Jinx wrote:

> > It's obvious why each column needs a high-current sink, but there's
> > something I'm missing here...
> 
> The display is also 3-dimensional. At each intersection of row and
> column is a choice of two colours

Yep, I understand that, but all that means is that, for each column of 6
bi-colour LEDs, there are actually 12 LEDs, and in the diagram at
http://freepdfhosting.com/uploads/6640a7ea8a.pdf it shows them being driven
by 12 independent outputs.

So - imagine there was no driver there.  Each of the 887's "high-side"
outputs, labeled D0-7 and A0-3, is connected to a single row of 7 LEDs.
Current will only be sunk from one of these at a time, as each column on
PORTC is pulled low, so each of the high-side outputs is only ever driving
as single LED at a time.

The only reason I can see for doing this through darlingtons would be if he
needs more than 25mA for a single LED - and as I write this, I remember that
being given as the reason, because of the LEDs being low efficiency and
having to be visible in broad daylight or whatever.  So in retrospect I
understand, but still think it's misguided - pumping >25mA through each LED
goes against all the discussion about battery life.  Better off with a pair
of higher-efficiency SMDs than the red/green LEDs.


David Meiklejohn
www.gooligum.com.au


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