Drawing with 64 LEDs: the 8×8 matrix with the MAX7219

2How one chip drives 64 LEDs

The MAX7219 does not keep all the LEDs on at the same time. It lights one row at a time and moves on to the next so quickly that your eye sees the whole picture: the datasheet gives a typical 800 scans per second through all eight rows. A single resistor on the module sets how much current flows through the LEDs, and Arduino only sends the orders: which row to change and which LEDs to light.

3Solder the header and turn the module

Solder the right-angle header into the five holes on the IN side, the one with DIN, with the pins pointing sideways, outward (guide no. 70). You only need the straight header on the OUT side if one day you want to add a second module.

Then hold the module with the input on the right. The “→IN” and “OUT→” labels on the circuit board end up upside down, and that is fine: the documentation of the library we are going to use says exactly that these modules should be held with the DIN side on the right and that the printed text may appear upside down.

4Connect the five wires

• VCC to 5V and GND to GND: the chip works between 4 and 5.5 V.
• DIN to pin 11 and CLK to 13: these are the SPI pins of the Arduino UNO, the fast link the library uses to send data, and they cannot be changed.
• CS to pin 10. This one you can move: just write the new number in the sketch.

6Draw a smiley face

// 8x8 matrix with the MAX7219: a smiley face and a heart, drawn row by row.
// Library: MD_MAX72XX by majicDesigns.
#include <Arduino.h>
#include <MD_MAX72xx.h>

// DIN to pin 11 and CLK to 13 (these are fixed), CS to 10
MD_MAX72XX matrix(MD_MAX72XX::FC16_HW, 10, 1);   // module type, CS pin, matrices

// eight numbers, one per row from the top: 1 = LED on, 0 = off
const byte SMILEY[8] = {
  0b00111100,
  0b01000010,
  0b10100101,
  0b10000001,
  0b10100101,
  0b10011001,
  0b01000010,
  0b00111100,
};
const byte HEART[8] = {
  0b00000000,
  0b01100110,
  0b11111111,
  0b11111111,
  0b01111110,
  0b00111100,
  0b00011000,
  0b00000000,
};

void draw(const byte picture[8]) {
  for (byte row = 0; row < 8; row++) {
    matrix.setRow(row, picture[row]);   // the bit on the left is the LED on the left
  }
}

void setup() {
  matrix.begin();
  matrix.control(MD_MAX72XX::INTENSITY, 2);   // brightness from 0 to 15
}

void loop() {
  draw(SMILEY);
  delay(2000);
  draw(HEART);
  delay(2000);
}

setRow(row, number) lights the LEDs of one row; INTENSITY sets the brightness from 0 to 15. FC16_HW tells the library how the matrix is wired to the chip.

7Brightness and current

The MAX7219 sets the brightness by keeping the LEDs on for part of the time: with INTENSITY at 0 they stay on for 1/32 of each scan, with 15 for 31/32 (datasheet). So even at 0 they do not go completely dark.

• With all the LEDs on at the maximum recommended current, 40 mA, the datasheet gives 330 mA typical for each chip. How much current actually flows in your module depends on the resistor fitted, and the maker does not state it.
• An Arduino powered from a computer's USB port usually has no more than 500 mA available. With four matrices fully lit at maximum you could reach 4 × 330 = 1.32 A (our calculation, worst case).
• That is why the brightness in the sketches is low, 2 and 1: you can see it well and the USB port can cope. For big, bright text use a separate 5 V power supply for the modules, with its GND joined to the Arduino GND, as the library author advises and as in guide no. 82.

8Scrolling text

The four-matrix module has four MAX7219 chips in a row, each one passing the data on to the next. It connects with the same five wires to the header on the right, the input one. The library counts the matrices from the right: number 0 is the one attached to Arduino.

// Text scrolling across the four-matrix module (32 x 8 LEDs).
// Libraries: MD_Parola and MD_MAX72XX by majicDesigns.
#include <Arduino.h>
#include <MD_Parola.h>

// DIN to pin 11, CLK to 13, CS to 10, as for the single matrix
MD_Parola message(MD_MAX72XX::FC16_HW, 10, 4);   // module type, CS pin, matrices

void setup() {
  message.begin();
  message.setIntensity(1);                                   // from 0 to 15
  message.displayScroll("Hello from Arduino!", PA_LEFT, PA_SCROLL_LEFT, 50);
}

void loop() {
  if (message.displayAnimate()) {   // true when the whole text has scrolled out
    message.displayReset();         // and then it starts again from the right
  }
}

displayScroll sets up the text: alignment, effect and speed, that is the milliseconds between one step and the next (the lower the number, the faster it runs). displayAnimate() must be called over and over in the loop, without delay: it is what moves the letters at the right moment.