>
> Greetings,
>         I am building a moving message display. It is 7 rows x 80 columns.
> Multiplexing will be accomplished by loading shift registers with column
> data and the powering sequential rows for 1.64mS each. Therefore, each row
> will be refreshed once every 11.5mS.

Would you mind telling us how you're going to sink the current for each
row? even at a nominal 20ma per LED you can have a max sink current of
1.6A per row. That's a whole bunch of current!

Oh and the perceived brightness is more a function of peak current through
the LED rather than average brightness. So typically an LED pulsed at
100ma at a low duty cycle will have a higher perceied brightness than a
20ma at a higher duty cycle.

That's why I usually drive columns instead of rows at a much higher frequency.
Using drivers like the Allegro UCN5801 sink driver at a peak 500ma per, each
LED can be pulsed at 70ma each. And with loading taking only 10us or so at
20 Mhz on a pic (Read column, write to latch, shift column driver) it leaves
almost 600 uS between columns to do other stuff. Works great interrupt driven.


>         Is the refresh rate frequent enough to eliminate flicker?

86 Hz? probably not. May work if you can get the LED's bright enough. Do
you have a 5-8A sink for each row available (60-100ma current per LED)?

>Can I
> refresh at a slower rate? I sure could use the extra time for other tasks,
> but I don't want a flickery display!!

This is easily determined by testint. Just wire up one 5x7 display and vary
the refresh rate. It becomes very apparent when flicker occurs.

BTW how did you pick your display numbers above?

Your best bet is to pump up the current and drop the duty cycle, the
perceived brightness will go up and you'll get some of the persistence effect
from the user's eye meaning that you can go longer before flicker is perceived.

BAJ