MAX7219 8x8 LED Matrix ATtiny85

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/*
 Here's some code for testing ::: 8) Used Arduino IDE 1.6.12
   
  Runs on Trinket or DigiSpark controller(ATtiny85) 
  or change the port pins to use Arduino UNO.
  
  One MAX7219 connected to an 8x8 LED matrix. Bit-Bangs the chip.

  Original Code Base: https://halfbyteblog.wordpress.com/2015/11/29/using-a-max7219-8x8-led-matrix-with-an-attiny85-like-trinket-or-digispark/
  
  Sketch uses 330 bytes (5%) of program storage space. Maximum is 6,012 bytes.
  Global variables use 24 bytes of dynamic memory.
  
  Modifed and fixed some coding errors - rhb.20220905 -
*/

#include <util/delay.h>

#define MAX7219_DIN     PB0 // DIN, pin 3 on the MAX7219 Board
#define MAX7219_CS      PB1 // CS,  pin 4 on the MAX7219 Board
#define MAX7219_CLK     PB2 // CLK, pin 5 on the MAX7219 Board

#define DATA_HIGH()  PORTB |= (1 << MAX7219_DIN)
#define DATA_LOW()   PORTB &= ~(1 << MAX7219_DIN)

#define CS_HIGH()    PORTB |= (1 << MAX7219_CS)
#define CS_LOW()     PORTB &= ~(1 << MAX7219_CS)


#define CLK_HIGH()   PORTB |= (1 << MAX7219_CLK)
#define CLK_LOW()    PORTB &= ~(1 << MAX7219_CLK)

#define INIT_PORT()   DDRB |= (1 << PB0) | (1 << PB1) | (1 << PB2)

/*
The same code can control an 8-digit 7-segment display. 

uint8_t  ZERO = 0b01111110;
uint8_t   ONE = 0b00110000;
uint8_t   TWO = 0b01101101;
uint8_t THREE = 0b01111001;
uint8_t  FOUR = 0b00110011;
uint8_t  FIVE = 0b01011011;
uint8_t   SIX = 0b00011111;
uint8_t SEVEN = 0b01110000;
uint8_t EIGHT = 0b01111111;
uint8_t  NINE = 0b01110011;
uint8_t   DOT = 0b10000000;
unit8_t BLANK = 0b00000000;

I wouldn't do it this way. The chip can decode BCD to 7-segment.
*/

uint8_t smile[8] = {
        0b00000000,
        0b01100110,
        0b01100110,
        0b00011000,
        0b00011000,
        0b10000001,
        0b01000010,
        0b00111100};

uint8_t sad[8] = {
        0b00000000,
        0b01100110,
        0b01100110,
        0b00011000,
        0b00011000,
        0b00000000,
        0b00111100,
        0b01000010,
};

void spi_send(uint8_t data) {
  uint8_t i;
  for (i = 0; i < 8; i++, data <<= 1) {
    CLK_LOW();
    if (data & 0x80)
      DATA_HIGH();
    else
      DATA_LOW();
    CLK_HIGH();
  }
}

void max7219_writec(uint8_t high_byte, uint8_t low_byte) {
  CS_LOW();
  spi_send(high_byte);
  spi_send(low_byte);
  CS_HIGH();
  //CLK_LOW();  // might not be needed rhb.
}

void max7219_clear(void) {
  uint8_t i;
  for (i = 0; i < 8; i++) {
    max7219_writec(i+1, 0);
  }
}

void max7219_init(void) {
  INIT_PORT();
  // Decode mode: none
  max7219_writec(0x09, 0);
  // Intensity: 3 (0-15)
  max7219_writec(0x0A, 1);
  // Scan limit: All 'digits' (rows) on
  max7219_writec(0x0B, 7);
  // Shutdown register: Display on
  max7219_writec(0x0C, 1);
  // Display test: off
  max7219_writec(0x0F, 0);
  max7219_clear();
}

uint8_t display[8];

void update_display(void) {
  uint8_t i;
  for (i = 0; i < 8; i++) {
    max7219_writec(i+1, display[i]);

  }
}

void image(uint8_t im[8]) {
  uint8_t i;
  for (i = 0; i < 8; i++)
    display[i] = im[i];
}

void set_pixel(uint8_t r, uint8_t c, uint8_t value) {
  switch (value) {
    case 0: // Clear bit
      display[r] &= (uint8_t) ~(0x80 >> c);
      break;
    case 1: // Set bit
      display[r] |= (0x80 >> c);
      break;
    default: // XOR bit
      display[r] ^= (0x80 >> c);
      break;
  }
}

int main(void) {
 
  uint8_t i;
  max7219_init();
  
  while(1) {
    
    image(sad);
    update_display();
    _delay_ms(1000);
    
    image(smile);
    update_display();
    _delay_ms(1000);

  }
}