Sequential Circuits

Q1: Maximum Frequency

Q- Find the maximum clock frequency of the above circuit if specifications of the flip-flop are as T (setup) = 5ns     T (hold time) = 4ns             T (CLK to Q) = 9ns and maximum delay of the combinational circuit is T (c-delay) = 13 ns. There is a clock skew of +3ns for 2nd FF in the above circuit.

Ans: We firstly represent the delays wrt edge of 1st FF as

And the delayed input must reach before the edge reaches 2nd flip-flop

The clock skew is basically the delay in clock signal reaching 2nd flip-flop. Hence this is quiet similar to the previous question of a buffer in the pathway of clock.

Hence Clock time period is T = T CLK to Q + cdelay + Setup time – Clock Skew

= 9 + 13 + 5 – 3 = 24 ns

And maximum frequency of the circuit is F max = 1 / 24 = 4.16 MHz  

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