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What standard rise time to use for rising of an input signal?

Latest Updated:04/01/2006

Question:

What standard rise time should I use for the rising of an input signal whose slow rising edge will trigger an interrupt?

Answer:

Devices do not have standard rise times for edge detection.
For rising edges, it suffices if the level changes from a level that can be detected as the low level to a level that can be detected as the high level.

However, from the system's perspective, if there is also variation in the timing by which this slow rising edge is detected, you should carefully check whether that variation is causing any problems.

Also, a slow rise time means that the signal is more susceptible to noise.
As is shown in the figure below, what should have been a rising edge may be detected as multiple rising edges due to noise effects.

It is also important to check for noise effects because when edge detection fails for a falling edge, if the falling edge is sluggish then noise effects may cause it to be detected as a rising edge, which can trigger interrupts at unexpected times.

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