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Why Hold time is not considered while calculating max freq?

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Full Member level 3
Feb 23, 2004
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calculating maximum clock to flip flop


While calculating the maximum frequency of the circuit why hold time is not considered? When does this hold time come into play?

setup time,hold time and max frequency

Hi carrot,

Actually the max frequency is limited by the total maximum delay of combination logic between two flops including flop's setup time. So setup time should be considered during max frequency calculation. On the contrary the hold time of a flop limits the minimum total delay that the combination logic between two flops can have. So hold time has nothing to do with max frequency calculation.

hold time analysis

both of hold time and setup time are important.
but when we calculate max frequency, it is not nessasery to use hold time. because when we find max frequency, hold time satisfied inherently.

calculating maximum working frequency

Hold Time is considered for the previous clock with respect to Max frequency, i.e it has already expired.

calculating maximum frequency

Maximum frequency of a circuit grossly depends on the combinational delay path(longest) between the two adjacent flip flops. Now to be exact there is also an extra time for C-Q of the first FF plus the SKEW between the two FF clocks and setup time, as these also come in the paths between two flip flops. As soon as setup time is maintained it means that the data has succesfully reached the FLop before the clock and is ready for sampling.

Now the flip flop takes some time to sample the given input once the clock has reached. This sampling time is the hold time and in terms of circuits its the time required for the input capacitances to charge and hold the charge.

Hence hold time is independent of frequency.

If you see it in a broad view setup time and hold time are very much related due to the fact that the clock edge decides who is who. If clock is little late the part of hold time comes into setup and vice versa.
min frequecy and max freq a flip flop

For timing analysis, setup timing (max speed/frequency) analysis use slow corner case, while hold timing analysis uses fast corner case. Think for a while, you'll know why.

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