diff --git a/bma42x/bma421-config-waspos.bin b/bma42x/bma421-config-waspos.bin
new file mode 100644
index 000000000..cc49293f4
Binary files /dev/null and b/bma42x/bma421-config-waspos.bin differ
diff --git a/bma42x/bma425-config-waspos.bin b/bma42x/bma425-config-waspos.bin
new file mode 100644
index 000000000..886fa2ef4
Binary files /dev/null and b/bma42x/bma425-config-waspos.bin differ
diff --git a/bma42x/bma42x.go b/bma42x/bma42x.go
new file mode 100644
index 000000000..46b5e9108
--- /dev/null
+++ b/bma42x/bma42x.go
@@ -0,0 +1,352 @@
+// Package bma42x provides a driver for the BMA421 and BMA425 accelerometer
+// chips.
+//
+// Here is a reasonably good datasheet:
+// https://datasheet.lcsc.com/lcsc/1912111437_Bosch-Sensortec-BMA425_C437656.pdf
+//
+// This driver was originally written for the PineTime, using the datasheet as a
+// guide. There is an open source C driver provided by Bosch, but unfortunately
+// it needs some small modifications to work with other chips (most importantly,
+// the "config file").
+// The InfiniTime and Wasp-OS drivers for this accelerometer have also been used
+// to figure out some driver details (especially step counting).
+package bma42x
+
+import (
+	_ "embed"
+	"errors"
+	"reflect"
+	"time"
+	"unsafe"
+
+	"tinygo.org/x/drivers"
+)
+
+// Driver for BMA421 and BMA425:
+// BMA421: https://files.pine64.org/doc/datasheet/pinetime/BST-BMA421-FL000.pdf
+// BMA425: https://datasheet.lcsc.com/lcsc/1912111437_Bosch-Sensortec-BMA425_C437656.pdf
+
+// This is the BMA421 firmware from the Wasp-OS project.
+// It is identical to the so-called BMA423 firmware in InfiniTime, which I
+// suspect to be actually a BMA421 firmware. I don't know where this firmware
+// comes from or what the licensing status is.
+// It has the FEATURES_IN command prepended, so that it can be written directly
+// using I2C.Tx.
+// Source: https://github.com/wasp-os/bma42x-upy/blob/master/BMA42X-Sensor-API/bma421.h
+//
+//go:embed bma421-config-waspos.bin
+var bma421Firmware string
+
+// Same as the BMA421 firmware, but for the BMA425.
+// Source: https://github.com/wasp-os/bma42x-upy/blob/master/BMA42X-Sensor-API/bma425.h
+//
+//go:embed bma425-config-waspos.bin
+var bma425Firmware string
+
+var (
+	errUnknownDevice     = errors.New("bma42x: unknown device")
+	errUnsupportedDevice = errors.New("bma42x: device not part of config")
+	errConfigMismatch    = errors.New("bma42x: config mismatch")
+	errTimeout           = errors.New("bma42x: timeout")
+	errInitFailed        = errors.New("bma42x: failed to initialize")
+)
+
+const Address = 0x18 // BMA421/BMA425 address
+
+type DeviceType uint8
+
+const (
+	DeviceBMA421 DeviceType = 1 << iota
+	DeviceBMA425
+
+	AnyDevice            = DeviceBMA421 | DeviceBMA425
+	noDevice  DeviceType = 0
+)
+
+// Features to enable while configuring the accelerometer.
+type Features uint8
+
+const (
+	FeatureStepCounting = 1 << iota
+)
+
+type Config struct {
+	// Which devices to support (OR the device types together as needed).
+	Device DeviceType
+
+	// Which features to enable. With Features == 0, only the accelerometer will
+	// be enabled.
+	Features Features
+}
+
+type Device struct {
+	bus               drivers.I2C
+	address           uint8
+	accelData         [6]byte
+	combinedTempSteps [5]uint8 // [0:3] steps, [4] temperature
+	dataBuf           [2]byte
+}
+
+func NewI2C(i2c drivers.I2C, address uint8) *Device {
+	return &Device{
+		bus:     i2c,
+		address: address,
+	}
+}
+
+func (d *Device) Connected() bool {
+	val, err := d.read1(_CHIP_ID)
+	return err == nil && identifyChip(val) != noDevice
+}
+
+func (d *Device) Configure(config Config) error {
+	if config.Device == 0 {
+		config.Device = AnyDevice
+	}
+
+	// Check chip ID, to check the connection and to determine which BMA42x
+	// device we're dealing with.
+	chipID, err := d.read1(_CHIP_ID)
+	if err != nil {
+		return err
+	}
+
+	// Determine which firmware (config file?) we'll be using.
+	// There is an extra check for the device before using the given firmware.
+	// This check will typically be optimized away if the given device is not
+	// configured, so that the firmware (which is 6kB in size!) won't be linked
+	// into the binary.
