Measuring digital coincidence

[LouAlder]

There is an add mode on oscilloscope to measure the logic coincidence signals. Somehow you use the add mode to compare digital signals with each other. What does it mean to measure logic coincidence signals on TTL/cmos logic boards?

 

[poor mystic]

Hi Lou
Welcome to Electronics Point

"What does it mean to measure logic coincidence signals on TTL/cmos logic boards?"

It's the kind of thing that might be useful in chasing faults on a digital circuit, where you might have a fault which is indirectly related to a logic level on another part of the board. At least, that's how I'd try and use it...

Mark

 

[hevans1944]

You trigger the 'scope on one channel with one of your digital signals and apply the other digital signal to the other channel. When you select add mode, you will see a stable waveform (triggered sweep) from the first channel with the second channel superimposed on it. If the second channel is synchronous with the first channel, you will be able to see any differences in timing, such as a delay from when the first channel is triggered and the rising (or falling) edge of the signal on the second channel.

 

[LouAlder]

Is that for testing logic racing issues problems? or timing issues?

How would you test for logic racing issues?

 

[poor mystic]

Hopefully, the circuit architecture does not allow logic races; a circuit which could fail in that way would be badly designed.

 

[LouAlder]

measure the logic coincidence signals means the logic timing are aligned

I'm not sure what coincidence means when measuring logic signals. Coincidence of what?

 

[hevans1944]

It can be handy to verify setup timing constraints, for example to see if the inputs to a flip-flop are stable before the clock edge occurs. It can also tell you if there is clock skew if one signal is derived from the clock and the other signal is also derived from the clock but possibly delayed or with jitter. However, a digital signal oscilloscope does a better job of resolving timing issues and usually accommodates several digital input channels. Do not confuse a digital signal oscilloscope with a digital logic analyzer. The latter displays logic levels instead of actual waveforms and is usually capable of triggering on digital patterns selected by the operator.

measure the logic coincidence signals means the logic timing are aligned

I'm not sure what coincidence means when measuring logic signals. Coincidence of what?

It generally means that the two signals, for some period of time (the coincidence interval) have stable logic levels. It can also mean that the edges of two logic transitions are aligned within specified time limits. It is not possible for two transition edges to be perfectly simultaneous, but this is a difficult statement to prove (it involves Special Relativity and the nature of simultaneity). You can take my word for it or do further research yourself. For logic circuits, you can arrange for two transitions to be "close enough" to be considered simultaneous for clocking purposes.

 

[poor mystic]

I'm not sure what coincidence means when measuring logic signals. Coincidence of what?

In the sense that things coincide... i.e. happen at the same time or synchronously.