A bit more on how this library is using Timer1 and Timer2 to run GSCLK and BLANK:

The PWM on the TLC5940 works by counting the number of GSCLK pulses; a setting of 2048 will mean the LED stays on for 2048 pulses and then turns off for the rest of the cycle.  After 4096 pulses, the counter needs to be reset by a pulse on BLANK.  In the library, I'm using Timer2 to generate the GSCLK pulses and Timer1 to generate the BLANK pulse.

I'm trying to fade multiple LEDs, independently, and at different rates. Is it possible to have the TLC5940 (or the arduino, for that matter) to run a series of "analogWrite" commands independently?

In other words, send one "fade channel0 from 4095 to 0 in 5000ms" and, before it's finished, send "fade channel1 from 0 to 4095 in 2500ms".

I know I could chain them together, so that as soon as channel0 finishes its fade, channel1 would begin...but that's not what I'm aiming for.

nphillips wrote on 20.08.2008 at 19:30:22:


I'm writing some improvements this weekend that changes to the way the library handles the BLANK pulses, and I think it would be pretty easy to add fading too:
1. When we pulse BLANK, check to see if any fades should happen, and calculate the PWM output
2. set an "updated" flag or something to let the user know they should call updatePWMs(). <- I don't think you could call updatePWMs() in 1. because it would put too much code in an interrupt, breaking Serial and stuff

The changes I'm making to the way BLANK pulses:
- Instead of outputting BLANK as a PWM on TIMER1, use TIMER1 to count GSCLK pulses and call an interrupt to pulse BLANK.

woot! Ti shipped me more TLC5940 samples today!

The project that I've been working on is building a 4096^3-colors version of SpokePOV that uses a secure-digital card for storage, so full video should be possible.

lwhi wrote on 22.08.2008 at 20:17:50:


I wasn't planning on multiplexing, so at least 6.  I'm assuming that a single rotation of the wheel will have ~30 PWM periods, which means one image is (24 bytes per tlc)*(6 tlc's)*(30 periods) = 4.32KB.  That comes out to ~231 frames per 1MB of SD card space.

You may have just ruined my weekend, acleone!

I'm at work, and don't have my arduino here to play with (and if I did, I'd probably get in trouble  ), but I can't wait to get home to try this stuff out. It looks exactly like what I wanted it to do.

You're awesome!

EDIT: WOW! I took a look at the code and examples, and if I'm understanding it right, it looks like you've really done some fantastic work. Now I want to try and leave work early!

At 10mph you're getting about 2 revolutions per second. With a single spokepov, that's too flickery to be called a persistent image. You'll probably want three spokes, and that only gets you the equivalent of a 6Hz update. And then both sides...and then two wheels. That's a lot of TLC5940s. 72 of them, actually.

You might not be happy with the brightness control. It's PWM, which will be visible on a moving LED. If you can achieve a 5MHz grayscale clock, you get almost 10 full PWM cycles for each of the 256 radial slices. That might be enough to not confuse the image you're trying to generate.

I know a guy who makes POV displays that have variable brightness, and he's actually using a simple resistor DAC for 3 bit brightness control on each LED. This doesn't use pulses to vary the apparent brightness, so it doesn't disrupt the virtual pixels on the POV display.