Hi everybody
if time period of a clock is 10ns and if uncertainty is as follows
clock_uncertainty -setup 3ns
clock_uncertainty -hold 1ns
then how tool will perform timing analysis for both setup and hold?
i mean is the tool will calculate the skew and jitters from clock uncertainty and perform setup and hold checks according to skew and jitters or directly it will subtract the uncertainty from time period for setup and will add for hold checks?..please clarify me..
thanks in advance
Hi,
The tool subtracts the uncertainty from the required time for setup analysis and adds the uncertainty to the required time for hold analysis.
Typically pre CTS your uncertainty includes your clock jitter + skew + design margin and post CTS uncertainty will be clock jitter + design margin.
Hi,
The tool subtracts the uncertainty from the required time for setup analysis and adds the uncertainty to the required time for hold analysis.
Typically pre CTS your uncertainty includes your clock jitter + skew + design margin and post CTS uncertainty will be clock jitter + design margin.
How is hold going to be affected by varying the effective clock period?
effective clock period for setup check = 10 -3 =7ns
clock_uncertainty -setup 3ns
clock period is 10ns.
for setup check in this case, we calculate the setup margin with effective clock period of 7ns?
ya, but why we have to calculate the setup margin with effective clock period of 7ns, actually uncertainty include skew+jitter+margin so if the skew is positive skew then effective clock period will be increase..but here irrespective of skew(either positive or negative) we are directly subtracting the uncertainty from clock period, why?
clock_uncertainty -setup 3ns
The 3ns includes skew+jitter+margin
You dont know the skew until you have built the clock tree. we take a pessimistic approach during synthesis and pre CTS.
uncertainty = skew+jitter+margin
the skew value can be positive or negative. It depends on how you define.
if the skew is useful, skew will be negative. otherwise, positive.
for hold, uncertainty = skew. it will not depend on jitter I think.
ya there is no issue of positive/negative for jitter, and one thing we have to notice is, jitter was not effect the hold, because we know that hold is calculated at same edge...But cant the jitter be either positive or negative. If its positive jitter there wont be any hold violation, but if its negative jitter there is a chance for hold violation to occur. Please correct if I am wrong.
ya there is no issue of positive/negative for jitter, and one thing we have to notice is, jitter was not effect the hold, because we know that hold is calculated at same edge...
U can think as the uncertainty will give less margin for timing slack. So that means u need to subtract this value from the RT for setup and add this margin to RT for hold.
setup slack = RT - AT
hold slack = AT - RT
Hope its clear to u.
U can think as the uncertainty will give less margin for timing slack. So that means u need to subtract this value from the RT for setup and add this margin to RT for hold.
setup slack = RT - AT
hold slack = AT - RT
Hope its clear to u.
Can some one pls explain jitter impact on hold
Jitter does not effect hold. It effects only setup.
Why because as hold is analysed on the same clock edge.
Hope it helped.
Cheers
vivek_p said:Can you please refer the below link and explain if possible
**broken link removed**
As per the pdf(link) , jitter impacts both setup and hold time on two different conditions
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