I am still getting my feet wet programming PIC microcontrollers and need
help with a project.  I would like to build a 4-digit counter using a
16F628.  For the oscillator, I would like to use the Internal 4MHz RC.
For inputs, I need three push-button switches: Up, Down, and Reset.  For
output, I need 4 common-cathode 7-segment LED displays.  I would like to
write code to debounce the switches.  This should be a fairly simple
project, but at this stage in my PIC experience, it would really help if
I could start with someone else's code that works and modify it.

To start with, I want to learn to multiplex 4 7-segment LED displays.  I
know how to write my own code to count from 0-9 on one display,
incrementing once per second.  Where I am getting stuck is multiplexing
4 displays at once.  I have tried the code in AN557, modifying it work
with the 16F628.  It compiles and partly works.  The PORTB outputs go
high and low as they should for a one-digit display.  PORTA pins remain
at ground.  So, I can connect the common cathode of one display to
ground and it counts up, one digit per second, but only on one display.

One problem I am facing is I don't really understand how to use the TMR0
interrupt.

If anyone is interesting in helping, I have posted the code at:
http://www.patrickrichards.com/picmicro/projects/multiplex7seg/multiplex
7seg.asm.

If there is a better way for me to approach this problem, any
suggestions would be appreciated.

I have searched the PICList Archive and while I do see some code for
driving 7-segment displays, I couldn't find any I could easily adapt to
my needs.

Thanks!
-Pat

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