Re: how to control LED array?


"Lord Garth" <LGarth@xxxxxxxxxxxx> wrote in message
news:9psbe.423$gd5.117@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>
> "Anthony Fremont" <spam@xxxxxxxxxxxx> wrote in message
> news:f1qbe.26530$AE6.26160@xxxxxxxxxxxxxxxxxxxxxxx
> >
> > "John Fields" <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote in message
> >
> > <snip for brevity>
> >
> > > The display will need to be multiplexed, and the most efficient
way to
> > > do that would be to broadside data, a byte at a time, into the row
> > > drivers and then to scan the columns. If you want to make a
> > > non-flickering display, then the pixel update rate should be
somewhat
> > > higher than 30Hz, say 50Hz just to make sure. That means that
since
> >
> > I disagree on the refresh rate. Having just experimented with PWM
on
> > some RGB LEDs, I can attest to the fact that 40Hz refresh is easily
> > visible as flicker at very low duty cycles. You _will_ see the
annoying
> > flickering. I suggest using at least 80Hz for completely flicker
free
> > operation. Before you get all upset, I know it seems awfully high
> > compared to what's acceptible for TV and movies, but I assure you
that
> > the flicker is much more visible on the LEDs. There is plenty of
advice
> > on the net about using refresh rates around 100Hz for this very
reason.
> > Just commenting for the benefit of the OP.
> >
> > Of course if the OP doesn't need low intensity levels, then a slower
> > refresh may be fine.
> >
>
> I was just thinking that a column multiplexing scheme as opposed to
> a row multiplexing scheme would be more likely to show strobing
> as one drove by. Is this indeed the case? (not that this is at all a
> concern
> to the OP and his 8x8 array)

I don't know if that would make any difference. My experiments were
with only one LED and flickering was clearly visible at >40Hz refresh
combined with low duty cycles. [I was using a software based 8 bit PWM
routine (~25mS period with ~100uS resolution)] When I upped the clock
speed to 8Mhz so that the period was ~12.5mS with ~50uS resolution the
flicker was not visible to me even with a 1/256 duty cycle. I was also
surprised by the brightness of the LED at 1/256 duty cycle, it was
clearly visible.

Disclaimer: I know that a linear duty cycle is not the most appropriate
method of PWM driving an LED, but it was easy to implement. With the
ISR running every 50uS, you don't have allot of time to jack around.
;-)

Using an 8Mhz internally clocked 16F88, I was able to achieve my goal of
imperceptibly varying the colors of an RGB LED, in a rainbow like
fashion, without generating shades of grey, or taking a week to cycle
thru all 16 million combinations. No biggy, but it looks allot better
than those cheesy things with the built in color sequencer. ;-) My 10
month old daughter really likes the effect and IMO it's going to look
really good on my scooter as full-color, adjustable accent lighting.
8-)

.

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