Measurement Computing PCI-2511 User Manual page 16

PCI-2511 User's Guide
Trigger after stable mode
In the trigger after stable mode, the output of the debounce module does not change state until a period of
stability has been achieved. This means that the input has an edge, and then must be stable for a period of time
equal to the debounce time.
The following time periods (T1 through T5) pertain to
the debounce module is required to have a period of stability after an incoming edge, in order for that edge to be
accepted (passed through to the counter module.) The debounce time for this example is equal to T2 and T5.
T1 – In the example above, the input signal goes high at the beginning of time period T1, but never stays

high for a period of time equal to the debounce time setting (equal to T2 for this example.)
T2 – At the end of time period T2, the input signal has transitioned high and stayed there for the required

amount of time—therefore the output transitions high. If the input signal does not stabilize in the high state
long enough, no transition would have appeared on the output and the entire disturbance on the input would
have been rejected.
T3 – During time period T3, the input signal remained steady. No change in output is seen.

T4 – During time period T4, the input signal has more disturbances and does not stabilize in any state long

enough. No change in the output is seen.
T5 – At the end of time period T5, the input signal has transitioned low and stayed there for the required

amount of time—therefore the output goes low.
Trigger before stable mode
In the trigger before stable mode, the output of the debounce module immediately changes state, but will not
change state again until a period of stability has passed. For this reason the mode can be used to detect glitches.
The following time periods (T1 through T6) pertain to the above drawing.
Figure 6. Debounce model block diagram
Figure 7. Debounce module – trigger after stable mode
Figure 8. Debounce module – Trigger before stable mode
7. In trigger after stable mode, the input signal to
Figure
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Functional Details