Jinx,

Don't you find that there is a big difference in brightness between
columns with different numbers of LEDs on?  It looks to me like a column
with 1 LED would run that one at 18+ mA (really bright), but with all 8 on
each LED would be driven at 1/8 that, or about 2.3mA.  This is why the
somewhat similar scanned array I built (smaller, it's 12x16) uses no
current limiting resistors and depends on the low duty cycle to determine
the current.  I used a transistor for each row & column drive, bases
driven by the PIC port.  I forget what the scan rate is, but I seem to
recall it being around 1kHz.

Downside is that if the CPU latches up whatever LEDs are on
are in trouble, but that's why I use the watchdog on a short leash!

Dale
--
"Curiosity is the very basis of education and if you tell me that
curiosity killed the cat, I say only the cat died nobly."
          - Arnold Edinborough


On Mon, 4 Feb 2002, Jinx wrote:

> > I have built an 8 x 128 leds designs as a matrix display,
> > the rows [8] goes to rb0  to rb7 then the columns goes to the
> > ra0 to ra3 through a cascade demultiplexer
>
> It sounds very similar to mine, except I use transistors for
> drive. Without them it would be very dim. I have to disagree
> slightly with others' comments about the required power.
> Although this uses normal 3mm LEDs it is quite bright and
> uses only 40mA (surprised me !!)  For 5mm ultrabrights, I
> agree, the power consumed will be higher, but what I found
> illustrates that it can be done at lower power (battery supply
> was a requirement). This one is strobing at 200Hz, based
> on a timer IRQ. It's a work in progress (displays messages
> but the s/w is unrefined), so can't help much further
>
> http://home.clear.net.nz/pages/joecolquitt/message.html

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.