trace delay

[mostafa222]

Hi. How i can caculate the trace delay? For example i read from some document, the usb track must be in 2ns or shorter delay. Or in uart, match the tx line to within 50ps.
What is the meaning of this? What should i do in pcb

[mairomaster]

That is the difference in time between the two (or N) signals going down two tracks. If one of the tracks is longer, it will take the signal more time to arrive at the destination, assuming both signals have the same propagation speed. The formula is:

t = L/V

t - time of travel
L - length of the track
V - propagation speed of the signal

Propagation speed you can estimate reasonably accurately with the Saturn PCB calculator (conductor impedance tab). Once you convert the maximum propagation delay required to a distance (length) tolerance, that will help you to length match the signal tracks. Length matching is a very common topic - you can read a lot about it here on the forum and on the web in general.

If your examples are real, 50ps for uart sounds ridiculously small to me.

[mostafa222]

Thank you very very much

[robertferanec]

PS: UART is slow and asynchronous. You do not need to do any length matching there.

[mairomaster]

Wow I didn't even think about that (facepalm) Thank you for the clarification.

[robertferanec]

I am pretty sure you knew that