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Honored Contributor I
1,400 Views

hold timing and clock frequency considerations

hello all, 

 

two questions for the experts: 

 

1. why hold time is not frequency depended? 

 

2. will it be reasonable to assume that design that fits timing requirements with 100MHz clock frequency will also work properly with lower frequency clock? 

 

thanks.
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Honored Contributor I
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1. Hold time is pretty small compared to setup time. It is usually setup that is violated. 

2. Yes - if it met for 100Mhz, it will work for anything lower (IO may be different).
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Honored Contributor I
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1. Hold time is based on the same clock edge, whereas setup time is based on consecutive clock edges. Thus hold time is not frequency dependent, whereas setup time will be. 

2. Tricky's answer is correct.
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Honored Contributor I
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thanks to both of you. 

 

can you please elaborate about your answer to my second question? 

why it is ok to define the main clock of the design, lets say 100MHz, but actually inject to the fpga 80MHz clock?
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Honored Contributor I
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Timing analysis checks to ensure the setup time is not violated. If you use a slower clock, the setup required period is the same, but the time between potential bit changes is now longer. So if it didnt break the setup timing before, it wont now.

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Honored Contributor I
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for setup requirements i understand.  

but what about the hold requirements? skew? and other parameters? 

 

from my understanding, the skew is tailored according to specific clock that defined in the sdc file.  

it doesn't matter if we inject lower frequency that specified?
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Honored Contributor I
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--- Quote Start ---  

for setup requirements i understand.  

but what about the hold requirements? skew? and other parameters? 

 

from my understanding, the skew is tailored according to specific clock that defined in the sdc file.  

it doesn't matter if we inject lower frequency that specified? 

--- Quote End ---  

 

 

hold is managed by fitter making sure that clock delay is never more than data delay (unless you gate the clock) 

skew and other delays are function of routing/temperature (not clock speed)
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