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svip
Adventurer
Adventurer
224 Views
Registered: ‎02-01-2008

timing analysis setup time interpretation

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Hello to all
Analyzing the post-implementation timing path report there is an aspect that is not clear for me. In order to simplify, let's suppose that there are not buffer on the clock line in order to not consider them in the STA.
In order to analyze the setup time in a path the equation:
tpff1+tpc<T-tsuff2 must be satisfied
where tpff1 is the propagation time of the starting flip flop, tpc is the propagation time of the combinational path, T is the timing required and finally, tsuff2 is the setup-time of the ending flip flop.
However, analyzing the post-implementation timing path it seems that the equation used by the tool is
tpff1+tpc<T+tsuff2
If I understand correctly the post-implementation report, the terms T and tsuff2 are added together and define the destination clock path (i am supposing clock delay equal to zero for both the flip flop involved in the path in order to simplify the analisys)
why they are added? shouldn't they be subtracted?
what am I doing wrong?
Thank you to all

 

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avrumw
Expert
Expert
192 Views
Registered: ‎01-23-2009

This is an idiosyncrasy of the timing report (that traces way back to how things were done in Synopsys).

The setup requirement of most of the flip-flops in the FPGA is negative (normally a small negative number).

When the timing report subtracts a negative number it doesn't make this clear - instead of doing something like 

- (-0.04)

This shows up as 

+ 0.04

In the timing report

So your analysis is correct - you are just being confused by the fact that the setup requirement is negative.

Avrum

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avrumw
Expert
Expert
193 Views
Registered: ‎01-23-2009

This is an idiosyncrasy of the timing report (that traces way back to how things were done in Synopsys).

The setup requirement of most of the flip-flops in the FPGA is negative (normally a small negative number).

When the timing report subtracts a negative number it doesn't make this clear - instead of doing something like 

- (-0.04)

This shows up as 

+ 0.04

In the timing report

So your analysis is correct - you are just being confused by the fact that the setup requirement is negative.

Avrum

View solution in original post