Simple AVR code for using a MAX7219 7-segment display driver

max7219-basic.c

/* MAX7219 Interaction Code
 * ---------------------------
 * For more information see
 * http://www.adnbr.co.uk/articles/max7219-and-7-segment-displays
 *
 * 668 bytes - ATmega168 - 16MHz
 */

// 16MHz clock
#define F_CPU 16000000UL

// Outputs, pin definitions
#define PIN_SCK                   PORTB5
#define PIN_MOSI                  PORTB3
#define PIN_SS                    PORTB2

#define ON                        1
#define OFF                       0

#define MAX7219_LOAD1             PORTB |= (1<<PIN_SS)
#define MAX7219_LOAD0             PORTB &= ~(1<<PIN_SS)

#define MAX7219_MODE_DECODE       0x09
#define MAX7219_MODE_INTENSITY    0x0A
#define MAX7219_MODE_SCAN_LIMIT   0x0B
#define MAX7219_MODE_POWER        0x0C
#define MAX7219_MODE_TEST         0x0F
#define MAX7219_MODE_NOOP         0x00

// I only have 3 digits, no point having the
// rest. You could use more though.
#define MAX7219_DIGIT0            0x01
#define MAX7219_DIGIT1            0x02
#define MAX7219_DIGIT2            0x03

#define MAX7219_CHAR_BLANK        0xF 
#define MAX7219_CHAR_NEGATIVE     0xA 

#include <avr/io.h>
#include <util/delay.h>



char digitsInUse = 3;

void spiSendByte (char databyte)
{
    // Copy data into the SPI data register
    SPDR = databyte;
    // Wait until transfer is complete
    while (!(SPSR & (1 << SPIF)));
}

void MAX7219_writeData(char data_register, char data)
{
    MAX7219_LOAD0;
        // Send the register where the data will be stored
        spiSendByte(data_register);
        // Send the data to be stored
        spiSendByte(data);
    MAX7219_LOAD1;
}

void MAX7219_clearDisplay() 
{
    char i = digitsInUse;
    // Loop until 0, but don't run for zero
    do {
        // Set each display in use to blank
        MAX7219_writeData(i, MAX7219_CHAR_BLANK);
    } while (--i);
}

void MAX7219_displayNumber(volatile long number) 
{
    char negative = 0;

    // Convert negative to positive.
    // Keep a record that it was negative so we can
    // sign it again on the display.
    if (number < 0) {
        negative = 1;
        number *= -1;
    }

    MAX7219_clearDisplay();

    // If number = 0, only show one zero then exit
    if (number == 0) {
        MAX7219_writeData(MAX7219_DIGIT0, 0);
        return;
    }
    
    // Initialization to 0 required in this case,
    // does not work without it. Not sure why.
    char i = 0; 
    
    // Loop until number is 0.
    do {
        MAX7219_writeData(++i, number % 10);
        // Actually divide by 10 now.
        number /= 10;
    } while (number);

    // Bear in mind that if you only have three digits, and
    // try to display something like "-256" all that will display
    // will be "256" because it needs an extra fourth digit to
    // display the sign.
    if (negative) {
        MAX7219_writeData(i, MAX7219_CHAR_NEGATIVE);
    }
}

int main(void)
{
    // SCK MOSI CS/LOAD/SS
    DDRB |= (1 << PIN_SCK) | (1 << PIN_MOSI) | (1 << PIN_SS);

    // SPI Enable, Master mode
    SPCR |= (1 << SPE) | (1 << MSTR)| (1<<SPR1);

    // Decode mode to "Font Code-B"
    MAX7219_writeData(MAX7219_MODE_DECODE, 0xFF);

    // Scan limit runs from 0.
    MAX7219_writeData(MAX7219_MODE_SCAN_LIMIT, digitsInUse - 1);
    MAX7219_writeData(MAX7219_MODE_INTENSITY, 8);
    MAX7219_writeData(MAX7219_MODE_POWER, ON);

    int i = 999;
    while(1)
    {
        MAX7219_displayNumber(--i);
        _delay_ms(10);

        if (i == 0) {
            i = 999;
        }
    }
}

Verry good, thank you about that.

HI! I've tested the code.... Works perfect, You need add a line disabling test mode, which is very probably be enabled after power on, e.g., as the first line.

Regards,
Evgeny

Great coding, very clear and neat comments too!
Thanks for that <3

Thank You! You save my life!
Great Code!

is this work with codevisionavr IDE?