II Xeon processor driving cluster controller

    • Cluster controller driving Pentium II Xeon processor

    • MIOC driving cluster controller

    • Cluster controller driving MIOC

    A designer using components other than those listedabove must evaluate any additional combinations ofdriver and receiver.

    Table 1 lists the AGTL+ component timings of thePentium II Xeon processor and MIOC defined at thepins. These timings are for reference only; obtaincomponent specifications from the Pentium® IIXeon™ Processor at 400 MHz and Intel® 450NXPCIset.

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    Table 1. Pentium® II Xeon™ Processor and MIOC AGTL+ Parameters1,2

    IC ParametersPentium® II Xeon™ Processor

    at 90 MHz or 100 MHz Bus MIOC

    Clock to Output maximum (TCO_MAX) 2.70 2.65

    Clock to Output minimum (TCO_MIN) 0.20 -0.15

    Setup time (TSU_MIN) 1.75 1.58

    Hold time (THOLD) 0.62 0.63

    NOTES1. All times in nanoseconds.2. Numbers in table are for reference only. These timing parameters are subject to change. Please check the appropriate

    component datasheets for valid timing parameter values.

    Table 2 gives an example AGTL+ initial maximum flighttime calculation for a 100 MHz, 4-way Pentium II Xeonprocessor / Intel 450NX PCIset design that does notinclude a cluster controller. Note that assumed values forclock skew and clock jitter were used. Clock skew andclock jitter values are dependent on the clockcomponents and distribution method chosen for aparticular design and must be budgeted into the initialtiming equations as appropriate for each design.

    Intel highly recommends adding margin as shown in the“MADJ” column to offset the degradation caused by SSOpushout and other multi-bit switching effects. The“Recommended TFLT_MAX” column contains therecommended maximum flight time after incorporatingthe MADJ value. If edge rate, ringback, and monotonicityrequirements are not met, flight time correction must beperformed as documented in the Pentium® II Processor

    Developer’s Manual with the additional requirementsnoted in Appendix A. The commonly used “textbook”equations used to calculate the expected signalpropagation rate of a board are included in Section 5.1.

    Simulation and control of baseboard design parameterscan ensure that signal quality and maximum andminimum flight times are met. Baseboard propagationspeed is highly dependent on transmission line geometryconfiguration (stripline vs. microstrip), dielectricconstant, and loading. This layout guideline includeshigh-speed baseboard design practices that may improvethe amount of timing and signal quality margin. Themagnitude of MADJ is highly dependent on baseboarddesign implementation (stackup, decoupling, layout,routing, reference planes, etc.) and needs to becharacterized and budgeted appropriately for eachdesign.

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    Table 2. Example TFLT_MAX Calculations for 100 MHz Bus1,2

    Driver Receiver







    RecommendedTFLT_MAX2 (ns)

    Pentium®II Xeon™processor

    Pentium IIXeon


    10.00 2.70 1.75 0.15 0.15 0.80 4.45

    Pentium IIXeon


    MIOC 10.00 2.70 1.58 0.15 0.15 0.80 4.63

    MIOC Pentium IIXeon


    10.00 2.65 1.75 0.15 0.15 0.50 4.80

    NOTES:1. All times in nanoseconds.2. The flight times in this column include margin to account for the following phenomena which Intel has observed when

    multiple bits are switching simultaneously. These multi-bit effects can adversely affect flight time and signal quality and aresometimes not accounted for in simulation. Accordingly, maximum flight times depend on the baseboard design andadditional adjustment factors or margins are recommended.

    • SSO pushout or pull-in• Rising or falling edge rate degradation at the receiver caused by inductance in the current return path, requiring

    extrapolation that causes additional delay.

    • Crosstalk on the PCB and internal to the package can cause variation in the signals.There are additional effects that may not necessarily be covered by the multi-bit adjustment factor and should bebudgeted as appropriate to the baseboard design. Examples include:

    • The effective board propagation constant (SEFF), which is a function of: Dielectric constant (εr) of the PCB material The type of trace connecting the components (stripline or microstrip) The length of the trace and the load of the components on the trace. (Note that the board propagation constant

    multiplied by the trace length is a component of the flight time but not necessarily equal to the flight time.)

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    Table 3. Example TFLT_MAX Calculations for 90 MHz Bus (Cluster Controller Design)1,2

    Driver ReceiverClk

    Period TCO_MAX TSU_MIN ClkSKEW ClkJITTER MADJRecommended


    Pentium®II Xeon™processor

    Pentium IIXeon


    11.11 2.70 1.75 0.15 0.15 0.80 5.56

    Pentium IIXeon


    MIOC 11.11 2.70 1.58 0.15 0.15 0.80 5.73

    MIOC Pentium IIXeon


    11.11 2.65 1.75 0.15 0.15 0.50 5.91

    Table 4. Example TFLT_MIN Calculations (Frequency Independent)1,2

    Driver Receiver THOLD ClkSKEW TCO_MIN MADJRecommended


    Pentium® IIXeon™


    Pentium IIXeon


    0.62 0.15 0.20 0.63 1.20

    Pentium IIXeon


    MIOC 0.63 0.15 0.20 0.62 1.20

    MIOC Pentium IIXeon


    0.62 0.15 -0.15 0.28 1.20

    NOTE:1. All times in nanoseconds.

    Table 3 gives a similar example maximum flight timecalculation for a 90 MHz AGTL+ 4-way Pentium IIXeon processor / Intel 450NX PCIset design thatincludes a cluster controller.

    Table 4 is an example calculation for minimum flighttime that is frequency independent. Intel highlyrecommends adding margin as shown in the “MADJ”column to offset the degradation caused by SSO pull-inand other multi-bit switching effects. The“Recommended TFLT_MIN” column contains therecommended minimum flight time after incorporatingthe MADJ value.

    Table 2, Table 3, and Table 4 are derived assuming:

    • CLKSKEW = 0.15 ns (PCB skew only – assumes zerodriver skew by tying clock driver outputs)

    • CLKJITTER = 0.15 ns

    3.3. Determine General Topology,Layout, and Routing Desired

    After the selecting the processor bus components andcalculating the timing budget, determine the approximatelocation of the devices on the baseboard. Estimate theprinted circuit board parameters from the placement andother information. Locate the processors, Intel 450NXPCIset, and cluster controller as required to meet timing.The “Double Star” and “Crow’s Foot” Topologiesillustrated in Figure 1 have been shown in simulation tobe successful for 6-load, 90 MHz bus operation. 100MHz bus operation requires that the cluster controller beremoved and replaced with AGTL+ termination or aterminator card.

