Hallo Reza,

in one your last replies you told, that 4k7 pull-up resistor presents a strong pull-up, eliminating noise, so that a debouncing scheme, you know several subsequent pollings must show the same result, is not needed.

I don't agree with that, for two reasons:

1. If you compare the source impedance of your combination 'key + pull-up', which is 4.7kOhm of course when key is not hit, with the source impedance of a standard 74HCMOS push-pull output, which is about 50Ohm, you will notice, that impedance is bigger by a factor of about 100! Means, the probability that a false trigger occurs due to charge injection via stray capacitance is greater by a factor of 100.

2. Copper traces connecting the keys of your keyboard to micro are rather long and present ideal antennas, best suited for picking up hum and noise.

From my understanding, the noise suppression characteristic of a debouncing scheme is more than needed, and it's not at all a good idea to accept a key as being hit with the first low level detected by your micro!!


Kai