Contamination delay

Also called: tcd, t_cd, minimum delay

The shortest time from an input change until a gate or circuit output can start to change. Written tcd; always at most tpd.

The contamination delay tcd is the minimum delay of a gate: the earliest moment after an input change that the output could begin to move. Until then, the output is guaranteed to still show its old value.

After an input changes, an output goes through three phases:

Before tcd: certainly the old value.
Between tcd and tpd: possibly changing. Don't trust it.
After tpd: certainly the new value.

The name helps: after tcd, the old value starts to be contaminated by the new one.

Why have two numbers? Real delays vary with temperature, supply voltage and manufacturing. The propagation delay is the slowest case and tcd the fastest, so tcd ≤ tpd always.

tcd matters whenever the question is "how soon could this change?" Its big use is the hold constraint: new data racing out of one flip-flop mustn't reach the next one too early. Along a path, contamination delays add, and the path with the smallest total is the short path.

Worked examples

Example

Three phases of one output

An AND gate has tcd = 15 ps and tpd = 45 ps. One input rises at t = 100 ps, which should make the output rise.

1.
100 to 115 ps (before tcd): the output is certainly still 0.
2.
115 to 145 ps: the output may be rising. It could read as 0 or 1.
3.
From 145 ps (after tpd): the output is certainly 1.
4.
The uncertain stretch lasts tpd − tcd = 30 ps.

Example