    Figure 2 shows a “Double Star” and “Crow’s Foot”topology modified to provide more noise and timingmargin for 5-load, 100 MHz operation. The modificationinvolves complete removal of the 6th stub and AGTL+

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    termination that should improve worst case flight timemargin on falling edge transition. Doing so will reducethe bus load, and should provide more noise and timingmargin. Perform simulations on the entire system bus toensure that ideal termination resistance at the MIOC ischosen for any given design. The termination at theMIOC may be decreased to 75 Ω to maintain an effectiveAGTL+ termination of 25 Ω.

    Six-load, cluster-capable systems may gain timing andsignal quality margin by using faster dielectric material(e.g., Getek) and better ground plane referencing forAGTL+ signals.

    • Place termination resistors at the MIOC and clustercontroller, which should be located at opposite endsof the AGTL+ network. Minimize the inductancebetween the VTT distribution and the terminationresistors.

    • The placement of the Pentium® II Xeon™ processor,MIOC and/or custom ASIC(s) on the Pentium II Xeonprocessor system bus must be carefully chosen. Usinga custom ASIC (with different timings than PentiumII Xeon processor or MIOC) on the system bus willrequire additional analog simulations to determine theoptimum location of each agent along the bus.

    175 ps

    Slot DSlot CSlot BSlot A

    MIOC Optional ClusterSEC Connector

    Slot DSlot CSlot BSlot A


    Cluster Controller orTerminator/Termination -

    - SEC Connector notRecommended



    175 ps 175 ps 175 ps

    525 ps 525 ps700 ps

    350 ps 350 ps117 ps 117 ps 117 ps

    805 ps 805 ps

    Cluster Controller orTerminator/Termination

    Times associated with net segmentsrepresent electrical distance whichdepend on transmission linegeometry, board dielectric,propagation speed, loading, etc.


    Figure 1. Example 6-load and 5-load Plus 6th Stub Termination Network Topology

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    175 ps

    Slot DSlot CSlot BSlot A


    Slot DSlot CSlot BSlot A


    Last terminationstub removed.



    175 ps 175 ps 175 ps

    525 ps 700 ps

    350 ps 350 ps117 ps 234 ps

    805 ps

    Last terminationstub removed.

    Times associated with net segmentsrepresent electrical distance whichdepend on transmission linegeometry, board dielectric,propagation speed, loading, etc.


    Figure 2. Example 5-load Network Topology Optimized for 100 MHz Bus

    3.4. Pre-Layout Simulation

    3.4.1. METHODOLOGY

    Pentium II Xeon processor designs require analogsimulations. Start simulations prior to layout. Pre-layoutsimulations provide a detailed picture of the working“solution space” that meets flight time and signal qualityrequirements. The layout recommendations in theprevious sections are based on pre-layout simulationsconducted at Intel. By basing board layout guidelines onthe solution space, the iterations between layout andpost-layout simulation can be reduced.

    Intel specifies signal quality at the device pads andtherefore recommends running simulations at the devicepads for signal quality. However, the core timings arespecified at the device pins, so simulation results at thedevice pins may be used to correlate simulationperformance against actual system measurements

    Pre-layout analysis includes a sensitivity analysis usingparametric sweeps. Parametric sweep analysis involvesvarying one or two system parameters while all otherssuch as driver strength, package, Z0, and S0 are constant.This way, the sensitivity of the proposed bus topology tovarying parameters can be analyzed systematically.Sensitivity of the bus to minimum flight time, maximumflight time, and signal quality should be covered.Suggested sweep parameters include trace lengths,termination resistor values, and any other factors thatmay affect flight time, signal quality, and feasibility oflayout. Minimum flight time and worst signal quality aretypically analyzed using fast I/O buffers andinterconnect. Maximum flight time is typically analyzedusing slow I/O buffers and slow interconnect.

    Outputs from each sweep should be analyzed todetermine which regions meet timing and signal qualityspecifications. To establish the working solution space,find the common space across all the sweeps that resultin passing timing and signal quality. The solution spaceshould allow enough design flexibility for a feasible,cost-effective layout.

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    Accurate simulations require that the actual range ofparameters be used in the simulations. Intel hasconsistently measured the cross-sectional resistivity ofPCB copper to be in the order of 1 ohm*mil2/inch, notthe 0.662 ohm*mil2/inch value for annealed copper thatis published in reference material. Using the 1ohm*mil2/inch value may increase the accuracy of lossysimulations.

    Positioning drivers with faster edges closer to the middleof the network typically results in more noise thanpositioning them towards the ends. We have also shownthat the worst-case noise margin can be generated bydrivers located in all positions (given appropriatevariations in the other network parameters). Therefore,we recommend simulating the networks from all driverlocations, and analyzing each receiver for each possibledriver.

    Analysis has shown that both fast and slow cornermodels must be run for both rising and falling edgetransitions. The fast corner is needed because the fastedge rate creates the most noise. The slow corner isneeded because the buffer’s drive capability will be aminimum, causing the VOL to shift up, which may causethe noise from the slower edge to exceed the availablebudget. Slow corner models may produce minimumflight time violations on rising edges if the transitionstarts from a higher VOL. So, Intel highly recommendschecking for minimum and maximum flight timeviolations with both the fast and slow corner models.

    The transmission line package models must be insertedbetween the output of the buffer and the net it is driving.Likewise, the package model must also be placedbetween a net and the input of a receiver model. This isgenerally done by editing your simulator’s netdescription or topology file.

    Intel has found wide variation in noise margins whenvarying the stub impedance and the PCB’s Z0 and S0.Intel therefore recommends that PCB parameters becontrolled as tightly as possible, with a sampling of theallowable Z0 and S0 simulated. The recommendedeffective line impedance (ZEFF) is 65 Ω +/-10%. Intelrecommends running uncoupled simulations using the Z0of the package stubs; and performing fully coupledsimulations if increased accuracy is needed or desired.Accounting for crosstalk within the device package byvarying the stub impedance was investigated and was notfound to be sufficiently accurate. This lead to thedevelopment of full package models for the componentpackages.

    3.5. Place and Route Board


    Estimate the number of layers that will be required. Thendetermine the expected interconnect distances betweeneach of the components on the AGTL+ bus. Be sure toconsider the guidelines in Section 3.3. Using theestimated interconnect distances, verify that theplacement can support the system timing requirements.

