This is a 4 digit decimal counter
which can operate as a free running counter or in count and hold
mode with manual reset. In either mode the counter can be
preset to count to a specified value. Clock edge and leading
zero suppression can also be configured.
The 7-segment display and indicator
LEDs are multiplexed. It will drive most common anode 7
segment LEDs. I used four single digit LEDs but a four digit
LED module could also be used.
In free running mode the overflow
output resets on the next clock pulse. Therefore the pulse duration is
directly related to the input clock frequency.
The clock input goes to a Schmitt
trigger input pin on the PIC. It will accept a 0-5V input
signal only. If it's used with a mechanical switch you should
use the debounce circuit shown on the schematic.
I've had the circuit clock reliably at
200Hz and it should be able to operate without missing a clock pulse
at frequencies up to 5Khz.