Arduino showing multiplexed LED matrix

Posted in: Art, Gadgets, Software, Tech

To control any more than 12 LEDs on an Arduino board requires a bit of trickery. The Diecimila with its Atmega168 chip has 14 digital output pins, but you somewhat lose two from the TX/RX serial pins on 0 and 1 leaving just 12 pins. Today I built an LED matrix with 20 lights using nine pins,

The technique is to light row by row, and rely on our persistence of vision effect. Playing around with the delay it seemed like 3ms for a total of 15ms (five rows) was about the point where flicker just started being noticeable.

Geeky detail after the fold...

The software pulls the row pins high one by one while the column pins are set low for every LED to light, sinking the current from the one high row pin. There's a 3ms delay, then the next row pin is brought high and the last one low.

The data for the little animation is a 2D byte array that looks like,

byte PATTERN[][5] = { 
  /* ... */
  {B0111,
   B1110,
   B1100,
   B1000,
   B0000,},
  /* ... */
};

('B' is a handy prefix for a binary number.)

You can see a diagonal band of 1's there in the early part of the wipe. Each byte's translated into pin writes by looking at the lowest bit in the byte and then shifting right and looking at the next. LOW is on.

void SetColumn(byte pattern) {
  for (int i = COL_COUNT-1; i >= 0; i--, pattern >>= 1) {
    digitalWrite(COL_PINS[i], pattern & 1 ? LOW : HIGH);
  }
}

A "frame" of the display is completed by lighting each row in turn,

void ShowPattern(byte pattern[]) {
  int last_row = ROW_COUNT-1;
  for (int row = 0; row < ROW_COUNT; last_row = row++) {
    digitalWrite(ROW_PINS[row], HIGH);
    digitalWrite(ROW_PINS[last_row], LOW);
    SetColumn(pattern[row]);
    delay(MULTIPLEX_DELAY_MS);
  }
}

The main loop draws each frame over and over until a certain amount of time has elapsed and then it goes onto the next one. millis() is a built-in that returns the number of milliseconds since the board was switched on. (This code would fail after about nine hours when the counter resets.)

void loop()
{
  for (int pos = 0; pos < PATTERN_SIZE; pos++) {
    long start = millis();
    while (millis() - start < PATTERN_DELAY_MS) {
      ShowPattern(PATTERN[pos]);
    }
  }
}

Notes

I actually had the circuit the other way around, cycling through column-by-column and the rows programmed in turn with a line of lights. This happened out to make writing patterns awkward so I flipped the orientation which made SetColumn pretty straightforward.

The matrix could've had more lights on it but the breadboard limited width a bit. I was quite proud of the tight wiring under the LEDS.

From Multiplexed LED Matrix

This was one of the projects where the bits that seemed like they'd be hard Just Worked and then silly things wasted time, like C mistakes with sizeof.

int ROW_PINS[] = {13, 12, 11, 10, 9};
int COL_PINS[] = {5, 4, 3, 2};
#define ROW_COUNT (sizeof(ROW_PINS)/sizeof(ROW_PINS[0]))
#define COL_COUNT (sizeof(COL_PINS)/sizeof(COL_PINS[0]))

Parameterizing these kinds of things made moving from a 3x4 board to 4x5, flipping the rows & columns, and using different pins trivial. An unfortunate cargo-cult effect in Arduino and Wiring code I've seen is folks not using #define and sizeof.

Some cute enhancements might be to have an array of structs, the struct containing additionally a delay time for each frame. We could also have, rather than 0/1, a wider range of values to indicate brightness. With that, (e.g. Perl/Python scripted) code could generate the animation sequences rapidly flipping frames to give an impression of anti-alias. Unfortunately I have other things to do... (yeah, I made the animations by hand in vi and pasted into the Arduino editor :-))

And finally, a Wiring-based pair of color LED matrixes are used as toilet signs... that change throughout the night [video]. More on this funny project.

Posted by Paul Makepeace at March 8, 2008 23:27 | TrackBack