Add i2c slave support to arduino firmware

src/arch/avr/i2c_slave.c

@@ -0,0 +1,87 @@
+// Based on https://github.com/thegouger/avr-i2c-slave
+
+#include <stdio.h>
+#include <avr/io.h>
+#include <util/twi.h>
+#include <avr/interrupt.h>
+
+#include <board/cpu.h>
+#include <arch/i2c_slave.h>
+
+static void (* volatile i2c_slave_new_cb)() = NULL;
+static void (* volatile i2c_slave_recv_cb)(uint8_t) = NULL;
+static uint8_t (* volatile i2c_slave_send_cb)() = NULL;
+
+void i2c_slave_init(uint8_t address, void (*new_cb)(), void (*recv_cb)(uint8_t), uint8_t (*send_cb)()){
+	// ensure correct behavior by stopping before changing callbacks or address
+	i2c_slave_stop();
+
+	// clear interrupts
+	cli();
+
+	// setup callbacks
+	i2c_slave_new_cb = new_cb;
+	i2c_slave_recv_cb = recv_cb;
+	i2c_slave_send_cb = send_cb;
+	// 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);
+
+	// set interrupts
+	sei();
+}
+
+void i2c_slave_stop(){
+	// clear interrupts
+	cli();
+
+	// clear acknowledge and enable bits
+	TWCR &= ~((1<<TWEA) | (1<<TWEN));
+	// clear address
+	TWAR = 0;
+	// remove callbacks
+	i2c_slave_new_cb = NULL;
+	i2c_slave_recv_cb = NULL;
+	i2c_slave_send_cb = NULL;
+
+	// set interrupts
+	sei();
+}
+
+ISR(TWI_vect) {
+	uint8_t status = TW_STATUS;
+	switch(status) {
+	case TW_SR_SLA_ACK:
+		// master has started a new transaction, call the new callback
+		if (i2c_slave_new_cb != NULL) {
+			i2c_slave_new_cb();
+		}
+		TWCR = (1<<TWIE) | (1<<TWINT) | (1<<TWEA) | (1<<TWEN);
+		break;
+	case TW_SR_DATA_ACK:
+		// received data from master, call the receive callback
+		if(i2c_slave_send_cb != NULL){
+			i2c_slave_recv_cb(TWDR);
+		}
+		TWCR = (1<<TWIE) | (1<<TWINT) | (1<<TWEA) | (1<<TWEN);
+		break;
+	case TW_ST_SLA_ACK:
+	case TW_ST_DATA_ACK:
+		// master is requesting data, call the send callback
+		if(i2c_slave_recv_cb != NULL) {
+			TWDR = i2c_slave_send_cb();
+		}
+		TWCR = (1<<TWIE) | (1<<TWINT) | (1<<TWEA) | (1<<TWEN);
+		break;
+	case TW_BUS_ERROR:
+		// some sort of erroneous state, prepare TWI to be readdressed
+		printf("TWI_vect bus error\n");
+		TWCR = 0;
+		TWCR = (1<<TWIE) | (1<<TWINT) | (1<<TWEA) | (1<<TWEN);
+		break;
+	default:
+		TWCR = (1<<TWIE) | (1<<TWINT) | (1<<TWEA) | (1<<TWEN);
+		break;
+	}
+}

src/arch/avr/include/arch/i2c_slave.h

@@ -0,0 +1,7 @@
+#ifndef _ARCH_I2C_SLAVE_H
+#define _ARCH_I2C_SLAVE_H
+
+void i2c_slave_init(uint8_t address, void (*new_cb)(), void (*recv_cb)(uint8_t), uint8_t (*send_cb)());
+void i2c_slave_stop();
+
+#endif // _ARCH_I2C_SLAVE_H