diff --git a/firmware/common/i2c_lpc.c b/firmware/common/i2c_lpc.c
index f7bf4393..607a1a0a 100644
--- a/firmware/common/i2c_lpc.c
+++ b/firmware/common/i2c_lpc.c
@@ -44,17 +44,23 @@ void i2c_lpc_transfer(i2c_bus_t* const bus,
 	uint8_t* const data_rx, const size_t count_rx
 ) {
 	const uint32_t port = (uint32_t)bus->obj;
-	i2c_tx_start(port);
-	i2c_tx_byte(port, (slave_address << 1) | I2C_WRITE);
-	for(size_t i=0; i<count_tx; i++) {
-		i2c_tx_byte(port, data_tx[i]);
+	size_t i;
+	bool ack = false;
+	if (data_tx && (count_tx > 0)) {
+		i2c_tx_start(port);
+		i2c_tx_byte(port, (slave_address << 1) | I2C_WRITE);
+		for(i=0; i<count_tx; i++) {
+			i2c_tx_byte(port, data_tx[i]);
+		}
 	}
 
-	if( data_rx ) {
+	if (data_rx && (count_rx > 0)) {
 		i2c_tx_start(port);
 		i2c_tx_byte(port, (slave_address << 1) | I2C_READ);
-		for(size_t i=0; i<count_rx; i++) {
-			data_rx[i] = i2c_rx_byte(port);
+		for(i=0; i<count_rx; i++) {
+			/* ACK each byte except the last */
+			ack = (i!=count_rx-1);
+			data_rx[i] = i2c_rx_byte(port, ack);
 		}
 	}
 
diff --git a/firmware/libopencm3 b/firmware/libopencm3
index 50c51c7b..0ca89139 160000
--- a/firmware/libopencm3
+++ b/firmware/libopencm3
@@ -1 +1 @@
-Subproject commit 50c51c7b5fd2f113440241bb13be3a0e64724b8d
+Subproject commit 0ca89139446586e743b842fb1074db0f2173f6dd