    The maximum network length between the bus agents isdetermined by the required bus frequency and themaximum flight time propagation delay on the PCB. Theminimum network length is independent of the requiredbus frequency. Table 2, Table 3, and Table 4 assumevalues for CLKSKEW and CLKJITTER, parameters that arecontrolled by the system designer. In order to reducesystem clock skew to a minimum, clock buffers whichallow their outputs to be tied together are recommended.Intel suggests running analog simulations to ensure thateach design has adequate noise and timing margin.


    Route the board satisfying the estimated space andtiming requirements. Stay within the solution space setfrom the pre-layout sweeps. Estimate the printed circuitboard parameters from the placement and otherinformation including the following general guidelines:

    • Distribute VTT with a power plane or a partial powerplane. If this cannot be accomplished, use as wide atrace as possible and route the VTT trace with thesame design rules as the AGTL+ traces.

    • Keep the overall length of the bus as short as possible(but don’t forget minimum component to componentdistances to meet hold times).

    • Plan to minimize crosstalk by:

    Routing the same type of AGTL+ I/O signals inisolated signal groups. I.e., route the data signalsin one group, the arbitration signals in anothergroup. Keep at least a 5:1 spacing to trace widthratio between each group

    Keeping at least a 25 mil space between AGTL+signals and non-AGTL+ signals (and at least a 5:1spacing to line width ratio).

    Keeping at least a 4:1 spacing to trace width ratiobetween AGTL+ signals in the same group.

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    Using a trace pitch to trace height ratio of 3:1between AGTL+ signals in the same group.

    Using a trace pitch to trace height ratio of 4:1between AGTL+ signals in different groups.

    Using a trace pitch to trace height ratio of 5:1between AGTL+ signals and non-AGTL+ signals

    Minimizing the dielectric process variation usedin the PCB fab.

    Eliminating parallel traces between layers notseparated by a power or ground plane.

    The spacing between the various bus agents causesvariations in trunk impedance and stub locations. Thesevariations cause reflections which can cause constructiveor destructive interference at the receivers. A reductionof noise may be obtained by a minimum spacing betweenthe agents. Unfortunately, a tighter spacing results inreduced component placement options and lower holdmargins. Therefore adjusting the inter-agent spacing maybe one way to change the network’s noise margin, butmechanical constraints often limit the usefulness of thistechnique. Always be sure to validate signal quality aftermaking any changes in agent locations or changes tointer-agent spacing.

    There are six AGTL+ signals that can be driven by morethan one agent simultaneously. These signals mayrequire more attention during the layout and validationportions of the design. When a signal is asserted (drivenlow) by two or more agents on the same clock edge, thetwo falling edge wave fronts will meet at some point onthe bus and can sum to form a negative voltage. Theringback from this negative voltage can easily cross intothe overdrive region. The signals are AERR#, BERR#,BINIT#, BNR#, HIT#, and HITM#.

    This document addresses AGTL+ layout. Chassisrequirements for cooling, connector location, memorylocation, etc., may constrain the system topology andcomponent placement location, therefore constrainingthe board routing. These issues are not directly addressedin this document.

    3.6. Post-Layout Simulation

    Following layout, extract the interconnect informationfor the board from the CAD layout tools. Run post-layoutsimulations to verify that the layout meets timing andnoise requirements. A small amount of “tuning” may berequired; experience at Intel has shown that sensitivityanalysis dramatically reduces the amount of tuningrequired. The post layout simulations should take into

    account the expected variation for all interconnectparameters.

    Intel specifies signal quality at the device pads andtherefore recommends running simulations at the devicepads for signal quality. However, Intel specifies coretimings at the device pins, so simulation results at thedevice pins should be used later to correlate simulationperformance against actual system measurements.


    Intersymbol Interference (ISI) refers to the distortion orchange in the waveform shape caused by the voltage andtransient energy on the network when the driver beginsits next transition.

    For example, ISI may occur when the line is driven high,low, and then high in consecutive cycles (the oppositecase is also valid). When the driver drives high on thefirst cycle and low on the second cycle, the signal maynot settle to the minimum VOL before the next risingedge is driven. This results in improved flight times inthe third cycle. ISI simulations for the topology given inthis section were performed by comparing flight timesfor the first and third cycle. ISI effects do not necessarilyspan only 3 cycles so it may be necessary to simulatebeyond 3 cycles for certain designs. After simulating andquantifying ISI effects, adjust the timing budgetaccordingly to take these conditions into consideration.


    AGTL+ crosstalk simulations can consider the Pentium IIXeon processor core package, MIOC package, and Slot 2connectors as non-coupled. Treat the traces on thePentium II Xeon processor cartridge and baseboard asfully coupled for maximum crosstalk conditions.Simulate the traces as lossless for worst case crosstalk,and lossy where more accuracy is needed. Evaluate bothodd and even mode crosstalk conditions.

    AGTL+ crosstalk simulation involves the followingcases:

    • Intra-group AGTL+ crosstalk

    • Inter-group AGTL+ crosstalk

    • CMOS to AGTL+ crosstalk


    Perform a Monte Carlo analysis to refine the passingsolution space region. A Monte Carlo analysis involves

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    randomly varying parameters (independent of oneanother) over their tolerance range. This analysis isintended to ensure that no regions of failing flight timeand signal quality exist between the extreme corner casesrun in pre-layout simulations. For the example topology,vary the following parameters during Monte Carlosimulations:

    • Trace lengths on baseboard

    • AGTL+ termination resistance RTT on Pentium® IIXeon™ processor cartridges

    • AGTL+ termination resistance RTT at the MIOC

    • AGTL+ termination resistance RTT at the ClusterController (if present)

    • AGTL+ termination resistance RTT on terminationcards or 6th stub (if present).

    • Z0 of traces on Pentium II Xeon processor cartridges

    • S0 of traces on Pentium II Xeon processor cartridges

    • Z0 of traces on baseboard

    • S0 of traces on baseboard

    • Fast and slow corner Pentium II Xeon processor I/Obuffer models

    • Fast and slow Pentium II Xeon processor packagemodels

    • Fast and slow corner Intel® 450NX PCIset I/O buffermodels

    • Fast and slow Intel 450NX PCIset package models

    Refer to the Pentium® II Xeon™ Processor at 400 MHzand electronic I/O buffer models for the parameter rangesof the Pentium II Xeon processor and Intel 450NXPCIset.

