On Mon, 15 Sep 1997, Shane Nelson wrote:

> Hi guys,
>
> I'm in need of some advice regarding multiplexing seven seg
> displays. I have 4 displays on 12 i/o pins, and have two
> concerns:
>
> 1) Visibility.
>         What is the optimum "on time" vs "off time" for best
> visibility, while at the same time having enough remaning cpu
> time to be able to handle various other tasks?
>
> 2) Current consumption.
>         How do I determine how much current the leds are going to
> need, and how can I keep these values w/in the current specs of
> the pic?  All the microchip literature seems to suggest that you
> can drive leds directly. I've been doing it that way but have a
> feeling I've been exceeding current specs on the i/o ports.
>
>
> Thanks for any help! :)
>
> -Shane.
>
PTM: I Have a proto on my table with PIC16C84 and 3 additional
chips (74HCxx and 4xxx). It has 5 x 7-segm LED, 25 keys, one beep, 2
digital inp and 10 digital outp. The board is eating 25 mA with the
LED's ON and 1 mA OFF. I use 10 mA per a LED segment. It the prosessor
stops the LED's don't destroy themselves. If You are absolute sure your
program is going to run flawless ad infinitum you can use even 1 A at
90% dead time.
The LED's are driven with 4511 decoder/latch/buffer so that there is a 1:5
mux in them with no dead time.
Using HighBright LED's you could have even more numbers. My friend made a
timer without external latch. With 1:4 mux and about 10% dead time
visibility is fine.

Negative point in this kind of drive is that you can't have ABC.. on the
LED's, only 123... Even a HEX-driver (0..9A..F) would be nice, but for
some odd reason these ABCDEF have been left out of all the cheap
LED-drivers.

(No, I'm not giving the schema out before Xmas :)

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PTM, pasi.mustalahti@utu.fi, ptmusta@utu.fi, http://www.utu.fi/~ptmusta
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