what does the propergation delay in datasheet mean in practise

[amateur_25]

Hi

I see in datasheet of 7400 series logic ic's they mension properagation delay and output transistion delays.

For example say the output transistion is 25ns, how do I know it'll work reliably if I stick a 1MHZ serial data bus through it?

This maybe a stupid question since 1MHZ is propbably not that fast but I want to understand those number instead of guessing and hoping for the best.

[tggzzz]

Look at the diagrams in the datasheet; they will define what the manufacturer is prepared to guarantee. If your device's datasheet doesn't have diagrams, use another manufacturer.

Draw a picture of your signal, in this case paying attention to the relative timing of various parts of your signal. Then add the effects specified in your device's datasheet to understand what the output will look like.

To determine whether that is sufficient, look at the diagrams in the datasheet of whatever will be receiving the signal.

If you are using "long lines" then the connections may affect the signal and you might need to treat them as transmission lines. See "Bogotin's Rules of Thumb" for examples.

[T3sl4co1l]

There is usually a diagram explaining what is measured.

It is usually the time measured between 50% points of the input and output waveform.

The maximum switching, or toggle, or clock, frequency of a device is usually on the order of the propagation delay (i.e., for a 32ns delay, ~32MHz).  But it can be greater or lesser, depending on internal design.

The very fastest logic is little slower than the speed of light across the package: an internal propagation delay  <100ps, yet the delay as measured at the pins is 300ps or more -- because the signal has to propagate along the pins to get there, first!  This is one of the more extreme cases, where the clock frequency can be much higher (10s GHz?) than the propagation delay, simply because the propagation delay, as measured at any useful location, is several wavelengths already.  (Needless to say, design of digital circuits at those frequencies requires considerable thought in matching up the delays.)

Tim