Measurement Computing USB-CTR04 - Manual (Page 14)

USB-CTR04 User's Guide Functional Details
Figure 8. Example of two debounce modes interpreting the same signal
Set the debounce time according to the amount of instability expected in the input signal. Setting a debounce
time that is too short may result in unwanted glitches clocking the counter. Setting a debounce time that is too
long may result in an input signal being rejected entirely. Some experimentation may be required to find the
appropriate debounce time for a particular application.
To see the effects of different debounce time settings, view the analog waveform along with the counter output.
This can be done by connecting the source to an analog input.
Use trigger before stable mode when the input signal has groups of glitches and each group is to be counted as
one. The trigger before stable mode recognizes and counts the first glitch within a group but rejects the
subsequent glitches within the group if the debounce time is set accordingly. Set the debounce time to
encompass one entire group of glitches, as shown in Figure 9.
Figure 9. Optimal debounce time for trigger before stable mode
Trigger after stable
mode behaves more like a traditional debounce function: rejecting glitches and only passing
state transitions after a required period of stability. Trigger after stable mode is used with electro-mechanical
devices like encoders and mechanical switches to reject switch bounce and disturbances due to a vibrating
encoder that is not otherwise moving.
The debounce time should be set short enough to accept the desired input pulse but longer than the period of the
undesired disturbance, as shown in Figure 10.
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