Measurement Computing WBK17 - Manual (Page 9)

WBK17, Counter/Encoder Module 987996 WBK17, pg. 9
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.
Debounce Module – Trigger Before Stable Mode
The following time periods (T1 through T6) pertain to the above drawing.
T1 – In the illustrated example, the Input signal is low for the debounce time (equal to T1); therefore when
the input edge arrives at the end of time period T1 it is accepted and the Output (of the debounce
module) goes high. Note that a period of stability must precede the edge in order for the edge to be
accepted.
T2 – During time period T2, the input signal is not stable for a length of time equal to T1 (the debounce
time setting for this example.) Therefore, the output stays “high” and does not change state during
time period T2.
T3 – During time period T3, the input signal is stable for a time period equal to T1, meeting the debounce
requirement. The output is held at the high state. This is the same state as the input.
T4 – At anytime during time period T4, the input can change state. When this happens, the output will also
change state. At the end of time period T4, the input changes state, going low, and the output follows
this action [by going low].
T5 – During time period T5, the input signal again has disturbances that cause the input to not meet the
debounce time requirement. The output does not change state.
T6 – After time period T6, the input signal has been stable for the debounce time and therefore any edge on
the input after time period T6 will be immediately reflected in the output of the debounce module.
Mode Comparison
The following example shows how the two modes interpret the same input signal (which exhibits glitches).
Notice that the Trigger Before Stable mode will recognize more glitches than the Trigger After Stable
mode. Use the bypass option to achieve maximum glitch recognition.
Example of Two Debounce Modes Interpreting the Same Signal