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

#include "USI_TWI_Master.h"

/*
Example using an ATtiny85, MCP23017, interrupts and I2C to illuminate
each of 8 LEDs when the appropriate one of 8 buttons are pressed.
A couple of further LEDs on the otherwise-unused ATtiny85 pins give
diagnostic info.

Connect Arduino (as ArduinoISP) to ATTiny85 for programming:
        13         7
        12         6
        11         5
        10         1

Connect ATtiny85:
  * Pin 2, via LED and resistor, to GND
  * Pin 3, via LED and resistor, to GND
  * Pin 4 to GND
  * Pin 5, via 4k7 resistor, to 5V
  * Pin 7, via 4k7 resistor, to 5V
  * Pin 8 to 5V

Connect MCP23017:
  * Pins 1-8 via buttons to GND.
  * Pin 9 to 5V
  * Pin 10 to GND
  * Pin 12 to ATtiny85 pin 7 (SCL)
  * Pin 13 to ATtiny85 pin 5 (SDA)
  * Pins 15-17 to GND
  * Pin 18 to ATtiny85 pin 1 (~RESET)
  * Pin 19 to ATtiny85 pin 6 (interrupt)
  * Pins 21-28, via LED and resistor, to GND

You need USI_TWI_Master.c and USI_TWI_Master.h from the zip file on
http://www.atmel.com/images/atmel-2561-using-the-usi-module-as-a-i2c-master_ap-note_avr310.pdf

Compile and upload with:
avr-gcc -mmcu=attiny85 -Wall -Werror -Os -DF_CPU=1000000UL -o bin.elf main.c USI_TWI_Master.c
avr-objcopy -j .text -j .data -O ihex bin.elf bin.hex
avrdude -b 19200 -c arduino -P /dev/ttyUSB0 -p attiny85 -e -U flash:w:bin.hex:i
*/

// MCP23017 slave address:
// 0 1 0 0 a2 a1 a0 R/~W
// 0 1 0 0 0 0 0 0 = 0x40 to write
// 0 1 0 0 0 0 0 1 = 0x41 to read
#define SLAVE_WRITE_ADDR 0x40
#define SLAVE_READ_ADDR  0x41

ISR(PCINT0_vect) {
    unsigned char messageBuf[3];
    unsigned char temp;

    while(!(PINB & 0x02)) {
        PORTB ^= 0b00010000;
        messageBuf[0] = SLAVE_WRITE_ADDR;
        messageBuf[1] = 0x13; // GPIOB
        temp = USI_TWI_Start_Transceiver_With_Data(messageBuf, 2);
        if(!temp) {
            PORTB |= 0b00001000;
        }
        messageBuf[0] = SLAVE_READ_ADDR;
        temp = USI_TWI_Start_Transceiver_With_Data(messageBuf, 2);
        if(!temp) {
            PORTB |= 0b00001000;
        }
        unsigned char result = messageBuf[1];
        messageBuf[0] = SLAVE_WRITE_ADDR;
        messageBuf[1] = 0x12; // GPIOA
        messageBuf[2] = result;
        temp = USI_TWI_Start_Transceiver_With_Data(messageBuf, 3);
        if(!temp) {
            PORTB |= 0b00001000;
        }
    }
}

int main(void) {
    unsigned char messageBuf[3];
    unsigned char temp;

    DDRB = 0b00011000; // Pins 2 and 3 are outputs
    PORTB = 0b00000010; // Turn the LEDs off and enable the
                        // interrupt pin (6) pull-up resistor

    USI_TWI_Master_Initialise();

    messageBuf[0] = SLAVE_WRITE_ADDR;
    messageBuf[1] = 0x0A; // IOCON register
    messageBuf[2] = 0x04; // Set interrupt pin to open-drain
    temp = USI_TWI_Start_Transceiver_With_Data(messageBuf, 3);
    if(!temp) {
        PORTB |= 0b00001000;
    }
    messageBuf[0] = SLAVE_WRITE_ADDR;
    messageBuf[1] = 0x00; // IODIRA register
    messageBuf[2] = 0x00; // set all of port A to outputs
    temp = USI_TWI_Start_Transceiver_With_Data(messageBuf, 3);
    if(!temp) {
        PORTB |= 0b00001000;
    }
    messageBuf[0] = SLAVE_WRITE_ADDR;
    messageBuf[1] = 0x03; // IPOLB register
    messageBuf[2] = 0xFF; // invert all of port B's inputs
    temp = USI_TWI_Start_Transceiver_With_Data(messageBuf, 3);
    if(!temp) {
        PORTB |= 0b00001000;
    }
    messageBuf[0] = SLAVE_WRITE_ADDR;
    messageBuf[1] = 0x0D; // GPPUB register
    messageBuf[2] = 0xFF; // enable pull-up resistor for all
                          // of port B's inputs
    temp = USI_TWI_Start_Transceiver_With_Data(messageBuf, 3);
    if(!temp) {
        PORTB |= 0b00001000;
    }
    messageBuf[0] = SLAVE_WRITE_ADDR;
    messageBuf[1] = 0x05; // GPINTENB register
    messageBuf[2] = 0xFF; // enable interrupt-on-change for all
                          // of port B's inputs
    temp = USI_TWI_Start_Transceiver_With_Data(messageBuf, 3);
    if(!temp) {
        PORTB |= 0b00001000;
    }

    GIMSK = 0x20; // Enable the PCINT interrupt
    PCMSK = 0b00000010; // Enable PCINT interrupts for pin 6

    set_sleep_mode(SLEEP_MODE_PWR_DOWN);

    sei();

    while(1) {
        sleep_mode();
    }
}