+	var firmware string
+	switch identifyChip(chipID) {
+	case DeviceBMA421:
+		if config.Device&DeviceBMA421 == 0 {
+			return errUnsupportedDevice
+		}
+		firmware = bma421Firmware
+	case DeviceBMA425:
+		if config.Device&DeviceBMA425 == 0 {
+			return errUnsupportedDevice
+		}
+		firmware = bma425Firmware
+	default:
+		return errUnknownDevice
+	}
+
+	// Reset the chip, to be able to initialize it properly.
+	// The datasheet says a delay is needed after a SoftReset, but it doesn't
+	// say how long this delay should be. The bma423 driver however uses a 200ms
+	// delay, so that's what we'll be using.
+	err = d.write1(_CMD, cmdSoftReset)
+	if err != nil {
+		return err
+	}
+	time.Sleep(200 * time.Millisecond)
+
+	// Disable power saving.
+	err = d.write1(_PWR_CONF, 0x00)
+	if err != nil {
+		return err
+	}
+	time.Sleep(450 * time.Microsecond)
+
+	// Start initialization (because the datasheet says so).
+	err = d.write1(_INIT_CTRL, 0x00)
+	if err != nil {
+		return err
+	}
+
+	// Write "config file" (actually a firmware, I think) to the chip.
+	// To do this, unsafely cast the string to a byte slice to avoid putting it
+	// in RAM. This is safe in this case because Tx won't write to the 'w'
+	// slice.
+	err = d.bus.Tx(uint16(d.address), unsafeStringToSlice(firmware), nil)
+	if err != nil {
+		return err
+	}
+
+	// Read the config data back.
+	// We don't do that, as it slows down configuration and it probably isn't
+	// _really_ necessary with a reasonably stable I2C bus.
+	if false {
+		data := make([]byte, len(firmware)-1)
+		err = d.readn(_FEATURES_IN, data)
+		if err != nil {
+			return err
+		}
+		for i, c := range data {
+			if firmware[i+1] != c {
+				return errConfigMismatch
+			}
+		}
+	}
+
+	// Enable sensors.
+	err = d.write1(_INIT_CTRL, 0x01)
+	if err != nil {
+		return err
+	}
+
+	// Wait until the device is initialized.
+	start := time.Now()
+	status := uint8(0) // busy
+	for status == 0 {
+		status, err = d.read1(_INTERNAL_STATUS)
+		if err != nil {
+			return err // I2C bus error.
+		}
+		if status > 1 {
+			// Expected either 0 ("not_init") or 1 ("init_ok").
+			return errInitFailed
+		}
+		if time.Since(start) >= 150*time.Millisecond {
+			// The datasheet says initialization should not take longer than
+			return errTimeout
+		}
+		// Don't bother the chip all the time while it's initializing.
+		time.Sleep(50 * time.Microsecond)
+	}
+
+	if config.Features&FeatureStepCounting != 0 {
+		// Enable step counter.
+		// TODO: support step counter parameters.
+		var buf [71]byte
+		buf[0] = _FEATURES_IN // prefix buf with the command
+		data := buf[1:]
+		err = d.readn(_FEATURES_IN, data)
+		if err != nil {
+			return err
+		}
+		data[0x3A+1] |= 0x10 // enable step counting by setting a magical bit
+		err = d.bus.Tx(uint16(d.address), buf[:], nil)
+		if err != nil {
+			return err
+		}
+	}
+
+	// Enable the accelerometer.
+	err = d.write1(_PWR_CTRL, 0x04)
+	if err != nil {
+		return err
+	}
+
+	// Configure accelerometer for low power usage:
+	//   acc_perf_mode=0   (power saving enabled)
+	//   acc_bwp=osr4_avg1 (no averaging)
+	//   acc_odr=50Hz      (50Hz sampling interval, enough for the step counter)
+	const accelConf = 0x00<<7 | 0x00<<4 | 0x07<<0
+	err = d.write1(_ACC_CONF, accelConf)
+	if err != nil {
+		return err
+	}
+
+	// Reduce current consumption.
+	// With power saving enabled (and the above ACC_CONF) the chip consumes only
+	// 14µA.
+	err = d.write1(_PWR_CONF, 0x03)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (d *Device) Update(which drivers.Measurement) error {
+	// TODO: combine temperature and step counter into a single read.
+	if which&drivers.Temperature != 0 {
+		val, err := d.read1(_TEMPERATURE)
+		if err != nil {
+			return err
+		}
+		d.combinedTempSteps[4] = val
+	}
+	if which&drivers.Acceleration != 0 {
+		// The acceleration data is stored in DATA8 through DATA13 as 3 12-bit
+		// values.
+		err := d.readn(_DATA_8, d.accelData[:]) // ACC_X(LSB)
+		if err != nil {
+			return err
+		}
+		err = d.readn(_STEP_COUNTER_0, d.combinedTempSteps[:4])
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// Temperature returns the last read temperature in celsius milli degrees (1°C
+// is 1000).
