Simple Max7219 7-Segment Timer on Arduino Uno

I was just playing around today and thought I'd write an Arduino Uno program that increments a timer displayed on a MAX7219 7-Segment Display.

It was a pretty straight forward process. I had to use the SPI library because none of the MAX7219 libraries that I found would work with my device.

Here is the source code. I used the online code editor from Arduino to develop my code in. It was super easy.

/*

*/
#include <SPI.h>
const int slaveSelectPin = 6;
// define 0-9 for the LED segments
const int digits[] = { 0x7e, 0x30, 0x6d, 0x79, 0x33, 0x5b, 0x5f, 0x70, 0x7f, 0x7b };
const int digitsdecimal[] = { 0x1fe, 0xb0, 0xed, 0xf9, 0xb3, 0xdb, 0xdf, 0xf0, 0xff, 0xfb};

int seconds = 0;
int minutes = 0;

void setup() {
    // set the slaveSelectPin as an output:
  pinMode(slaveSelectPin, OUTPUT);
  
  digitalWrite(slaveSelectPin, HIGH);
  SPI.begin();
  
  initializeMax7219();
}

void loop() {
    seconds++;
    if(seconds == 599)
    {
      seconds = 0;
      minutes++;
      if(minutes == 10)
      {
        minutes = 0;
      }
    }
    updateDisplay(minutes, seconds);
    delay(100);
}

void updateDisplay(int minutes, int seconds)
{
  // get our individual digit values
  int tenths = seconds % 10;
  int secsOnes = seconds / 10  % 10;
  int secsTens = seconds / 100 % 10;
  int mins = minutes % 10;
  
  // update the first digit with our ones place value
  sendMessage(0x01, digits[tenths]);

  // update our second digit with our tens place value
  sendMessage(0x02, digitsdecimal[secsOnes]);
  
  // update the third digit with our hundreds place value
  sendMessage(0x03, digits[secsTens]);

  // update our fourth digit with our thousands place value
  sendMessage(0x04, digitsdecimal[mins]);
}

void initializeMax7219()
{
  // turn off test mode
  sendMessage(0x0F, 0);
  // set operation mode to normal
  sendMessage(0x0C, 1); 
  // set scan limit (number of digits)
  sendMessage(0x0B, 0x03); // use 4 digits
  // set brightness to 100%
  sendMessage(0x0A, 0x0F);
  // set no decode mode
  sendMessage(0x09, 0);
}

void sendMessage(int address, int data)
{
  SPI.beginTransaction(SPISettings(10000000, MSBFIRST, SPI_MODE3));
  digitalWrite(slaveSelectPin, LOW);
  //  send in the address and value via SPI:
  SPI.transfer(address);
  SPI.transfer(data);
  // take the SS pin high to de-select the chip:
  digitalWrite(slaveSelectPin, HIGH);
  SPI.endTransaction();
}

It only counts up to 9 minutes and 59.9 seconds until it resets back to zero. With a few modifications, you can add more minutes, and even hours. I'll leave that as an exercise for the reader.

Here is the datasheet for the MAX7219.

There's probably a better way to do clock math involving a mod on 60 and dividing by 60, but this one worked with my tenth of a second code.

Pin 6 is connected to the CS pin on the MAX7219 module, pin 11 to the DIN, and pin 13 to the CLK.