machine: refactor pin handling to auto-detect pin mode

With this change, it's no longer necessary to set a specific pin mode:
it will get autodetected in the Configure() call.

Tested on an ItsyBitsy M4 with the mpu6050 example in the drivers repo.

src/machine/board_feather-m4.go

@@ -67,10 +67,10 @@ const (
 
 // I2C on the Feather M4.
 var (
-	I2C0 = I2C{Bus: sam.SERCOM2_I2CM,
+	I2C0 = I2C{
-		SDA:     SDA_PIN,
+		Bus:    sam.SERCOM2_I2CM,
-		SCL:     SCL_PIN,
+		SERCOM: 2,
-		PinMode: PinSERCOM}
+	}
 )
 
 // SPI pins

src/machine/board_itsybitsy-m4.go

@@ -69,10 +69,10 @@ const (
 
 // I2C on the ItsyBitsy M4.
 var (
-	I2C0 = I2C{Bus: sam.SERCOM2_I2CM,
+	I2C0 = I2C{
-		SDA:     SDA_PIN,
+		Bus:    sam.SERCOM2_I2CM,
-		SCL:     SCL_PIN,
+		SERCOM: 2,
-		PinMode: PinSERCOM}
+	}
 )
 
 // SPI pins

src/machine/board_metro-m4-airlift.go

@@ -77,10 +77,10 @@ const (
 
 // I2C on the Metro M4.
 var (
-	I2C0 = I2C{Bus: sam.SERCOM5_I2CM,
+	I2C0 = I2C{
-		SDA:     SDA_PIN,
+		Bus:    sam.SERCOM5_I2CM,
-		SCL:     SCL_PIN,
+		SERCOM: 5,
-		PinMode: PinSERCOMAlt}
+	}
 )
 
 // SPI pins

src/machine/board_pybadge.go

@@ -105,10 +105,10 @@ const (
 
 // I2C on the ItsyBitsy M4.
 var (
-	I2C0 = I2C{Bus: sam.SERCOM2_I2CM,
+	I2C0 = I2C{
-		SDA:     SDA_PIN,
+		Bus:    sam.SERCOM2_I2CM,
-		SCL:     SCL_PIN,
+		SERCOM: 2,
-		PinMode: PinSERCOM}
+	}
 )
 
 // SPI pins

src/machine/board_pyportal.go

@@ -118,10 +118,10 @@ const (
 
 // I2C on the PyPortal.
 var (
-	I2C0 = I2C{Bus: sam.SERCOM5_I2CM,
+	I2C0 = I2C{
-		SDA:     SDA_PIN,
+		Bus:    sam.SERCOM5_I2CM,
-		SCL:     SCL_PIN,
+		SERCOM: 5,
-		PinMode: PinSERCOMAlt}
+	}
 )
 
 // SPI pins

src/machine/machine_atsamd51.go

@@ -800,9 +800,7 @@ func handleSERCOM0_2() {
 // I2C on the SAMD51.
 type I2C struct {
 	Bus    *sam.SERCOM_I2CM_Type
-	SCL     Pin
+	SERCOM uint8
-	SDA     Pin
-	PinMode PinMode
 }
 
 // I2CConfig is used to store config info for I2C.
@@ -835,12 +833,33 @@ const (
 const i2cTimeout = 1000
 
 // Configure is intended to setup the I2C interface.
-func (i2c I2C) Configure(config I2CConfig) {
+func (i2c I2C) Configure(config I2CConfig) error {
 	// Default I2C bus speed is 100 kHz.
 	if config.Frequency == 0 {
 		config.Frequency = TWI_FREQ_100KHZ
 	}
 
+	// Use default I2C pins if not set.
+	if config.SDA == 0 && config.SCL == 0 {
+		config.SDA = SDA_PIN
+		config.SCL = SCL_PIN
+	}
+
+	sclPinMode, sclPad, ok := findPinPadMapping(i2c.SERCOM, config.SCL)
+	if !ok || sclPad != 1 {
+		// SCL must be on pad 1, according to section 36.4 of the datasheet.
+		// Note: this is not an exhaustive test for I2C support on the pin: not
+		// all pins support I2C.
+		return ErrInvalidClockPin
+	}
+	sdaPinMode, sdaPad, ok := findPinPadMapping(i2c.SERCOM, config.SDA)
+	if !ok || sdaPad != 0 {
+		// SDA must be on pad 0, according to section 36.4 of the datasheet.
+		// Note: this is not an exhaustive test for I2C support on the pin: not
+		// all pins support I2C.
+		return ErrInvalidDataPin
+	}
+
 	// reset SERCOM
 	i2c.Bus.CTRLA.SetBits(sam.SERCOM_I2CM_CTRLA_SWRST)
 	for i2c.Bus.CTRLA.HasBits(sam.SERCOM_I2CM_CTRLA_SWRST) ||
@@ -866,8 +885,10 @@ func (i2c I2C) Configure(config I2CConfig) {
 	}
 
 	// enable pins
-	i2c.SDA.Configure(PinConfig{Mode: i2c.PinMode})
+	config.SDA.Configure(PinConfig{Mode: sdaPinMode})
-	i2c.SCL.Configure(PinConfig{Mode: i2c.PinMode})
+	config.SCL.Configure(PinConfig{Mode: sclPinMode})
+
+	return nil
 }
 
 // SetBaudRate sets the communication speed for the I2C.