At 01:14 PM 2/10/97 +0000, Byron A Jeff wrote:
>>
>> 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
>
 I agree. A bunch of current. I plan on using TIP120 darlingtons. A robust power
supply may be one of my smaller challenges as this is only my second pic
project.
The display numbers were picked based on a compromise between wanting a larger
display and the available ram in a 16c63.

Thanks to all who have responded with comments!  Jeff