Improve Charlieplexing Performance With Interrupts


[Dmitry] was shopping for LEDs and accidentally pulled the trigger on the wrong type. Since he didn’t want to be wasteful, he figured he should at least take the time to build something with them.

A LED matrix display was the obvious project choice, but he only had a PIC16F688 at his disposal. Since the micro controller only has 11 output pins, charlieplexing was the only way he would be able to light the entire matrix.

While testing his LED array, he found charlieplexing to be a bit disappointing. The fact that the LEDs can get relatively dim, depending on the number of units lit at any particular time struck him as annoying.

In order to improve the performance of his charlieplexed array, he first decided to scan through all of the LEDs rather than just those that needed to be lit. This ensured that all of his LEDs had the same 1/110 duty cycle and were always as bright as possible. He also chose to use interrupts when lighting the LEDs. This meant that his code does not need to take into consideration any specific timing requirements to maintain persistence of vision. He also double-buffers the display to help reduce flicker.

He says that he ran into certain constraints with the PIC chip he chose, so he used a handful of lookup tables to ensure smooth operation of his display. He was quite satisfied with the results, and we think that the interrupt-driven display looks like it works just fine from where we’re standing as well.

Be sure to stick around for a quick video explaining and demonstrating his single-chip LED matrix.


20 thoughts on “Improve Charlieplexing Performance With Interrupts

  1. Note to haters: This is the same guy who rewrote the Palm TX ROM to take out most of the suck and replace it with nifty. What’ve you done that’s half that awesome?

  2. @Ian: I just wanted to toss it out there in case anybody felt like going “oh lol charlieplexing 101 what a looser”.

    @M4 et cetera: Wikipedia sez it was somebody called Charlie Allen who worked for Maxim. With 24 pins you could charlieplex up to 552 LEDs, which would be awesome.

  3. “Note to haters: This is the same guy who rewrote the Palm TX ROM to take out most of the suck and replace it with nifty. What’ve you done that’s half that awesome?”

    Put a Corvette LS3 engine in a Mazda Miata, I utterly destroy the wannabee Honda and Mitsubishi in town as well as Every Dodge challenger, charger, mustang that eve dares to rev at me.

    What have you done?

  4. Awesome, fartface. Did you use Flyin’ Miata parts to do your conversion? FM weren’t the first ones to put an LS engine in a Miata, but with their help it’s a hell of a lot easier.

  5. This project book marked, a Dimtry’s page lead to book mark two more outside of his.

    Seeing how the comments got off track about haters. True, haters are going to hate, but that doesn’t mean they shouldn’t be called out on it. But if you decide to do so please don’t use too broad brush to determine hate.

  6. It’s funny how you end up with the display technology that was used in the old 8bit systems like the appleII and c64 and such, a display driver scans a set of positions and you ‘poke’ values in the positions.
    Full circle

    1. i am running into this very problem driving 126 LEDs,; just not as bright as I would have hoped. I don’t have a lot of experience with the electronics… how would i use a transistor in the circuit? would i supply power to the LED’s seperately from my microcontroller? any insight would be greatly appreciated.

  7. Driving 552 LEDs with charlieplexing would result in a 1/552 duty cycle… that would be VERY dim unless you seriously overdrive the LEDs. For instance, if an LED has a nominal drive current of 20 mA to produce its standard brightness, you’d theoretically need to drive it with 552 * (20 mA) = 11 A to get the same apparent visual brightness… I doubt any LED would survive that.

    I say with LEDs it’s “go bright or go home!”

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