Another related issue is the digital filter methods.
Right now I have two choice. I am using a simple +1/-1
method now. The filter counter will be increased by
1 (up to 8) or decreased by 1 (down to 0). The output
will be triggered when the counter reaches 8 (or 6 and then
promoted to 8) or 0. Another slightly different scheme 
is to trigger the output when the counter reaches
6 (then promote it to 8) or 2 (then downgrade it to
0).

The other idea is to use a shift register, every time 
when the MCU detected the signal, it shifts a 1 into the 
shift register. Only 8 consecutive detected signal (or
undetected signal) will trigger the output. It will reject 
the noise better but then it will slow the response time 
significantly even in the low noise situation.

Any better solutions?

Regards,
Xiaofan
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