Schematic 18

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UW-Madison: ECE 555/755 Cadence Tutorial-I Prepared By: Ranjith Kumar

TUTORIAL I

CADENCE SCHEMATIC SIMULATION USING SPECTRE

Cadence Virtuoso Schematic editing provides a design environment comprising tools to

create schematics, symbols and run simulations. This tutorial will take you through the varioussteps involved in the creation of a schematic using Virtuoso schematic editor. The steps are

explained in context of a simple inverter and then later with two inverters to show hierarchy.

To have all your cadence related work in a folder, create a new folder cadence using

the command mkdir. Change your working directory to the folder created using the cd command. To set-up the cadence working environment, run the command fixcadence

o If you already have a cadence environment set, skip this step. All your cadence

environment settings will have to be reactivated after executing this command.o Setting of cadence working environment includes adding additional library paths.o You dont need to add any additional library paths for this course. NCSU

libraries are already set for you to use in this course To invoke a cadence, type icfb & at the command prompt.

o This opens a command interface window (CIW) as shown in Fig. 1.

Fig. I. Command Interface Window

We will use the NCSU CDK which automatically starts the library manager and youshould see the NCSU libraries as shown in Fig. 2.

ECE 555

Updated By : Vinod Reddy for 0.18u on 09/11/08


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Fig 2. Library Manager.

Creating a new library:o Select File --> New --> Library in Library manager.

o Enter a library name in the new library creation window as shown in Fig. 3. PressOK.

Fig. 3. New Library Creation Window.

o Then a new window opens to configure the new library with a technology file.The window is shown in Fig. 4.

o Select Attach to an existing techfile and press OK.


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Fig. 4. Window for configuring the technology file.

OK.

Creating a new schematic:o Select File --> New --> Cell view in library manager and choose library under

which you would like to create the new cell view.o Enter a cell view name and choose Composer-Schematic as the tool. View name

should be schematic exactly according to Fig. 6.

Fig. 6. Creating a new cell

o Under Technology Library select NCSU_TechLib_tsmc02d and presso A new window opens to attach the library file.



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o Click OK. A blank composer-schematic window should open.

Placing instances on schematic:o To draw the inverter, you have to add a PMOS transistor, a NMOS transistor,

GND, power supply, pins for input and output, and wire them together. To addcomponents to your schematic:

From Composer-Schematic menu, select Add --> Instance. This bringsoutAdd instance form with Component browser.

Fig. 7. Component Browser.

After selecting pmos4, place the pmos transistor by using a single leftmouse click in the composer schematic window.

If you need to get out of add instance mode, just press the Esc key. Follow similar steps as above to place a nmos transistor in the composer

instantiate a nmos transistor.

DD

Select the component vdd for supply voltage. Place it in the schematic. Select the component gnd for ground voltage. Place it in the schematic. If you make a mistake and need to get out of add instance mode, just

press the Esc key.

So far, you should have schematic as shown in Fig. 8.

Select library as NCSU Analog Parts. The component browser is shown in Fig. 7.

Select P_Transistors and then select pmos4 to instantiate a pmos transistor.

schematic window. You need to select nmos4 under N_Transistors to

To add the V and GND to the schematic, select Supply Nets from the

NCSU Analog Parts in the component browser.


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Fig. 8. Composer Schematic Window after the components is placed.

Defining the devices:

o After the instances are placed, we have to define the device widths and lengths.This can be done by

Selecting the device.

A device can be selected by a single left mouse click on thedevice that has to be defined. A solid white box appears on the

selected device. Press the letter Q in the keyboard. An Edit Object Properties window opens up as shown in Fig. 9.


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Fig. 9. Edit object properties window.

width. Press OK.

width. Press OK.

After defining the devices, you should have schematic as Fig. 10.

For a PMOS, select model name as tsmc18dP and define its length and

For a NMOS, select model name as tsmc18dN and define its length and

The minimum width for the devices in this technology is 27 0nm.

The minimum length for the devices in this technology is 180nm.


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Connecting wires with pins:o After the instances are defined and placed, you can connect them by wires to

form the inverter. To add wires,

From Composer-Schematic menu, select Add --> Wire (narrow). Click left mouse button on the starting point, and click left mouse button

on as many transition points as you want. Then if you want to end wire,just double click.

o After wiring, you need to create pins for input and output nodes. In order to dothis,

From Composer-Schematic menu, select Add --> Pin. AnAdd Pin form appears. The window is shown in Fig. 11.

Fig. 11. Window to add a pin.