    3.7. Validation

    Build systems and validate the design and simulationassumptions.


    Note that the AGTL+ specification for signal quality is atthe pad of the component. The expected method ofdetermining the signal quality is to run analogsimulations for the pin and the pad. Then correlate thesimulations at the pin against actual systemmeasurements at the pin. Good correlation at the pinleads to confidence that the simulation at the pad isaccurate. Controlling the temperature and voltage to

    correspond to the I/O buffer model extremes shouldenhance the correlation between simulations and theactual system.


    As defined earlier in Section 2.3., flight time is the timedifference between a signal crossing VREF at the inputpin of the receiver, and the output pin of the drivercrossing VREF were it driving a test load. The timings inthe tables and topologies discussed in this guidelineassume the actual system load is 25 Ω and is equal to thetest load. This may not be the case in a particular designand this section describes how to correlate the designload to the test load in simulation and in validation.








    I/O Buffer








    I/O Buffer











    Figure 3. Test Load vs. Actual System Load

    Figure 3 above shows the different configurations forTCO testing and flight time simulation. The flip-floprepresents the logic input and driver stage of a typicalAGTL+ I/O buffer. TCO timings are specified at thedriver pin output. TFLIGHT-SYSTEM is usually reported bya simulation tool as the time from the driver pad startingits transition to the time when the receiver’s input pinsees a valid data input. Since both timing numbers (TCOand TFLIGHT-SYSTEM) will include propagation time fromthe pad to the pin, it is necessary to subtract this time(TREF) from the reported flight time to avoid doublecounting. TREF is defined as the time that it takes for thedriver output pin to reach the measurement voltage,VREF, starting from the beginning of the driver transitionat the pad. TREF must be generated using the same testload for TCO. Intel provides this timing value in theAGTL+ I/O buffer models.

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    In this manner, the following valid delay equation issatisfied:

    Valid Delay Equation

    Valid Delay = TCO + TFLIGHT-SYS - TREF = TCO-


    This valid delay equation is the total time from when thedriver sees a valid clock pulse to the time when thereceiver sees a valid data input.


    When a measurement is made on the actual system, TCOand flight time do not need correction since these are theactual numbers. These measurements include all of theeffects pertaining to the driver-system interface and thesame is true for the TCO. Therefore the addition of themeasured TCO and the flight time must be equal to thevalid delay calculated above.

    4.0. THEORY

    4.1. AGTL+

    AGTL+ is the electrical bus technology used for thePentium II Xeon processor bus. This is an incident waveswitching, open-drain bus with external pull-up resistorsthat provide both the high logic level and termination ateach load. The Pentium II Xeon processor AGTL+drivers contain a full cycle active pull-up device toimprove system timings. The AGTL+ specificationdefines:

    • Termination voltage (VTT).

    • Receiver reference voltage (VREF) as a function oftermination voltage (VTT).

    • Pentium® II Xeon™ processor Termination resistance(RTT).

    • Input low voltage (VIL).

    • Input high voltage (VIH).

    • NMOS on resistance (RONN).

    • Edge rate specifications.

    • Ringback specifications.

    • Overshoot/Undershoot specifications.

    • Settling Limit.

    The complete AGTL+ specification can be found in thePentium® II Xeon™ Processor at 400 MHz. Layoutrecommendations for the AGTL+ bus can be found inSection 3.0. of this document.

    4.2. Timing Requirements

    The system timing for AGTL+ is dependent on manythings. Each of the following elements combine todetermine the maximum and minimum frequency theAGTL+ bus can support:

    • The range of timings for each of the agents in thesystem.

    Clock to output [TCO]. (Note that the system loadis likely to be different from the “specification”load therefore the TCO observed in the systemmay not be the same as the TCO from thespecification.)

    The minimum required setup time to clock[TSU_MIN] for each receiving agent.

    • The range of flight time between each component.This includes:

    The velocity of propagation for the loaded printedcircuit board [SEFF].

    The board loading impact on the effective TCO inthe system.

    • The amount of skew and jitter in the system clockgeneration and distribution.

    • Changes in flight time due to crosstalk, noise, andother effects.

    4.3. Noise Margin

    The goal of this section is to describe the total amount ofnoise that can be tolerated in a system (the noise budget),identify the sources of noise in the system, andrecommend methods to analyze and control the noise sothat the allowed noise budget is not exceeded.

    There are several sources of noise which must beaccounted for in the system noise budget, including:

    • VREF variation

    • VCCCORE variation

    • Variation in VTT• Crosstalk

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    • Ringback due to impedance variation along thenetwork, termination mismatch, and/or stubs on thenetwork

    • Simultaneous Switching Output Effects

    The total noise budget is calculated by taking thedifference in the worst case specified input level and theworst case driven output level.

    Sections 4.3.1. and 4.3.2. discuss calculating noisemargin. These sections do not discuss ringback tolerantreceivers which can increase the effective noise margin.See the appropriate component datasheets forinformation about ringback tolerance.


    Equation 5 below shows a method for calculating fallingedge noise margin when the Pentium II Xeon processoris driving. An example calculation follows.

    Equation 5. Low Level Noise Margin


    ⇒ VREF_MIN - 100 mV - VOLMAX

    ⇒ [ [ 2/3 ( VTT_MIN) ] - 1% ] - 100 mV - VOLMAX

    Symbols for Equation 5 are:

    • VILMAX is the maximum specified valid input lowlevel from the component specification. For thisexample, 100 mV below the reference voltage isassumed.

    • VOLMAX is the maximum output low level thecomponent will drive. This VOLMAX maximum

    condition corresponds to the slow corner componentsand models.

    • VREF_MIN is the minimum valid voltage referenceused for the threshold reference.

    • VTT_MIN is the minimum termination voltage.

    For the following example calculations for low level andhigh level noise margin, an RON_MAX equal to 12.5 Ω isassumed, along with VREF and VTT toleranceassumptions. These specs should be obtained from thePentium® II Xeon™ Processor at 400 MHz.

    Solving for VREF_MIN with 1% VREF uncertainty:

    VREF_MIN = [ 2/3 ( VTT_MIN) ] - 1%

    = [ 2/3 (1.5 V - 9%) ] - 1%

    = [ 2/3 (1.37 V) ] - 1%

    = 901 mV

    The output low current in the case of VTT_MIN, can becalculated as shown below:

    I = V/R = 1.37/(25 Ω + 12.5 Ω) = 36.5 mA

    Then the VOLMAX for VREF_MIN is (36.5 mA * 12.5 Ω) =456 mV

    Then, Noise MarginLOW LEVEL

    = (VREF_MIN-100 mV) - VOLMAX= (901 mV - 100 mV) - 456 mV= 345 mV

    These example calculations are for an effectivetermination resistance of 25 Ω. These calculations do notinclude any resistive drop along the trace.

