qmk-firmware/drivers/avr/i2c_slave.c

/* Library made by: g4lvanix
 * Github repository: https://github.com/g4lvanix/I2C-slave-lib
 */

#include <avr/io.h>
#include <util/twi.h>
#include <avr/interrupt.h>
#include <stdbool.h>

#include "i2c_slave.h"

void i2c_init(uint8_t address){
    // load address into TWI address register
    TWAR = (address << 1);
    // set the TWCR to enable address matching and enable TWI, clear TWINT, enable TWI interrupt
    TWCR = (1 << TWIE) | (1 << TWEA) | (1 << TWINT) | (1 << TWEN);
}

void i2c_stop(void){
    // clear acknowledge and enable bits
    TWCR &= ~((1 << TWEA) | (1 << TWEN));
}

ISR(TWI_vect){
    uint8_t ack = 1;
    // temporary stores the received data
    //uint8_t data;
    
    switch(TW_STATUS){
        case TW_SR_SLA_ACK:
            // The device is now a slave receiver
            slave_has_register_set = false;
            break;

        case TW_SR_DATA_ACK:
            // This device is a slave receiver and has received data
            // First byte is the location then the bytes will be writen in buffer with auto-incriment
            if(!slave_has_register_set){
                buffer_address = TWDR;

                if (buffer_address >= RX_BUFFER_SIZE){ // address out of bounds dont ack
                    ack = 0;
                    buffer_address = 0;
                }
                slave_has_register_set = true; // address has been receaved now fill in buffer
            } else {
                rxbuffer[buffer_address] = TWDR;
                buffer_address++;
            }
            break;

        case TW_ST_SLA_ACK:
        case TW_ST_DATA_ACK:
            // This device is a slave transmitter and master has requested data
            TWDR = txbuffer[buffer_address];
            buffer_address++;
            break;

        case TW_BUS_ERROR:
            // We got an error, reset i2c
            TWCR = 0;
        default:
            break;
    }

    // Reset i2c state mahcine to be ready for next interrupt
    TWCR |= (1 << TWIE) | (1 << TWINT) | (ack << TWEA) | (1 << TWEN);
}