Fig 10. Composer Schematic Window after the components are defined



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Type in "IN as the pin name, and make sure the Direction is "input" asshown Fig. 11.

Move cursor to schematic window. Then, click left mouse button toplace pin.

Move cursor back to Add Pin form, change "Direction" to "output".Type in "OUT" as the pin name. Then, repeat above step.

The pins for global signals are automatically given. Final schematic view of inverter will be similar as Fig. 12. Don't forget to save by selecting Design --> Check and Save in

Composer-Schematic menu.

Fig. 12. Completed schematic.

Creating a Symbol:o Select Design --> Create Cellview --> From Cell view in the Virtuoso

Schematic Editing Window. A new symbol creation window will open as shown

in Fig. 13. Press OK to create a symbol for the schematic you created.o A new Virtuoso Symbol editing window then appears as shown in Fig. 14.o If you want to name the part, then select @partName with a single left mouse

click. A white box appears over the @partName indicating that it is selected.o Press Q in the keyboard. An Edit Object Properties window opens as shown

in Fig. 15.o Change the partname to the name of your interest.o Don't forget to save by selecting Design --> Check and Save in Symbol Editing

menu.



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Fig. 13. Creating a Symbol

Fig. 14. Virtuoso Symbol Editing Window.

Fig. 15. Edit Object Properties Window


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Creating a Schematic using an existing instance:o Select File --> New --> Cell view in library manager and choose library under

which you would like to create the new cell view.

o Enter a cell view name and choose Composer-Schematic as the tool. View nameshould be schematic exactly according to Fig. 16.

Fig. 16. Creating a new cell

o To instantiate an already existing symbol, From Composer-Schematic menu, select Add --> Instance. This brings

outAdd instance form with Component browser.

Select library as ece555. The component browser is shown in Fig. 17.

Fig. 17. Component Browser

Place two instances of the inverter in parallel. Draw wires and connectthem as shown in Fig. 18.

A thick blue line indicates a wide wire (used for multiple bits).

A thin blue line indicates a narrow wire.


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Fig. 18. Schematic with existing instances

o To add wire labels on the bus. From Composer-Schematic menu, select Add --> Wire Name. The wire

name window in shown in Fig. 19.

Type a name as shown in the figure and select Bus Expansion ON. Place labels on each wire in the input side. Do a similar naming for the

output wires.

The named wires are shown in Fig. 20.

Fig. 19. Wire Name Window


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o After wiring, you need to create pins for input and output nodes. In order to do this, From Composer-Schematic menu, select Add --> Pin. AnAdd Pin form appears. Type in "A as the pin name, and make sure the Direction is "input".

Move cursor to schematic window. Then, click left mouse button to place pin. Move cursor back toAdd Pin form, change "Direction" to "output". Type in

"B as the pin name. Then, repeat above step. Final schematic view of inverter will be similar as Fig. 21. Since the global sources namely, VDD and GND are not in the schematic, we

can add them to the schematic. The initialization of the global sources in the schematic would enable us to

control the voltage sources when simulating the circuit. The new schematic isshown in Fig. 22.

Don't forget to save by selecting Design --> Check and Save in Composer-

Schematic menu.

Fig. 20. Schematic after named wires.


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Fig. 21. Complete Schematic for two inverters.

Fig. 22. Complete Schematic for two inverters with global sources.


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CIRCUIT SIMULATION

We will be using SPECTRE circuit simulator to simulate the designed circuit. This partof the tutorial will illustrate the steps to be followed before simulating a single inverter.

Simulating a bigger circuit with more devices is similar to the process described below.

Close all the windows and open the schematic of a single inverter.

From Composer-Schematic menu, select Tools --> Analog Environment. This bringsout Virtuoso Environment window as Fig. 23.

Fig. 23. Virtuoso Analog Environment.

Select Setup --> Simulator/Directory/Host.o A Choosing Simulator/Directory/Host window appears like Fig. 24. Make sure

the simulator is set to spectre.o If you need to change your simulation directory (place where all the simulation

files are created and stored), you can change the project directory.o Click on OK when you are done.

Fig. 24. Setting up the simulator.


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Select Setup --> Model Path.

o A Setting Model Path window appears like in Fig. 25. Browse to the path

where the model card is present and Click ADD and you will have awindow similar to the one shown in Fig. 25.

Fig. 25. Setting up the model path.

Then, in Virtuoso Environment window, select Analyses --> Choose and fill it as

shown in Fig. 26.o This is the setting to run a transient analysis for 20ns.

