Very true, but I was speculating for a device I created a couple
months ago that needed one A/D, so I used a 16F872, but then had
a bunch of other I/O left over, so I thought I could make use of
them to get some real power into the LED's.  Those were run at
20% duty-cycle (2x5 multiplexed bargraph).

I also use SIP resistors with a single common on the common side
of the mux, but if I used DIP resistor networks, I could run say
R1 from RB0 to LED anode, R2 from RB1 to the same LED anode, then
the common to the transitor, etc.  This way, any brief unexpected
moment of conflicting outputs to a single LED would have the
protection of 2R between them.

But's it's all speculation for now, and just my mind working
overtime on a boring Sunday.

Cheers,
-Neil.




-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Spehro Pefhany
Sent: Sunday, May 19, 2002 9:12 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: On the subject of LED drivers...


At 07:40 PM 5/19/02 -0500, you wrote:
>Just started looking thru my bin of old TTL parts for a 74xx244 or some
>other driver to experiment with driving a higher current thru my 4 mux'ed
>LED's.  But then I thought -- if I have extra ports on the PIC, why can't
>I run these in pairs for each LED segment?  At 25mA max source/sink
>on a 16F872, I could get 50mA to the LED's.  And to prevent the
>condition of 2 pins of a pair being out of sync, I would use 2 pins on the
>same port for each LED ... RB0 and RB1 for segment a, RB2 and RB3
>for segment b, etc.

Yes, you could do that.. But I'd suggest getting a brighter display,
port pins are generally too valuable to waste 8 of them (the driver
would be cheaper). There is some worry about what could happen if the
port (through noise or whatever) gets set so the outputs are fighting
each other, but you'll be updating the port every few milliseconds
anyway.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the
reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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