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Author Topic: Flaky Atmega328p + 6xTLC5940 setup  (Read 632 times)
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Hi all,

I've done a lot of searching and researching (ha, puns) regarding my problem and hope that the community can help me and point out what I'm doing wrong/stupidly. smiley

I'm using, as the subject says, 6 TLC5940s daisy chained from a stand-alone atmega328p (with arduino bootloader and using the arduino IDE/TLC5940 library).  When I run the BasicUse sketch, all seems good, but I find if I turn on more than a few LEDs simultaneously the setup does one of:  lock-up, get flakey (twitchy, reset like behaviour) or starts flashing LEDs very rapidly.

Some notes on my setup:

  • Circuit is faithfully based on:  http://tlc5940arduino.googlecode.com/svn/wiki/images/breadboard-arduino-tlc5940.png
  • The atmega328p has a 20Mhz crystal on it and additions from the multiple TLC5940 diagram in "Demystifying the TLC5940" - not the pinouts, but the additional components (diodes, resistors, capacitors and the aforementioned crystal).
  • I was afraid that my amp draw might be too much and am currently (ha, another pun) using an ATX power supply (20A max at 5V) rather than the 1200mA @ 5V switching supply I used originally - no change.
  • If I do  Tlc.setAll(4095) (or even 1024 for that matter) the LEDs all turn on and flicker rapidly until the whole thing goes dark.

I did a quick and dirty random lighting sketch along these lines:

Code:
#include "Tlc5940.h"

void setup()
{
  Tlc.init();
  randomSeed(analogRead(0));
}

void loop()
{
  Tlc.clear();
  for (int i=0; i<32; i++)
  {
    Tlc.set(random(0,95),random(2047,4095));
  }
  Tlc.update();
  delay(500);
}

...the setup will work at first, but generally drifts into unstable land as described above.

Other things I've thought about and need guidance on:

  • Looking at tlc_config.h, I have set the NUM_TLCS to 6 but I'm concerned about TLC_PWM_PERIOD (default 8192) and TLC_GSCLK_PERIOD (default 3) as they seem to be set for a microcontroller clock of 16Mhz.  Should I change this?  Could this be part of my issue?  More often than not, when things get flaky, I see the results coming from the latter TLC5940s in the chain.
  • I'm using and arduino uno to program the atmega328p using this shield:  http://www.adafruit.com/products/462 ...and setting the board as an arduino uno, should I be setting it to something different (as I'm using a stand alone chip at 20MHz)?   For details on the settings, see:  http://www.smacula.co.uk/2011/10/adjusting-tlc5940-libraries-to-run.html
  • I read one posting that talked about putting a capacitor across GSCLK/SCLK to cut down on noise, but that strikes me a little odd, no?
  • The length of wire from the board to the LEDs is about 18" right now - but that doesn't seem too long to me, based on reading what other people have done (read: many feet long).

Any thoughts or guidance would be appreciated, I'm building a disco dance floor for my wife's 40th birthday party (this Saturday) and while I could hook up something that regularly resets the boards, I'd rather do things right.

Thanks in advance.
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My guess is that the LEDs are either sucking away all your power causing a temporary board reset OR dumping that much power into GND is temporarily raising GND causing the TLC chips to read bad values on the IO lines.  See http://effluviaofascatteredmind.blogspot.com/2012/05/tlc5940-tlc5941-and-arduino.html.  You need a bunch of various sized caps to do "decoupling" in a big way.  Try 1000uF, 100uF and a ceramic at a couple of uF (I didn't really bother to figure out whether all of these are needed...)

20mhz chips is not an issue; the TLCs can handle clocks up to 30mhz and the fastest a 20mhz chip can clock an IO line is < 10mhz.

Cheers!
Andrew
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The decoupling caps you need are 0.1uF ceramic. On each chip as close to the power supply pins as possible. The best is to solder a surface mount cap directly across the two pins.
A photo of your setup would help define other problems of layout or construction.
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My direct experience with the 5941 chip is that the datasheet recommended .1uF is not enough.  I believe it has to do with fluctuations in the shared GND when the chip turns on its 80mA LEDs and so suddenly dumps a lot of power on the GND plane.  But this is mostly guesswork because my scope is limited to 100Mhz.
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Hey guys, thanks for the replies - responses in-line.

The decoupling caps you need are 0.1uF ceramic. On each chip as close to the power supply pins as possible. The best is to solder a surface mount cap directly across the two pins.
A photo of your setup would help define other problems of layout or construction.

I do have those, actually.  As per this diagram.

NB: I'm using the pin-outs from atmega, as per the examples in with the arduino TLC5940 library.

Quote from: AndrewStone
My direct experience with the 5941 chip is that the datasheet recommended .1uF is not enough.  I believe it has to do with fluctuations in the shared GND when the chip turns on its 80mA LEDs and so suddenly dumps a lot of power on the GND plane.  But this is mostly guesswork because my scope is limited to 100Mhz.

I'm only drawing 18mA per channel - I'm using a 2.2K ohm IREF resistor - or are you talking of an aggregate load?  Do you think I still need beefier caps?
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Here's a picture of a board, without all the wiring yet, beyond the ground/vcc wiring.

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I added 100uF caps in parallel with the existing caps - and the boards work flawlessly now.

Grumpy_Mike gets another notch in his "it's always a decoupling problem" belt and AndrewStone gets props for the 100uF suggestion. smiley
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