Brian Aase wrote:

> Here's an interesting (and somewhat PIC-based) situation;
> A customer has a requirement for a product that would have
> an array of bicolor LEDs, the kind with two leads. Since the
> necessary states of each LED include Off as well as red
> and green, the obvious but messy way to drive them involves
> connecting each LED lead to an output port. I.E. each LED
> uses up two outputs.  This works great for just a few LEDs
> but uses up port pins really fast!  We're talking some 50 or so
> LEDs here.  :-(
>
> One variation I thought of would be to connect one side of
> each LED to a source of V+/2 and then tristate the output
> to get the Off condition.  Better, but it still uses too many
> i/o pins.  The code needed to do mutliplexing for power
> reduction is a little scary too, but that can be dealt with.
>
> Can anyone suggest a way to further reduce the i/o pin count
> by some sort of clever matrix connection?  My poor tired brain
> is just drawing a blank on this.

Ultimately, the pin count is the count of the LED pins...
But, You can set up a series of latches to accomplish driving an array of
LEDs.
Row = 1, Column =1 LED is OFF, Row = 0, Column =0 LED is OFF
Row = 1, Column =0 LED is RED, Row = 0, Column =1 LED is GREEN
and you can have Row(n) by Column (m) of LEDs.
So the I/O pins required are the number of bits in the Row and columns
latches (8!),
and the address bits for the latch(es).

>
>
> Brian Aase
>
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  |  __O    Thomas C. Sefranek  tcs@cmcorp.com
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