The counter is triggered by a negative edge on its
CLOCKinput. That is, the CLOCK positive pin is
momentarily connected to ground. The Clock input is
debounced in the software to prevent false triggering
when mechanical switches inputs are used. The debounce
time is approx 15msec for each low/high or high/low
transition, giving a maximum count rate of 30 to 35 counts
per second. The counters may be easily mounted together
so the displays can merge together. You can work out for
yourself how many months this dual unit will count for
before an overflow is generated.