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    VTTNoise Margin

    BCLKSetup window

    +100 mV

    -100 mV


    Figure 4. Rising Edge Noise Margin


    Equation 6 below shows a method for calculating risingedge noise margin when the Pentium II Xeon processoris driving. An example calculation follows.

    Equation 6. High Level Noise Margin

    Noise MarginHIGH LEVEL = VOH_MIN - VIH_MIN ⇒

    VTT_MIN - (VREF_MAX + 100 mV)

    Symbols for Equation 6 are:

    • VIH_MIN is the minimum specified valid input highlevel from the component specification. For this

    example, 100 mV above the reference voltage isassumed.

    • VOH_MIN is the minimum output high level thecomponent will drive.

    • VTT_MIN is the minimum termination voltage. This isassumed to be 1.5 V - 9%, or 1.37 V.

    • VREF_MAX is the maximum valid voltage referenceused for the threshold reference. Since VREF isdefined as a function of VTT, the maximum VREF withVTT_MIN is 2/3 * (1.37 V) + 1% = 922 mV.

    • VOH_MIN for AGTL+ signals is VTT_MIN.• Then Noise MarginHIGH LEVEL

    = VTT_MIN - (VREF_MAX + 100 mV)

    = 1.37 V - 922 mV - 100 mV

    = 348 mV

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    Signal Propagates in bothdirections on agressor line.


    Figure 5. Propagation on Aggressor Network

    4.4. Crosstalk Theory

    AGTL+ signals swing across a smaller voltage range andhave a correspondingly smaller noise margins thantechnologies that have traditionally been used in personalcomputer designs. This requires that designers usingAGTL+ be more aware of crosstalk than they may havebeen in past designs.

    Crosstalk is caused through capacitive and inductivecoupling between networks. Crosstalk appears as bothbackward crosstalk and as forward crosstalk. Backwardcrosstalk creates an induced signal on a victim networkthat travels in a direction opposite that of the aggressor’ssignal. Forward crosstalk creates a signal that travels inthe same direction as the aggressor’s signal. On theAGTL+ bus, a driver on the aggressor network is not

    necessarily at the end of the network, therefore it sendssignals in both directions on the aggressor’s network.The signal propagating in each direction causes crosstalkon the victim network. Figure 5 shows a driver on theaggressor network and a receiver on the victim networkwhich illustrates this effect. Figure 6 shows twoaggressors on each side of the victim. Additionalaggressors are possible in the z-direction, if adjacentsignal layers are not routed in mutually perpendiculardirections. Because crosstalk coupling coefficientsdecrease rapidly with increasing separation, it is rarelynecessary to consider aggressors which are at least fiveline widths separated from the victim. The maximumcrosstalk occurs when all the aggressors are switching inthe same direction at the same time. There is crosstalkinternal to the IC packages, which can also affect thesignal quality.

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    82453 CPU ClusterCPUCPUCPU


    Agent 1 Agent 6






    Figure 6. Aggressor and Victim Networks









    Figure 7. Transmission Line Geometry: (A) Microstrip (B) Stripline

    Backward crosstalk is present in both stripline andmicrostrip geometry (see Figure 7). A way to rememberwhich geometry is stripline and which is microstrip isthat a stripline geometry requires stripping a layer awayto see the signal lines. The backward coupled amplitudeis proportional to the backward crosstalk coefficient, theaggressor’s signal amplitude, and the coupled length ofthe network up to a maximum which is dependent on therise/fall time of the aggressor’s signal. Backwardcrosstalk reaches a maximum (and remains constant)

    when the propagation time on the coupled networklength exceeds one half of the rise time of the aggressor’ssignal. Assuming the ideal ramp on the aggressor from0% to 100% voltage swing, and the rise time on anunloaded coupled network, then:

    Length for Max Backward CrosstalkRise Time

    Board Delay PerUnit Length= ×


    An example calculation if fast corner fall time is 1.5 V/nsand board delay is 175 ps/inch (2.1 ns/foot) follows:

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    Table 5. Example Backward Crosstalk Coupling Factors with εr = 4.5, VOH_MAX = 1.5 V, and Z0 = 65 ΩSpace:Width:Thickness Coupling Factor Maximum Crosstalk

    24:4:8 0.65% 9.8 mV

    20:4:8 1.3% 19. 5 mV

    16:4:8 1.75% 26.2 mV

    14:4:8 2.5% 37.5 mV

    12:4:8 3.4% 51.0 mV

    8:4:8 6.55% 98.2 mV

    4:4:8 13.5% 202.5 mV

    Fall time = 1.5 V/1.5 V/ns = 1 ns

    Length of maximum backward crosstalk =

    ½ * 1 ns * 1000 ps/ns /175 ps/in = 2.86 inches

    Agents on the AGTL+ bus drive signals in each directionon the network. This will cause backward crosstalk fromsegments on two sides of a driver. The pulses from thebackward crosstalk travel toward each other and willmeet and add at certain moments and positions on thebus. This can cause the voltage (noise) from crosstalk todouble.

    Table 5 provides example coupling factors for variousstripline space to width to dielectric thickness ratios (seeFigure 6) with dielectric constant εr = 4.5, VOH_MAX =1.5 V, and Z0 = 65 Ω. Note that the fast edge rates offalling edges place limits on the maximum coupledlength allowable, and Table 5 illustrates the potentialconsequences of maximum coupled lengths. Also, itshould be noted that multiple parallel coupled lines willincrease the impact on the noise budget.

    Forward crosstalk is absent in stripline topologies, butpresent in microstrip. (This is for the ideal case with auniform dielectric constant. In actual boards, forwardcrosstalk is nearly absent in stripline topologies, butabundant in microstrip.) The forward coupled amplitudeis proportional to the forward crosstalk coefficient, theaggressor’s signal edge rate (dv/dt), and the couplednetwork’s electrical length. The forward crosstalkcoefficient is also a function of the geometry. Unlikebackward crosstalk, forward crosstalk can grow withcoupled section length, and may transition in a directionsimilar to or opposite to that of the aggressor’s edge.Unlike backward crosstalk, forward crosstalk on thevictim signal will continue to grow as it passes throughmore coupled length before the aggressor’s wave front isabsorbed by the termination.


