>
> >        the 6 LEDs utilizing a common pin on one side. Something like:
> >
> >         Port pin---+-[220ohm]--+---[LED]------  Port pin
> >                    |
> >                    +-[220ohm]--+---[LED]------- Port pin
> >                    |
> >                    +-[220ohm]--+---[LED]------- Port pin
> >                    |
> >                    +-[220ohm]--+---[LED]------- Port pin
>
> Hi,
>
>    the problem with this design arises when you want all four LEDs running.

I definitely do not want all four LEDs running at the same time. That defeats
the original spec which is that each bi-polar LED can have a color independant
of the others. If all four are driven simulteanously then only two of the
possible 4 colors can be achieved (One color and off).

The plan is to multiplex them: i.e. turn on one LED at a time with the corrent
color, show that color for a while, turn that LED off and move on to the next
one which can be turned on with a completely different color. If you do this
fast enough, the eye blends the flashing LED's into a solid (but dimmer)
color.

And as Rick Miller pointed out in another post, with careful programming
it's not even necessary to have one resistor per led but one resistor
period. The current will only flow through the LED activated. And it seems
that the order of the LED and resistor isn't important. That's good to know.

> If we allow for 10mA through each LED the common pin has to source and sink
> 40mA. According to the 16C5X data sheet in front of me the specification is
> sink 25mA and source 20mA. In addition to which there are limits on total
> currents for each port, and for the total device.

I'd pull the full 20ma through the currently activated LED.. It's only one
LED, not all 6 that turn on at once so there's no need to limit the current to
10 ma
>
> One way around this is to use the common pin to drive a pair of transistors
> but that increases the part count and we are still faced with a 40mA per
> port limitation. Perhaps someone has a better solution.

Correct. If brightness is a real problem (and in this application it isn't)
then each port pin would have to drive a transistor pair. But that raises
the next question: Can a transistor pair be tri-stated? If so how would you
do it?

Thanks for the reply,

BAJ