View Full Version : Only the Propeller for me. But Can I ...

Areal Person
09-20-2007, 05:24 AM
Hi, http://forums.parallax.com/images/smilies/turn.gif

Ok, most/some know by now what I'm working on...

But if you dont, here it is.

A 6” x 6” Modular RGB Matrix dbl-sided PCB that supports 3 LED’s per pixel (a red, green & blue)
centered at a 19mm DOT pitch.· It has 192 Led’s per 6” panel with standard anode, cathode traces used for individual row/column Led selection. NOTE: 4 of these 6” panels work together to make 1-1’ sq. module.

Here's a small & simple COMPLETLY DIY Proof of concept·I did using only the Propeller, The production version is going to go the PCB manufracturer·! I'm not doing it.

Now the project & preformance needs to be high quality, as good or better as the real thing !

Ok, right, now you know the problem.

Now·this is what I need help on.

What I·want to do:
I want to use·ONLY the Propeller chip for driving the led panels (save basic circuit rqmts.).
there are 4 panels per 1' module…
I need to control 768 led’s with a Propeller only solution. I figure 4 SMD Propellers on a custom designed controller
would do this. The four would need to be programmed to work corrently together (Obvisouly etc. etc.)·
there would be some general coordanation and inner-chip communication on the tasks (coorprative processing)
for reading the data to be displayed and where it goes etc... All that stuff...

I know I can use LED PWM driver chips with dot correction etc... But really do I need to ?
Can't the propeller (If I use enough working together) Do it all ? What can't it do that I would need ?

QUESTION: http://forums.parallax.com/images/smilies/confused.gif
Can I use only Propeller chips to drive this type display ? I would like to a 15-20 sec video clip at 30fps ? can I ? Why not ?
There will need to be at least 2 duty cycles, 1 for blending/dimming to a 65k color for a Pixel. The other for refresh
60Hz-70Hz maybe, tasking etc.

Please tell me why this will not work ?

Thank you for all the help, I can't do it without you!
Areal Person

I have a tree growing out of my head, but

what do you expect ? I'm a programmer.

Mike Green
09-20-2007, 05:50 AM
A couple of questions/concerns ...
1) The Propeller has limited I/O drive capability. Sure each I/O pin is capable of sourcing or sinking 30-40ma, but the total current for the chip is only 300ma. If you want to multiplex the LEDs, you'll need a higher peak current than the Propeller can handle. What are you going to use to drive the LEDs? Some transistors would be cheap and would work.
2) What kind of resolution are we talking about? BTX has been working on a VGA resolution display using the Propeller. The code for that is really tight on speed.
3) Where's the video coming from? Is it a VGA signal? If not, what?

09-20-2007, 06:06 AM
We had similar discussions in the Charlieplexing threads. My experience is , that a "very bright" LED rated 500-800 mcd @20 mA works very fine @ 1mA (most likely with 30 mCd http://forums.parallax.com/images/smilies/smile.gif )

Multiplexing considerations can take into acount that you generally will NOT have a 10 fold brightness when the whole screen is on with contrast to 10% of the scren being on....

So one design decision is: How bright (max!) should the screen become, converting this to a current value. This is one landmark as to how many Props to be used, or what drivers....
768 LEDs can be Charlieplexed by 4 Props each controlling 2 clusters à 110 LEDs needing 11+11 pins each.

That will add up to 40mA * 2 * 4 =320 mA = 620 mW for 2V LEDs

Looked from another side: This will be the brightness of 8 very bright LEDS alltogether ....

Areal Person
09-20-2007, 09:29 PM
Yes, you raise important fundemental design issues, that I now can address, these are the issues I'm
currently working to define. I'll bump the post in a day or so with a follow up.
I've got to start wrighting the spec. for the driver board.

Thank you very much for your help & support.
I really appreciate it.


I have a tree growing out of my head, but

what do you expect ? I'm a programmer.