machine/samd21: use pins specified in I2CConfig

Instead of configuring machine.I2C0, machine.I2C1, etc. statically,
allow the pins to be set using machine.I2CConfig. This will also
automatically configure the correct pin mode for each pin instead of
having to specify that manually.

7 files changed, 52 insertions, 31 deletions

diff --git a/src/machine/board_arduino_nano33.go b/src/machine/board_arduino_nano33.go
index 00cc387c2..fba4e200f 100644
--- a/src/machine/board_arduino_nano33.go
+++ b/src/machine/board_arduino_nano33.go
@@ -110,10 +110,10 @@ const (
 
 // I2C on the Arduino Nano 33.
 var (
-	I2C0 = I2C{Bus: sam.SERCOM4_I2CM,
-		SDA:     SDA_PIN,
-		SCL:     SCL_PIN,
-		PinMode: PinSERCOMAlt}
+	I2C0 = I2C{
+		Bus:    sam.SERCOM4_I2CM,
+		SERCOM: 4,
+	}
 )
 
 // SPI pins
diff --git a/src/machine/board_circuitplay_express.go b/src/machine/board_circuitplay_express.go
index ae1bca9d5..6f7e34a1a 100644
--- a/src/machine/board_circuitplay_express.go
+++ b/src/machine/board_circuitplay_express.go
@@ -94,15 +94,15 @@ const (
 // I2C on the Circuit Playground Express.
 var (
 	// external device
-	I2C0 = I2C{Bus: sam.SERCOM5_I2CM,
-		SDA:     SDA_PIN,
-		SCL:     SCL_PIN,
-		PinMode: PinSERCOM}
+	I2C0 = I2C{
+		Bus:    sam.SERCOM5_I2CM,
+		SERCOM: 5,
+	}
 	// internal device
-	I2C1 = I2C{Bus: sam.SERCOM1_I2CM,
-		SDA:     SDA1_PIN,
-		SCL:     SCL1_PIN,
-		PinMode: PinSERCOMAlt}
+	I2C1 = I2C{
+		Bus:    sam.SERCOM1_I2CM,
+		SERCOM: 1,
+	}
 )
 
 // SPI pins (internal flash)
diff --git a/src/machine/board_feather-m0.go b/src/machine/board_feather-m0.go
index 8c4dc7595..57704cc58 100644
--- a/src/machine/board_feather-m0.go
+++ b/src/machine/board_feather-m0.go
@@ -73,10 +73,10 @@ const (
 
 // I2C on the Feather M0.
 var (
-	I2C0 = I2C{Bus: sam.SERCOM3_I2CM,
-		SDA:     SDA_PIN,
-		SCL:     SCL_PIN,
-		PinMode: PinSERCOM}
+	I2C0 = I2C{
+		Bus:    sam.SERCOM3_I2CM,
+		SERCOM: 3,
+	}
 )
 
 // SPI pins
diff --git a/src/machine/board_itsybitsy-m0.go b/src/machine/board_itsybitsy-m0.go
index 48a7a0954..0d7248c62 100644
--- a/src/machine/board_itsybitsy-m0.go
+++ b/src/machine/board_itsybitsy-m0.go
@@ -75,10 +75,10 @@ const (
 
 // I2C on the ItsyBitsy M0.
 var (
-	I2C0 = I2C{Bus: sam.SERCOM3_I2CM,
-		SDA:     SDA_PIN,
-		SCL:     SCL_PIN,
-		PinMode: PinSERCOM}
+	I2C0 = I2C{
+		Bus:    sam.SERCOM3_I2CM,
+		SERCOM: 3,
+	}
 )
 
 // SPI pins
diff --git a/src/machine/board_trinket.go b/src/machine/board_trinket.go
index 7eec1f6f1..d5acf8ee6 100644
--- a/src/machine/board_trinket.go
+++ b/src/machine/board_trinket.go
@@ -79,10 +79,10 @@ const (
 
 // I2C on the Trinket M0.
 var (
-	I2C0 = I2C{Bus: sam.SERCOM2_I2CM,
-		SDA:     SDA_PIN,
-		SCL:     SCL_PIN,
-		PinMode: PinSERCOMAlt}
+	I2C0 = I2C{
+		Bus:    sam.SERCOM2_I2CM,
+		SERCOM: 2,
+	}
 )
 
 // I2S pins
diff --git a/src/machine/machine.go b/src/machine/machine.go
index e55b522d5..a7863d8ff 100644
--- a/src/machine/machine.go
+++ b/src/machine/machine.go
@@ -5,6 +5,8 @@ import "errors"
 var (
 	ErrInvalidInputPin  = errors.New("machine: invalid input pin")
 	ErrInvalidOutputPin = errors.New("machine: invalid output pin")
+	ErrInvalidClockPin  = errors.New("machine: invalid clock pin")
+	ErrInvalidDataPin   = errors.New("machine: invalid data pin")
 )
 
 type PinConfig struct {
diff --git a/src/machine/machine_atsamd21.go b/src/machine/machine_atsamd21.go
index 32febd020..1c5b676b1 100644
--- a/src/machine/machine_atsamd21.go
+++ b/src/machine/machine_atsamd21.go
@@ -519,10 +519,8 @@ func defaultUART1Handler() {
 
 // I2C on the SAMD21.
 type I2C struct {
-	Bus     *sam.SERCOM_I2CM_Type
-	SCL     Pin
-	SDA     Pin
-	PinMode PinMode
+	Bus    *sam.SERCOM_I2CM_Type
+	SERCOM uint8
 }
 
 // I2CConfig is used to store config info for I2C.
@@ -552,11 +550,30 @@ 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
 	}
+	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 27.5 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 27.5 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)
@@ -582,8 +599,10 @@ func (i2c I2C) Configure(config I2CConfig) {
 	}
 
 	// enable pins
-	i2c.SDA.Configure(PinConfig{Mode: i2c.PinMode})
-	i2c.SCL.Configure(PinConfig{Mode: i2c.PinMode})
+	config.SDA.Configure(PinConfig{Mode: sdaPinMode})
+	config.SCL.Configure(PinConfig{Mode: sclPinMode})
+
+	return nil
 }
 
 // SetBaudRate sets the communication speed for the I2C.