Timing and Constraints

“The software is the lens through which the user views the FPGA.”

-Bill Carter

Outline• Basic Timing (comb. and sequential)• Block timing models

– LUTs– BRAM– Multipliers

• Some standard design timing tricks• Constraints

– Timing– Geometric (pinning & arrangement – Combinations

• Best tools– Experience– Insight

Propagation Delay

LUT Tpd is called “Tilo”

Flop Timing

LUT Logic Timing Parameters

Basic LUT Logic Timing

Distributed RAM Timing Model

Dist. RAM Parameters

LUT SRL Timing Model

SRL Timing Parameters

LUT w. Carry Chain Focus

Carry Chain Timing

BRAM Timing Model

BRAM Timing Params

Multiplier Model & Delay Variation

Multiplier Timing Params

Routing Delays

• FPGA datasheets do not give details on routing delay. Hence:– Routing delays not known to designer until

design is placed and routed– Delays for early silicon are frequently still

under analysis– Software maintains best source for the real

timing

• FPGA datasheets do provide times associated with incremental silicon blocks

Virtex style logic tile

Comment:CLE with IMUX and OMUX is what weCall the “CLB”

What you may find inside the Interconnect block

Little black splotchesAre muxes or littlePIPs to make selectableAttachments….

Some standard timing tricks

• Load splitting (aka fanout reduction)– Identify sites driving large number of loads– Insert buffered version of the signal with multiple

buffers each handling a piece of the total load– Result usually faster

• Pipelining– Insert flip flop stages to reduce setup time restrictions– Increases clocking speed, at expense of added

latency

Pipelining Idea

Pipeline solutionEach flip stage can operate at fasterRate than before, but result goes validAfter TWO clocks.

Constraints

• More options than we will discuss today• High level, global constraints = big payoff• Will compare a couple of designs across

multiple constraints/combinations to illustrate:– 32 bit adder (inherent internal constraint)

• Combinational suggests tPD constraints

– 32 bit shifter (very malleable)• Sequential suggests Fmax or cycle constraints

Spartan 3S50TQ144

Big Adder

module Big_Adder1( input [31:0] A, input [31:0] B, output [32:0] SUM );

assign SUM = A + B;endmodule

Big Adder (unconstrained)

Adder with 20 nsec tPD constraint

Change constraint

• Original unconstrained looks ~same as the nominal 20 nsec constraint.

• 20 nsec constraint came in at 11.83 nsec.

• Push it down a little, to say 11 nsec and see what happens……

Adder with 11 ns constraint

original new

Timing Improvement Wizard

Screen 1

TIW

Screen 2

TIW

Screen 3

Bad news ~78% of theDelay is due to logicSuggests need for fasterpart

Note

Several bitsAre out of spec

Interesting…

Faster designMeets timeWithoutShift to right?

Comment:RecompiledOn -5 version(original =-4)

Comments

• Free pinning, free routing gave a result and revealed that 11.83 nsec possible

• Free pinning, constrained to 11 nsec revealed 11 nsec is NOT possible (for -4 part)~78% time spent in silicon delay

~22% time spent in routing delay

Faster part (-5) hits 11 nsec, with centered design.

Faster part won’t hit 10 nsec when constrained

(please experiment for yourself!)

“Big_Shifter” Codemodule Big_Shifter( input C, input ALOAD, input SI, input [31:0] D, output SO );reg [31:0] tmp; always @(posedge C or posedge ALOAD) begin if (ALOAD) tmp = D; else begin tmp = {tmp[30:0], SI}; end end assign SO = tmp[31];endmodule

Big Shifter (unconstrained)

Run Failed….chunk of PAR report

Revised Constraints

Revised layout

Constraint revised again

Didn’t run: PAR report advice

Hmmm…based on slack revise to:

PAR report from revised setup/hold times

Bingo!

clock

Clock net

Serial in

Serial out

From “FloorPlan IO Pre-Synthesis”

Just defining at the BANK level(versus explicit PADs)

Placing half pins on Bank 0

Shifted the design aroundBut still met timing…..

Closing Comments• The ISE constraints guide is online• It has timing, placement, grouping, relational and

synthesis level constraints for both VHDL and Verilog

• MOST designers prefer to have a design.ucf file as a separate item.

• Best results most often by writing in RTL with .ucf file

• Best approach is to experiment using small designs to see what the results are– Examine various reports– Look at “world view”– Pay attention to advice from S/W

Footnote: WebPack 11.1 v. of shifter