i2c: implement target mode for rp2040 and nrf

src/machine/i2c.go

@@ -23,6 +23,37 @@ var (
 	errI2CSignalStopTimeout  = errors.New("I2C timeout on signal stop")
 	errI2CAckExpected        = errors.New("I2C error: expected ACK not NACK")
 	errI2CBusError           = errors.New("I2C bus error")
+	errI2COverflow           = errors.New("I2C receive buffer overflow")
+	errI2COverread           = errors.New("I2C transmit buffer overflow")
+)
+
+// I2CTargetEvent reflects events on the I2C bus
+type I2CTargetEvent uint8
+
+const (
+	// I2CReceive indicates target has received a message from the controller.
+	I2CReceive I2CTargetEvent = iota
+
+	// I2CRequest indicates the controller is expecting a message from the target.
+	I2CRequest
+
+	// I2CFinish indicates the controller has ended the transaction.
+	//
+	// I2C controllers can chain multiple receive/request messages without
+	// relinquishing the bus by doing 'restarts'.  I2CFinish indicates the
+	// bus has been relinquished by an I2C 'stop'.
+	I2CFinish
+)
+
+// I2CMode determines if an I2C peripheral is in Controller or Target mode.
+type I2CMode int
+
+const (
+	// I2CModeController represents an I2C peripheral in controller mode.
+	I2CModeController I2CMode = iota
+
+	// I2CModeTarget represents an I2C peripheral in target mode.
+	I2CModeTarget
 )
 
 // WriteRegister transmits first the register and then the data to the