When reading a value from a button (or any external pin), the signal may not be stable during the first clock cycles after the release. So, the bumping input signal can lead a reset manager circuit to a very instable state, and avoiding to initialize correctly the system.
A debouncer addresses this scenario and propagates the input to the system in a safe way. Its behavior is pretty simple:
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The module is composed first by a synchronization stage. Indeed, the signal catched from the pad is not in same clock domain than the reset manager or the rest of the system. Refer to the 2FFDs synchronizer document for further explanation.
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Once synchronized, the input signal is passed into a XNOR gate. This gate will drive a
logical 1if its two inputs are equal, otherwise alogical 0. The first input will be the synchronized signal, the second the same signal delayed with a basic FFD. This permits to detect when the input pad is flipping. -
A timer will observe the XNORed signal and count on each cycle when stable. If the XNOR output is stable enough long (setup with a parameter), the debounced output drives the synchronized input (from the first stage). Otherwise it continues to drive the previous value and stay with a zero value.