So you have 21 segment pins (not counting the decimal-point pins), plus one 
common?  Ouch!  Will you app need to have 7 segments on all 3 digits?  I ask 
cause perhaps your app may only need to display 0-100 for example, where the 
leftmost digit only needs 2 segments.  If not, you'll probably have to use 
some external devices -- either shift registers, 4-bit-to-7-seg decoders, or 
some arrangement of logic gates.

Try this in reverse -- start with the pins you have on the PIC, subtract the 
power pins, oscillator pins, MCLR, the input pins, etc.  Then with what you 
have left, subtract 2 for the shift register (data and clock).  The remaining 
go to LED segments, and the left-over segments get driven with the shift 
register.  The number of shift-register outputs will be decided by this.

Also check the 16C57 spec to see if it can source enough current to drive all 
those segments at once.  In addition to the 7-seg display, you'll end up with 
a lot of resistors.  Adding up the cost of the shift register, resistors, 
extra assembly time, etc, if it were me, I'd ditch the LED's on Ebay and pick 
up something multiplexed, unless there is something very specific about these 
displays that you need.

Cheers,
-Neil.


On Sunday 11 September 2005 01:52 pm, William "Chops" Westfield scribbled:
> My problem is that I have a single three-digit non-multiplexed
> display (actually, quite a few of them) with a SINGLE common.  It
> would be nice to drive it with a 16c57 (20 I/Os) and still have a
> pin or two left over for inputs.
>
> BillW


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