Fig. 26. Setting up transient analysis

o A copy of the model card for TSMC 0.18um CMOS Technology is available at /pong/usr2/v/vinodren/models /spectre/modelfile18


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In Virtuoso Environment window, select Outputs --> To Be Plotted --> Select On

Schematic.o Then, go back to schematic and click on the wires attached input and output of

inverter. The wires should change colors. Press Esc key to exit of selection mode.The signals should be added in the outputs window, as shown in Fig. 27.

Fig. 27. Setting up the output plots.

Select Setup --> Stimuli.

o A Setting Analog Stimuli window appears like in Fig. 28.o To set-up a input pulse at the pin IN

Select Stimulus Type Inputs

Check Enable

Set function to pulse

Set Voltage 1 to 0.0

Set Delay time to 0

Press Changeo To set-up a global inputs

Select Stimulus Type Global Sources

Check Enable

Set function to dc

Press Changeo Press OKo In Virtuoso Environment window, select Simulation --> Run.

The plot of the simulation should appear inthe Waveform Window,

similar to Fig. 28.

Set Voltage 2 to 1.8

Set Rise Time to 0.05n

Set Fall Time to 0.05n

Set Pulse Width to 2n

Set Period to 4n

Set DC Voltage to 1.8V


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Fig. 28. Final Output Plots.

To calculate the propagation delay using the calculator tool provided in Cadenceo In the waveform window, select Tools -> Calculator.o Double-click on delay to find the propagation delay between two signals. The

calculator window looks as shown in Fig. 29.

Fig. 29. Calculator Tools set to calculate the propagation delay between two signals

o Select wave in Selection Choices.o Place the cursor in Signal 1 and select the IN signal in the plot.o Place the cursor in Signal 2 and select the OUT signal in the plot.


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DD

o Set Edge Number 1 to 2o Set Edge Type 1 to risingo Press >>> to setup the calculator with more optionso Set Periodicity 1 to 1

DD

o Set Edge Number 2 to 2o Set Edge Type 2 to fallingo Set Periodicity 2 to 1o Press OKo Press Evalo The propagation delay between the two signals will be displayed as shown in Fig.

30.

The above set-up calculates the High-to-Low propagation delay between the signals. A

similar set-up with a change in the Edge Types can be used to calculate the Low-to-High propagation delay.

Fig. 30. Indicating the High-to-Low Propagation delay of the designed inverter

The circuit simulation set-up illustrates a step-by step procedure for doing a transient analysis.A similar procedure with a change in the analysis type can be used to perform a dc analysis.

o Set Threshold Value 1 to 0.9 [ V /2 (i.e.) 1.8/2]

o Set Threshold Value 2 to 0.9 [ V /2 (i.e.) 1.8/2]



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Technique II:The other option is to write the stimuli in a file and add it to the simulation whenever

needed.

A sample stimulus file for the above inverter is given below

A copy of the stimulus file is available at

V2 gnd! 0 0

The above commands can be stored in a file and can be added used during simulation.

In the Virtuoso Environment window, select Setup -> Simulation Files. A simulation

file setup window appears as shown in Fig. 32.

Fig. 32. Simulation File set-up.

In the Stimulus File enter the exact location of the file with all the stimulus values and

press OK.

Run the simulation as before.o Note: You still have to setup the analysis, technology file selection, and the

output waves to be plotted.

V1 vdd! 0 DC 1.8

VIN in 0 pulse (0 1.8 0 0.05n 0.05n 2n 4n)

o / pong/ usr 2/ v/ vi nodren/ ece555/ sampl e_st i mul i


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Syntax of some important statements

Pulse Signal: The definition of a pulse is as follows:pulse ( voltage_1 voltage_2 offset_time rise_time fall_time duty_cycle period)

cycle of 2n and a period of 4n

Piece-Wise Linear: The definition is as follows

pwl ( time1 voltage1 time2 voltage2 time3 voltage3 time4 voltage4..)

interval 4ns to 4.1ns and remains stationary afterwards.

Ex: pulse( 0 1.8 0 0.05n 0.05n 2n 4n)

Produces a pulse from 0V to 1.8V with 0 offset and 0.05n rise and fall time with a duty

Ex: pwl ( 0 0 2n 0 2.1n 1.8 4n 1.8 4.1n 0)

Produces a signal that stays at 0v from time 0s to 2ns and then rises to 1.8volts in the timeinterval 2ns to 2.1ns and stays at 1.8v from 2.1n to 4ns and goes from 1.8v to 0v in the time

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Schematic 18