led_matrix.ino

#include <SPI.h>

#define CHIP_SELECT_PIN 10

void setup() {
  // Setup SPI communication for the MAX7221.
  pinMode(CHIP_SELECT_PIN, OUTPUT);
  SPI.begin();

  // Test for 1 second.
  send(0x0F, 0x01);
  delay(1000);
  send(0x0F, 0x00);

  // Turn off decoding.
  send(0x09, 0x00);

  // Set scan limiter to display all registers.
  send(0x0B, 0x07);

  // Set display intensity (0-F).
  send(0x0A, 0x04);

  // Turn on display.
  send(0x0C, 0x01);

  // Clear all the display registers.
  send(0x01, 0x00);
  send(0x02, 0x00);
  send(0x03, 0x00);
  send(0x04, 0x00);
  send(0x05, 0x00);
  send(0x06, 0x00);
  send(0x07, 0x00);
  send(0x08, 0x00);
}

void loop() {
  send(0x01, random(256));
  send(0x02, random(256));
  send(0x03, random(256));
  send(0x04, random(256));
  send(0x05, random(256));
  send(0x06, random(256));
  send(0x07, random(256));
  send(0x08, random(256));
  delay(1000);
}

void send(byte address, byte value) {
  digitalWrite(CHIP_SELECT_PIN, LOW);
  SPI.transfer(address);
  SPI.transfer(value);
  digitalWrite(CHIP_SELECT_PIN, HIGH);
}

This has been superseded by https://github.com/nixpulvis/max7221