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

src/board/arduino/mega2560/battery.c

@@ -6,6 +6,10 @@ uint8_t smbus_read(uint8_t address, uint8_t command, uint16_t * data) {
     return i2c_get(address, command, (uint8_t *)data, 2);
 }
 
+uint8_t smbus_write(uint8_t address, uint8_t command, uint16_t data) {
+    return i2c_set(address, command, (uint8_t *)&data, 2);
+}
+
 void battery_debug(void) {
     uint16_t data = 0;
     uint8_t err = 0;
@@ -15,9 +19,9 @@ void battery_debug(void) {
         err = smbus_read(A, V, &data); \
         if (err) { \
             printf("ERROR %02X\n", err); \
-            return; \
+        } else { \
+            printf("%04X\n", data); \
         } \
-        printf("%04X\n", data); \
     }
 
     printf("Battery:\n");

src/board/arduino/mega2560/include/board/i2c.h

@@ -1,6 +1,8 @@
 #ifndef _BOARD_I2C_H
 #define _BOARD_I2C_H
 
+#include <arch/i2c.h>
+
 void i2c_init(unsigned long baud);
 
 #endif // _BOARD_I2C_H

src/board/arduino/mega2560/main.c

@@ -1,6 +1,7 @@
 #include <stdio.h>
 
 #include <arch/gpio.h>
+#include <arch/i2c_slave.h>
 #include <arch/uart.h>
 #include <board/battery.h>
 #include <board/i2c.h>
@@ -10,6 +11,16 @@ void init(void) {
     i2c_init(50000);
 }
 
+static void i2c_slave_new() {}
+
+static void i2c_slave_recv(uint8_t data) {
+    printf("%c", data);
+}
+
+static uint8_t i2c_slave_send() {
+    return 0;
+}
+
 struct Gpio LED = GPIO(B, 7);
 
 int main(void) {
@@ -22,7 +33,9 @@ int main(void) {
     battery_debug();
 
     for (;;) {
+        i2c_slave_init(0x76, i2c_slave_new, i2c_slave_recv, i2c_slave_send);
         int c = getchar();
+        i2c_slave_stop();
         if (c == '\r') {
             putchar('\n');
             battery_debug();