In general, I don't like to tie things like this to code execution speed, so
I use a timer to generate a 1mS system int and drive all the events at 10mS
and larger timing from that.

So in keyboard scanning, when I see a change on the inputs at the hardware
level, I load a byte in SRAM with the debounce time.
If I see another change, I reload the byte.

The 1mS tick int decs that byte to 0.
If the byte gets to 0, then I update the image of the keyboard, and signal
in a bit that the keyboard has changed.
When the key routine picks up the information, it clears this bit, which
prevents it from seeing a single long press as multiple single presses (and
provides a way to do auto-repeat)
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