    To minimize crosstalk (and the “cost” of crosstalk) interms of noise margin budget:

    • Route adjacent trace layers in different directions(orthogonal preferred) to minimize the forward andbackward crosstalk that can occur from parallel traceson adjacent layers. This reduces the source ofcrosstalk.

    • Maximize the spacing between traces. Where traceshave to be close and parallel to each other, minimizethe distance that they are close together, andmaximize the distance between sections that haveclose spacing. Routing close together could occurwhere multiple signals have to route between a pair ofpins. When this happens the signals should be spreadapart where possible. Also note that routing multiplelayers in the same direction between reference planescan result in parallel traces that are close enough toeach other to have significant crosstalk.

    • Minimize the variation in board impedance (Z0). Forthe example topologies covered in this guideline,65 Ω +/- 10% was assumed.

    • Minimize the nominal board impedance within theAGTL+ specification while maintaining the sametrace width/spacing ratio. For a given dielectricconstant, this reduces the trace width/trace heightratio, which reduces the backward and forwardcrosstalk coefficients. Having reduced crosstalkcoefficients reduces the magnitude of the crosstalk.

    • Minimize the dielectric constant used in the PCBfabrication. As above, all else being equal, this putsthe traces closer to their reference planes and reducesthe magnitude of the crosstalk.

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    • Watch out for voltage doubling at a receiving agent,caused by the adding of the backward crosstalk oneither side of a driver. Minimize the total networklength of signals that have coupled sections. If therehas to be closely spaced/coupled lines, place themnear the center of the net. This will cause the point intime that voltage doubling occurs to be before thesetup window.

    • Route synchronous signals that could be driven bydifferent components in separate groups to minimizecrosstalk between these groups. The Pentium® IIXeon™ processor uses a split transaction bus with sixindependent sub buses (arbitration, request, error,snoop, response, and data). This implies that in agiven clock cycle, each sub bus could be driven by adifferent agent. If these two agents are at the oppositeprocess corner (one fast and one slow), thenseparating the bus types will reduce the impact ofcrosstalk.

    Simulation shows that space to line to dielectric ratios ofless than 3:1:2 can produce excessive crosstalk betweennetworks on the Pentium II Xeon processor bus. This isdue to the lower voltage swing of AGTL+, highfrequencies (even with the controlled edge rate buffers)and likely long parallel traces. Also, while rising edgerates are controlled, falling edge rates are not as wellcontrolled.


    The use of conventional “pull-up” resistor networks forIntel 450NX PCIset and cluster controller terminationmay not be suitable. These networks have a commonpower or ground pin at the extreme end of the package,shared by 13 to 19 resistors (for 14- and 20-pincomponents). These packages generally have too muchinductance to maintain the voltage/current needed at eachresistive load. Intel recommends using discrete resistors,resistor networks that have separate power/ground pinsfor each resistor, or working with a resistor networkvendor to obtain resistor networks that have acceptablecharacteristics.


    5.1. Textbook Timing Equations

    The textbook equations used to calculate the propagationrate of a PCB are the basis for spreadsheet calculations

    for timing margin based on the component parameters.These equations are:

    Equation 7. Intrinsic Impedance



    0= (Ω)

    Equation 8. Stripline Intrinsic Propagation Speed

    S STRIPLINE r0 1017_ . *= ε (ns/ft)

    Equation 9. Microstrip Intrinsic PropagationSpeed

    S MICROSTRIP r0 1017 0 475 0 67_ . * . * .= +ε


    Equation 10. Effective Propagation Speed



    = +0 10

    * (ns/ft)

    Equation 11. Effective Impedance











    Equation 12. Distributed Trace Capacitance




    = (pF/ft)

    Equation 13. Distributed Trace Inductance

    L Z S0 12 0 0= ∗ ∗ (nH/ft)

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    Symbols for Equation 7 through Equation 13 are:

    • S0 is the speed of the signal on an unloaded PCB inns/ft. This is referred to as the board propagationconstant.

    • S0_MICROSTRIP and S0_STRIPLINE refer to the speed ofthe signal on an unloaded microstrip or stripline traceon the PCB in ns/ft.

    • Z0 is the intrinsic impedance of the line in Ω and is afunction of the dielectric constant (εr), the line width,line height and line space from the plane(s). Theequations for Z0 are not included in this document.See the MECL System Design Handbook by WilliamR. Blood, Jr. for these equations.

    • C0 is the distributed trace capacitance per unit lengthof the network in pF/ft.

    • L0 is the distributed trace inductance per unit lengthof the network in nH/ft.

    • CD is the sum of the capacitance of all devices andstubs divided by the length of the network’s trunk,not including the portion connecting the end agents tothe termination resistors in pF/ft.

    • SEFF and ZEFF are the effective propagation constantand impedance of the PCB when the board is“loaded” with the components.

    5.2. Effective Impedance andTolerance/Variation

    The impedance of the PCB needs to be controlled whenthe PCB is fabricated. The method of specifying controlof the impedance needs to be determined to best suit eachsituation. Using stripline transmission lines (where thetrace is between two reference planes) is likely to givebetter results than microstrip (where the trace is on anexternal layer using an adjacent plane for reference withsolder mask and air on the other side of the trace). This isin part due to the difficulty of precise control of thedielectric constant of the solder mask, and the difficultyin limiting the plated thickness of microstrip conductors,which can substantially increase crosstalk.

    • The effective line impedance (ZEFF) is recommendedto be 65 Ω +/-10%, where ZEFF is defined byEquation 11.

    5.3. Power/Reference Planes, PCBStackup, and High FrequencyDecoupling


    Designs using the Pentium II Xeon processor requireseveral different voltages. The following paragraphsdescribe some of the impact of three common methodsused to distribute the required voltages. Refer to thePentium® II Xeon™ Processor Power DistributionGuidelines for more information on power distribution.

    The most conservative method of distributing thesevoltages is for each of them to have a dedicated plane. Ifany of these planes are used as an “AC ground”reference, then the plane needs to be AC coupled to thesystem ground plane. This method may require moretotal layers in the PCB than other methods. 1 ounce/ft2thick copper is recommended for all power and referenceplanes.

