Minimum clock-to-Q delay

Also called: tcq(min), minimum clock-to-Q, tccq, clock-to-Q contamination delay

The shortest time after a flip-flop's clock edge before Q can start to change. Written tcq(min); it is the flip-flop's contamination delay.

A flip-flop's Q doesn't jump the instant the clock rises. The minimum clock-to-Q delay tcq(min) is the earliest moment after the edge that Q could begin to change. Until then, Q is guaranteed to still show its old value.

It's the flip-flop's version of a contamination delay, just as the clock to q delay tcq is its version of a propagation delay. After an edge:

before tcq(min): Q is certainly old;
between tcq(min) and tcq: Q may be changing;
after tcq: Q is certainly new.

Where it's used: the hold constraint. New data leaves the launching flip-flop no earlier than tcq(min), so tcq(min) + tcd ≥ th. A larger tcq(min) makes hold easier to meet.

Some textbooks call it tccq (clock-to-Q contamination) and call tcq tpcq.

Worked example

Example

How long is Q uncertain?

A flip-flop has tcq(min) = 20 ps and tcq = 75 ps. The clock rises at t = 400 ps.

1.
Q certainly holds its old value until 400 + 20 = 420 ps.
2.
Q certainly shows its new value from 400 + 75 = 475 ps.
3.
In between, for 75 − 20 = 55 ps, Q is uncertain.

Common mistakes

Using tcq(min) in the setup check. Setup needs the slowest case, tcq.
Confusing tcq(min) with hold time. Hold time is a requirement on D; tcq(min) is a delay on Q.

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Minimum clock-to-Q delay is taught in Timing and Sequential Logic.

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