# propagation delay in logic gates

#### shiva kumar

Joined Jun 27, 2008
11
Hi,,I have a query regarding the propagation delay in a gate,propagation delay is generally measured between 50 percent points on the input and output waveforms, why cannot we measure it between 20% or 80% points on the waveforms ? Is there any specific reason for opting 50% points ??

Regards,
Shiva.

#### MrChips

Joined Oct 2, 2009
31,068
In theory, if the rise times and fall times are the same then it does not matter where you choose. You have to pick a common point somewhere and the 50% point is the best because the slope is at the maximum.

#### holnis

Joined Nov 25, 2011
49
It is possible to characterize various output transition times (e.g. 10%, 50%, and 90%) as a function of the input transition time, If it is needed.

On the other hand and by definition: Propagation Delay, Is the time between a change in an input and a low to high (or high to low) change on the output. Measured from 50% point on input signal to 50% point on the output signal.