nrf: add I2C timeout

This commit adds I2C timeouts for nrf51 and nrf52 (but not yet for
others like nrf52840).

Tested on the PineTime, where I now got a timeout instead of hanging and
resetting due to a watchdog reset.

src/machine/machine_nrf.go

@@ -203,6 +203,8 @@ func (uart *UART) handleInterrupt(interrupt.Interrupt) {
 	}
 }
 
+const i2cTimeout = 0xffff // this is around 29ms on a nrf52
+
 // I2CConfig is used to store config info for I2C.
 type I2CConfig struct {
 	Frequency uint32
@@ -261,11 +263,17 @@ func (i2c *I2C) Configure(config I2CConfig) error {
 }
 
 // signalStop sends a stop signal to the I2C peripheral and waits for confirmation.
-func (i2c *I2C) signalStop() {
+func (i2c *I2C) signalStop() error {
+	tries := 0
 	i2c.Bus.TASKS_STOP.Set(1)
 	for i2c.Bus.EVENTS_STOPPED.Get() == 0 {
+		tries++
+		if tries >= i2cTimeout {
+			return errI2CSignalStopTimeout
+		}
 	}
 	i2c.Bus.EVENTS_STOPPED.Set(0)
+	return nil
 }
 
 var rngStarted = false

src/machine/machine_nrf5x.go

@@ -72,12 +72,17 @@ func (i2c *I2C) Tx(addr uint16, w, r []byte) (err error) {
 
 	if len(r) == 0 {
 		// Stop the I2C transaction after the write.
-		i2c.signalStop()
+		err = i2c.signalStop()
 	} else {
 		// The last byte read has already stopped the transaction, via
 		// TWI_SHORTS_BB_STOP. But we still need to wait until we receive the
 		// STOPPED event.
+		tries := 0
 		for i2c.Bus.EVENTS_STOPPED.Get() == 0 {
+			tries++
+			if tries >= i2cTimeout {
+				return errI2CSignalStopTimeout
+			}
 		}
 		i2c.Bus.EVENTS_STOPPED.Set(0)
 	}
@@ -87,12 +92,17 @@ func (i2c *I2C) Tx(addr uint16, w, r []byte) (err error) {
 
 // writeByte writes a single byte to the I2C bus and waits for confirmation.
 func (i2c *I2C) writeByte(data byte) error {
+	tries := 0
 	i2c.Bus.TXD.Set(uint32(data))
 	for i2c.Bus.EVENTS_TXDSENT.Get() == 0 {
 		if e := i2c.Bus.EVENTS_ERROR.Get(); e != 0 {
 			i2c.Bus.EVENTS_ERROR.Set(0)
 			return errI2CBusError
 		}
+		tries++
+		if tries >= i2cTimeout {
+			return errI2CWriteTimeout
+		}
 	}
 	i2c.Bus.EVENTS_TXDSENT.Set(0)
 	return nil
@@ -100,11 +110,16 @@ func (i2c *I2C) writeByte(data byte) error {
 
 // readByte reads a single byte from the I2C bus when it is ready.
 func (i2c *I2C) readByte() (byte, error) {
+	tries := 0
 	for i2c.Bus.EVENTS_RXDREADY.Get() == 0 {
 		if e := i2c.Bus.EVENTS_ERROR.Get(); e != 0 {
 			i2c.Bus.EVENTS_ERROR.Set(0)
 			return 0, errI2CBusError
 		}
+		tries++
+		if tries >= i2cTimeout {
+			return 0, errI2CReadTimeout
+		}
 	}
 	i2c.Bus.EVENTS_RXDREADY.Set(0)
 	return byte(i2c.Bus.RXD.Get()), nil