Electronic – Falling edge detector sometimes doesn’t work

edge detectionflipflop

I have a falling edge detector built based on a D flip-flop as shown in the following figure:


Components: 74hc74, 74ls04n, 74hc08

A sample output is shown below (I invert the output since I need it as an active low):

logic analyzer output

My question is why it sometimes doesn't detect the falling edge?

Thank you all so much!

The problem was the logic analyzer sampling rate and frequency as @Elliot Alderson mentioned.

I re-configured the PulseView and finally could capture the output correctly (indeed there was nothing wrong with the circuit itself)

logic analyzer output

Best Answer

My guess is that the pulses are too short for the logic analyzer to catch them. Logic analyzers sample the signals at a constant rate, and if the time between samples is longer than your pulse width then the logic analyzer simply won't see it.

Try decreasing the sampling time significantly, or switch to an oscilloscope.