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.