#define F_CPU 8000000UL
 
#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay_basic.h>
#include "usiTwiSlave.h"
 
#define I2C_SLAVE_ADDR  0x10
#define LED PB1
 
// Somewhere to store the values the master writes to i2c register 2 and 3.
static volatile uint8_t i2cReg2 = 0;
static volatile uint8_t i2cReg3 = 0;
 
// A callback triggered when the i2c master attempts to read from a register.
uint8_t i2cReadFromRegister(uint8_t reg)
{
    switch (reg)
    {
        case 0:
        return 10;
        case 1:
        return i2cReg2;
        case 2:
        return i2cReg3;
        default:
        return 0xff;
    }
}
 
// A callback triggered when the i2c master attempts to write to a register.
void i2cWriteToRegister(uint8_t reg, uint8_t value)
{
    switch (reg)
    {
        case 2:
        i2cReg2 = value;
        break;
        case 3:
        i2cReg3 = value;
        break;
    }
}
 
int main()
{
    // Set the LED pin as output.
    DDRB |= (1 << LED);
 
    usiTwiSlaveInit(I2C_SLAVE_ADDR, i2cReadFromRegister, i2cWriteToRegister);
    
    sei();
 
    while (1)
    {
        // This is a pretty pointless EXAMPLE which allows me to test writing to two i2c registers: the
        // LED is only lit if both registers have the same value.
        if (i2cReg2 == i2cReg3)
        PORTB |= 1 << LED;
        else
        PORTB &= ~(1 << LED);
    }
}