    A second method of power distribution is to use partialplanes in the immediate area needing the power, and toplace these planes on a routing layer on an as-neededbasis. These planes still need to be decoupled to groundto ensure stable voltages for the components beingsupplied. This method has the disadvantage of reducingarea that can be used to route traces. These partial planesmay also change the impedance of adjacent trace layers.(For instance, the impedance calculations may have beendone for a microstrip geometry, and adding a partialplane on the other side of the trace layer may turn themicrostrip into a stripline.)


    The type and number of layers for the PCB need to bechosen to balance many requirements. Many of theserequirements include:

    The maximum trace resistance for AGTL+ signal pathsshould not exceed 2 ohms. Depending on the trace widthchosen and PCB vendor’s process tolerance, this mayrequire 1 ounce/ft2 thick copper instead of 1/2 ounce/ft2thickness. A higher trace resistance increases the voltagedrop along the trace, which reduces the falling edge noisemargin.

    • Providing enough routing channels to support theminimum and maximum timing requirements of thecomponents.

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    • Providing stable voltage distribution for each of thecomponents.

    • Providing uniform impedance for the Pentium® IIXeon™ processor bus and other signals as needed.

    • Provide a ground plane under the principalcomponent side of the baseboard. Preferably underboth sides if active components are mounted on bothsides.

    • Minimizing coupling/crosstalk between the networks.

    • Minimizing RF emissions.

    • Maximizing PCB yield.

    • Minimizing PCB cost.

    • Minimizing cost to assemble PCB.

    The following baseboard stackup recommendationsshould help reduce the amount of SimultaneousSwitching Output (SSO) effects experienced.

    It is recommended that baseboard stackup be arrangedsuch that AGTL+ signal routes do not traverse multiplesignal layers, as this can create discontinuities in thesignal’s return path. It is also recommended that eachAGTL+ signal have a single reference plane for theentire route. Figure 8 shows the ideal case where aparticular signal is routed entirely within the same signallayer, with a ground layer as the single reference plane.

    Ground Plane

    Signal Layer A


    Figure 8. One Signal Layer and One ReferencePlane

    When it is not possible to route the entire AGTL+ signalon a single layer, there are methods to reduce the effectsof layer switches whereby the signal still references thesame plane (see Figure 9). Figure 10 shows anothermethod of minimizing layer switch discontinuities, butmay be less effective than Figure 9. In this case, thesignal still references the same type of reference plane(ground). In such a case, it is good practice to stitch (i.e.,connect) the two ground planes together with vias in thevicinity of the signal transition via.

    Signal Layer A

    Signal Layer B

    Ground Plane


    Figure 9. Layer Switch with One Reference Plane

    Signal Layer A

    Signal Layer B



    Ground Plane

    Ground Plane


    Figure 10. Layer Switch with Multiple ReferencePlanes (same type)

    When routing and stackup constraints require that anAGTL+ signal reference multiple planes, one method ofminimizing adverse effects is to add high-frequencydecoupling wherever the transitions occur, as shown inFigure 11 and Figure 12. Such decoupling should, again,be in the vicinity of the signal transition via and usecapacitors with minimal effective series resistance (ESR)and effective series inductance (ESL). Dual vias for thesecaps are recommended since via inductance maysometimes be higher than the actual capacitorinductance.

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    Signal Layer A

    Ground Plane

    Power Plane

    Signal Layer B




    Figure 11. Layer Switch with Multiple ReferencePlanes









    Signal Layer A



    Figure 12. One Layer with Multiple ReferencePlanes


    This section contains several high frequency decouplingrecommendations that will improve the return path for anAGTL+ signal. These design recommendations will verylikely reduce the amount of SSO effects.

    Just as layer switching and multiple reference planes cancreate discontinuities in an AGTL+ signal return path,discontinuities may also occur when a signal transitionsbetween the baseboard and cartridge. Therefore,providing adequate high-frequency decoupling acrossVCCCORE and ground at the Slot 2 connector interface on

    the baseboard will minimize the discontinuity in thesignal’s reference plane at this junction. Please note thatthese additional high-frequency decoupling capacitorsare in addition to the high-frequency decoupling alreadyon the processor.

    Transmission line geometry also influences the returnpath of the reference plane. The following are decouplingrecommendations that take this into consideration:

    • A signal that transitions from a stripline to anotherstripline should have close proximity decouplingbetween all four reference planes.

    • A signal that transitions from a stripline to amicrostrip (or vice versa) should have closeproximity decoupling between the three referenceplanes.

    • A signal that transitions from a stripline or microstripthrough vias or pins to a component (chipset, etc.)should have close proximity decoupling across allinvolved reference planes to ground for the device.

    5.3.4. SLOT 2 CONNECTOR

    Internal studies indicate that the use of thermal reliefs onthe connector pin layout pattern (especially ground pins)should be minimized. Such reliefs (cartwheels or wagon-wheels) increase the net ground inductance and reducethe integrity of the ground plane to which many signalsare referenced. Increased ground inductance has beenshown to aggravate SSO effects. Also, the anti-paddiameter (clearance holes in the planes) for the signalpins should be minimized since large anti-pads alsoreduce the integrity of the ground plane and increaseinductance.

    Some additional layout and EMI-reduction guidelinesregarding the Slot 2 connector follow:

    • Extend power/ground planes up to the Slot 2connector pins.

    • Extend the reference planes for AGTL+ and othercontrolled-impedance signals up to the Slot 2connector pins.

    • Minimize or remove thermal reliefs on power/groundpins.

    • Route VTT power with the widest signal trace or mini-plane as possible. Place decoupling caps across VTTand ground in the vicinity of the connector pins.

    • Use a ground plane under the principal componentside of the baseboard (and secondary side if itcontains active components).

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    • Distribute decoupling capacitors across power andground pins evenly around the connector (less than0.5 inch spacing) on the primary and secondary sides.

    • Minimize serpentine traces on outer layers.

    5.4. Clock Routing

    Analog simulations are required to ensure clock netsignal quality and skew is acceptable. The clock skew inPentium II Xeon processor based systems must be kept toa minimum (The calculations and simulations for theexample topologies in this document have a total clockskew of 150 ps and 150 ps of clock jitter). For a givendesign, the clock distribution system, including the clockcomponents, must be evaluated to ensure these samevalues are valid assumptions. The Pentium® II Xeon™Processor at 400 MHz specifies clock signal qualityrequirements for Pentium II Xeon processor systems. Tohelp meet these specifications, follow these generalguidelines:

    • Tie clock driver outputs.

