why hold time is not included in critical frequency calc.

[srik_naidu]

hi,
can anyone explain me why hold time is not included in the calc of critical freq.
is there any particular reason if so please explain me that reason.

waiting for reply,

with regards,
srik.

 

[circuit]

Re: why hold time is not included in critical frequency calc

say you have two D FFs, output of 1st FF goes to second. the 1st FF would have transmitted the data to the output on the 1st clock and the next FF would take it on the second clock. Now there is a clock-Q delay + the propagation delay before Q1 reached 2nd FF. Also 2nd FF has a set up time constraint. So effectively the time period

T should be > T(setup) + T(clock-q) + T(prop delay)
which means f is < 1/() which defines the max frequency of the system. But hold time does not affect the max frequency.

 

[Yun Lin]

The hold time don't affect the frequency. That is, if there is a hold time issue, you can't fix it by slow down the clock. So it's more diffcuity to improve the hold time issue.

 

[amarnath]

Re: why hold time is not included in critical frequency calc

The hold time don't affect the frequency. That is, if there is a hold time issue, you can't fix it by slow down the clock. So it's more diffcuity to improve the hold time issue.

i think u are incorrect,hold time violations can be corrected by adding delays in the path of the data that can satisfy the hold time equation:

Thold<D(c-q)min+Tlogic(min)

regards
amarnath

 

[aryajur]

Re: why hold time is not included in critical frequency calc

I think generally the propagation delay is more than the hold time, in case the propagation delay is less than the hold time, I think we should consider that in determining the maximum clock frequency for reliable operation. That is to say that we should consider hold time or propagation delay whichever is more.

 

[power-twq]

Re: why hold time is not included in critical frequency calc

because hold time has no effect to operating frequency.

best regards

hi,
can anyone explain me why hold time is not included in the calc of critical freq.
is there any particular reason if so please explain me that reason.

waiting for reply,

with regards,
srik.

 

[arvkum02]

Re: why hold time is not included in critical frequency calc

If you see hold time consatraint , it comes at the same egde of the operating clock i.e there is no time period involved in hold delay calculation that's why hold doesn't effect max frequency.

If you see set up constraint , it comes b/w two diffrent edge of operating clock , the allowable time diffrence between these edge determines your max frequency.

Once the design is closed ,lets say without hold fix , there will be functional failures even if you reduce the clock period because clock period increment increase the time diffrence b/w two diffrent edges, it would not effect the the same clock edge.

Thanks
Arvind