diff --git a/internal/legacy/i2clegacy.go b/internal/legacy/i2clegacy.go
index 284343c..ace7727 100644
--- a/internal/legacy/i2clegacy.go
+++ b/internal/legacy/i2clegacy.go
@@ -2,14 +2,13 @@ package legacy
 
 import "tinygo.org/x/drivers"
 
-type I2C struct {
-	i2c drivers.I2C
+func ReadRegister(bus drivers.I2C, addr uint8, reg uint8, data []byte) error {
+	return bus.Tx(uint16(addr), []byte{reg}, data)
 }
 
-func ReadRegister(i2c drivers.I2C, addr uint8, reg uint8, buf []byte) error {
-	return i2c.Tx(uint16(addr), []byte{reg}, buf)
-}
-
-func WriteRegister(i2c drivers.I2C, addr uint8, reg uint8, buf []byte) error {
-	return i2c.Tx(uint16(addr), append([]byte{reg}, buf...), nil)
+func WriteRegister(bus drivers.I2C, addr uint8, reg uint8, data []byte) error {
+	buf := make([]uint8, len(data)+1)
+	buf[0] = reg
+	copy(buf[1:], data)
+	return bus.Tx(uint16(addr), buf, nil)
 }
diff --git a/mcp23017/device.go b/mcp23017/device.go
index 6138c14..444a314 100644
--- a/mcp23017/device.go
+++ b/mcp23017/device.go
@@ -9,6 +9,7 @@ package mcp23017
 import (
 	"errors"
 
+	"tinygo.org/x/drivers"
 	"tinygo.org/x/drivers/internal/legacy"
 )
 
@@ -77,19 +78,12 @@ const (
 // address pins).
 var ErrInvalidHWAddress = errors.New("invalid hardware address")
 
-// I2C represents an I2C bus. It is notably implemented by the
-// machine.I2C type.
-type I2C interface {
-	ReadRegister(addr uint8, r uint8, buf []byte) error
-	WriteRegister(addr uint8, r uint8, buf []byte) error
-}
-
 // New returns a new MCP23017 device at the given I2C address
 // on the given bus.
 // It returns ErrInvalidHWAddress if the address isn't possible for the device.
 //
 // By default all pins are configured as inputs.
-func NewI2C(bus I2C, address uint8) (*Device, error) {
+func NewI2C(bus drivers.I2C, address uint8) (*Device, error) {
 	if address&hwAddressMask != hwAddress {
 		return nil, ErrInvalidHWAddress
 	}
@@ -117,7 +111,7 @@ type Device struct {
 
 	// bus holds the reference the I2C bus that the device lives on.
 	// It's an interface so that we can write tests for it.
-	bus  I2C
+	bus  drivers.I2C
 	addr uint8
 	// pins caches the most recent pin values that have been set.
 	// This enables us to change individual pin values without
diff --git a/mcp23017/multidevice.go b/mcp23017/multidevice.go
index fa13520..100dc68 100644
--- a/mcp23017/multidevice.go
+++ b/mcp23017/multidevice.go
@@ -1,5 +1,7 @@
 package mcp23017
 
+import "tinygo.org/x/drivers"
+
 // All is a convenience value that represents all pins high (or all mask bits one).
 var All = PinSlice{0xffff}
 
@@ -12,7 +14,7 @@ type Devices []*Device
 // NewI2CDevices returns a Devices slice holding the Device values
 // for all the given addresses on the given bus.
 // When more than one bus is in use, create the slice yourself.
-func NewI2CDevices(bus I2C, addrs ...uint8) (Devices, error) {
+func NewI2CDevices(bus drivers.I2C, addrs ...uint8) (Devices, error) {
 	devs := make(Devices, len(addrs))
 	for i, addr := range addrs {
 		dev, err := NewI2C(bus, addr)
diff --git a/tester/device16.go b/tester/device16.go
index 4c6c81c..b424462 100644
--- a/tester/device16.go
+++ b/tester/device16.go
@@ -75,6 +75,16 @@ func (d *I2CDevice16) WriteRegister(r uint8, buf []byte) error {
 
 // Tx implements I2C.Tx.
 func (bus *I2CDevice16) Tx(w, r []byte) error {
-	// TODO: implement this
-	return nil
+	switch len(w) {
+	case 0:
+		bus.c.Fatalf("i2c mock: need a write byte")
+		return nil
+	case 1:
+		return bus.ReadRegister(w[0], r)
+	default:
+		if len(r) > 0 || len(w) == 1 {
+			bus.c.Fatalf("i2c mock: unsupported lengths in Tx(%d, %d)", len(w), len(r))
+		}
+		return bus.WriteRegister(w[0], w[1:])
+	}
 }
diff --git a/tester/device8.go b/tester/device8.go
index 114cf53..7fa5a40 100644
--- a/tester/device8.go
+++ b/tester/device8.go
@@ -38,6 +38,9 @@ func (d *I2CDevice8) ReadRegister(r uint8, buf []byte) error {
 	if d.Err != nil {
 		return d.Err
 	}
+	if len(buf) == 0 {
+		d.c.Fatalf("no register buffer to read into")
+	}
 	d.assertRegisterRange(r, buf)
 	copy(buf, d.Registers[r:])
 	return nil
@@ -55,8 +58,18 @@ func (d *I2CDevice8) WriteRegister(r uint8, buf []byte) error {
 
 // Tx implements I2C.Tx.
 func (bus *I2CDevice8) Tx(w, r []byte) error {
-	// TODO: implement this
-	return nil
+	switch len(w) {
+	case 0:
+		bus.c.Fatalf("i2c mock: need a write byte")
+		return nil
+	case 1:
+		return bus.ReadRegister(w[0], r)
+	default:
+		if len(r) > 0 || len(w) == 1 {
+			bus.c.Fatalf("i2c mock: unsupported lengths in Tx(%d, %d)", len(w), len(r))
+		}
+		return bus.WriteRegister(w[0], w[1:])
+	}
 }
 
 // assertRegisterRange asserts that reading or writing the given