    • Have equal electrical length and type of traces on thePCB (microstrip and stripline may have differentpropagation velocities).

    • Maintain consistent impedance for the clock traces.

    Minimize the number of vias in each trace.

    Minimize the number of different trace layersused to route the clocks.

    Keep other traces away from clock traces.

    • Lump the loads at the end of the trace if multiplecomponents are to be supported by a single clockoutput.

    • Have equal loads at the end of each network.

    The ideal way to route each clock trace is on the samesingle inner layer, next to a ground plane, isolated fromother traces, with the same total trace length, to the sametype of single load, with an equal length ground traceparallel to it, and driven by a zero skew clock driver.When deviations from ideal are required, going from asingle layer to a pair of layers adjacent to power/groundplanes would be a good compromise. The fewer numberof layers the clocks are routed on, the smaller theimpedance difference between each trace is likely to be.Maintaining an equal length and parallel ground trace forthe total length of each clock ensures a low inductanceground return and produces the minimum current pathloop area. (The parallel ground trace will have lowerinductance than the ground plane because of the mutualinductance of the current in the clock trace.)

    5.5. Conclusion

    AGTL+ routing requires a significant amount of effort.Planning ahead and leaving the necessary time availablefor correctly designing a board layout will provide thedesigner with the best chance of avoiding the moredifficult task of debugging inconsistent failures causedby poor signal integrity. Intel recommends planning alayout schedule that allows time for each of the tasksoutlined in this document.

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    Acceptable signal quality must be maintained over alloperating conditions to ensure reliable operation. SignalQuality is defined by four parameters: Overshoot,Undershoot, Settling Limit, and Ringback which arespecified in the Pentium® II Xeon™ Processor at 400MHz and Intel® 450NX PCIset. Timings are measured atthe pins of the driver and receiver, while signal integrityis observed at the receiver chip pad. When signalintegrity at the pad violates the following guidelines andadjustments need to be made to flight time, the adjustedflight time obtained at the chip pad can be assumed tohave been observed at the package pin, usually with asmall timing error penalty.


    To account for noise sources that may affect the way anAGTL+ signal becomes valid at a receiver, VREF isshifted by ∆VREF for measuring minimum and maximumflight times. The VREF Guardband region is bounded byVREF-∆VREF and VREF+∆VREF. ∆VREF has a value of100 mV, which accounts for the following noise sources:

    • 50 mV for motherboard coupling

    • 35 mV for VTT noise

    • 15 mV for VREF noise

    The example topology covered in this guideline assumesringback tolerance within 20 mV of VREF. Since VREF is

    guardbanded by 100 mV, this places the absoluteringback limits at:

    • 1.12 V for rising edge ringback

    • 0.88 V for falling edge ringback

    A violation of these ringback limits requires flight timecorrection as documented in the Pentium® II ProcessorDeveloper’s Manual.


    The overdrive region is the voltage range, at a receiver,from VREF to VREF + 200 mV for a low-to-high goingsignal and VREF to VREF - 200 mV for a high-to-lowgoing signal. The overdrive regions encompass the VREFGuardband. So, when VREF is shifted by ∆VREF fortiming measurements, the overdrive region does not shiftby ∆VREF. Figure 13 below depicts this relationship.Corrections for edge rate and ringback are documentedin the Pentium® II Processor Developer’s Manual.However, there is an exception to the documentedcorrection method. The Pentium® II ProcessorDeveloper’s Manual states that extrapolations should bemade from the last crossing of the overdrive region backto VREF. Simulations performed on this topology shouldextrapolate back to the appropriate VREF Guardbandboundary, and not VREF. So, for maximum rising edgecorrection, extrapolate back to VREF + ∆VREF. Formaximum falling edge corrections, extrapolate back toVREF - ∆VREF.

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    VREF + 200 mV

    VREF GuardbandVREF - 100 mV

    VREF + 100 mV ∆∆VREF∆∆VREF

    VREF - 200 mV

    Overdrive Region (200 mV)

    Overdrive Region (200 mV)


    Figure 13. Overdrive Region and VREF Guardband


    The minimum flight time for a rising edge is measuredfrom the time the driver crosses VREF when terminated toa test load, to the time when the signal first crosses VREF-∆VREF at the receiver (see Figure 14). Maximum flighttime is measured to the point where the signal first

    crosses VREF+∆VREF, assuming that ringback, edge rate,and monotonicity criteria are met. Corrections forviolations of these criteria are described later in thissection. Similarly, minimum flight time measurementsfor a falling edge are taken at the VREF+∆VREF crossing;maximum flight time is taken at the VREF-∆VREFcrossing.


    VREF + 200 mVOverdrive Region

    VREF GuardbandVREF - 100 mV

    VREF + 100 mV

    Driver Pin into Test Load

    Receiver Pin



    ∆VREF (100 mV)



    Figure 14. Rising Edge Flight Time Definition

  • UNITED STATES, Intel Corporation2200 Mission College Blvd., P.O. Box 58119, Santa Clara, CA 95052-8119

    Tel: +1 408 765-8080

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    Tel: +852 2844-4555

    CANADA, Intel Semiconductor of Canada, Ltd.190 Attwell Drive, Suite 500Rexdale, Ontario M9W 6H8

    Tel: +416 675-2438

    1.0. INTRODUCTION2.0. ABOUT THIS DOCUMENT2.1. Document Organization2.2. References2.3. Definition of Terms

    3.0. AGTL+ DESIGN GUIDELINE3.1. Determine Components3.2. Initial Timing Analysis3.3. Determine General Topology, Layout, and Routing Desired3.4. Pre-Layout Simulation3.4.1. METHODOLOGY3.4.2. SIMULATION CRITERIA




    4.0. THEORY4.1. AGTL+4.2. Timing Requirements4.3. Noise Margin4.3.1. FALLING EDGE OR LOW LEVEL NOISE MARGIN4.3.2. RISING EDGE OR HIGH LEVEL NOISE MARGIN


    5.0. MORE DETAILS AND INSIGHTS5.1. Textbook Timing Equations5.2. Effective Impedance and Tolerance/Variation5.3. Power/Reference Planes, PCB Stackup, and High Frequency Decoupling5.3.1. POWER DISTRIBUTION5.3.2. REFERENCE PLANES AND PCB STACKUP5.3.3. HIGH FREQUENCY DECOUPLING5.3.4. SLOT 2 CONNECTOR

    5.4. Clock Routing5.5. Conclusion