+func (d *Device) Temperature() int32 {
+	// The temperature value is a two's complement number (meaning: signed) in
+	// units of 1 kelvin, with 0 being 23°C.
+	return (int32(int8(d.combinedTempSteps[4])) + 23) * 1000
+}
+
+// Acceleration returns the last read acceleration in µg (micro-gravity).
+// When one of the axes is pointing straight to Earth and the sensor is not
+// moving the returned value will be around 1000000 or -1000000.
+func (d *Device) Acceleration() (x, y, z int32) {
+	// Combine raw data from d.accelData (stored as 12-bit signed values) into a
+	// number (0..4095):
+	x = int32(d.accelData[0])>>4 | int32(d.accelData[1])<<4
+	y = int32(d.accelData[2])>>4 | int32(d.accelData[3])<<4
+	z = int32(d.accelData[4])>>4 | int32(d.accelData[5])<<4
+	// Sign extend this number to -2048..2047:
+	x = (x << 20) >> 20
+	y = (y << 20) >> 20
+	z = (z << 20) >> 20
+	// Scale from -512..511 to -1000_000..998_046.
+	// Or, at the maximum range (4g), from -2048..2047 to -2000_000..3998_046.
+	// The formula derived as follows (where 512 is the expected value at 1g):
+	//   x = x * 1000_000      / 512
+	//   x = x * (1000_000/64) / (512/64)
+	//   x = x * 15625         / 8
+	x = x * 15625 / 8
+	y = y * 15625 / 8
+	z = z * 15625 / 8
+	return
+}
+
+// Steps returns the number of steps counted since the BMA42x sensor was
+// initialized.
+func (d *Device) Steps() (steps uint32) {
+	steps |= uint32(d.combinedTempSteps[0]) << 0
+	steps |= uint32(d.combinedTempSteps[1]) << 8
+	steps |= uint32(d.combinedTempSteps[2]) << 16
+	steps |= uint32(d.combinedTempSteps[3]) << 24
+	return
+}
+
+func (d *Device) read1(register uint8) (uint8, error) {
+	d.dataBuf[0] = register
+	err := d.bus.Tx(uint16(d.address), d.dataBuf[:1], d.dataBuf[1:2])
+	return d.dataBuf[1], err
+}
+
+func (d *Device) readn(register uint8, data []byte) error {
+	d.dataBuf[0] = register
+	return d.bus.Tx(uint16(d.address), d.dataBuf[:1], data)
+}
+
+func (d *Device) write1(register uint8, data uint8) error {
+	d.dataBuf[0] = register
+	d.dataBuf[1] = data
+	return d.bus.Tx(uint16(d.address), d.dataBuf[:2], nil)
+}
+
+func unsafeStringToSlice(s string) []byte {
+	// TODO: use unsafe.Slice(unsafe.StringData(...)) once we require Go 1.20.
+	sh := (*reflect.StringHeader)(unsafe.Pointer(&s))
+	return unsafe.Slice((*byte)(unsafe.Pointer(sh.Data)), len(s))
+}
+
+func identifyChip(chipID uint8) DeviceType {
+	switch chipID {
+	case 0x11:
+		return DeviceBMA421
+	case 0x13:
+		return DeviceBMA425
+	default:
+		return noDevice
+	}
+}
diff --git a/bma42x/registers.go b/bma42x/registers.go
new file mode 100644
index 000000000..aea75d58a
--- /dev/null
+++ b/bma42x/registers.go
@@ -0,0 +1,73 @@
+package bma42x
+
+const (
+	// I2C registers
+	_CHIP_ID         = 0x00
+	_ERR_REG         = 0x02
+	_STATUS          = 0x03
+	_DATA_0          = 0x0A
+	_DATA_1          = 0x0B
+	_DATA_2          = 0x0C
+	_DATA_3          = 0x0D
+	_DATA_4          = 0x0E
+	_DATA_5          = 0x0F
+	_DATA_6          = 0x10
+	_DATA_7          = 0x11
+	_DATA_8          = 0x12
+	_DATA_9          = 0x13
+	_DATA_10         = 0x14
+	_DATA_11         = 0x15
+	_DATA_12         = 0x16
+	_DATA_13         = 0x17
+	_SENSORTIME_0    = 0x18
+	_SENSORTIME_1    = 0x19
+	_SENSORTIME_2    = 0x1A
+	_EVENT           = 0x1B
+	_INT_STATUS_0    = 0x1C
+	_INT_STATUS_1    = 0x1D
+	_STEP_COUNTER_0  = 0x1E
+	_STEP_COUNTER_1  = 0x1F
+	_STEP_COUNTER_2  = 0x20
+	_STEP_COUNTER_3  = 0x21
+	_TEMPERATURE     = 0x22
+	_FIFO_LENGTH_0   = 0x24
+	_FIFO_LENGTH_1   = 0x25
+	_FIFO_DATA       = 0x26
+	_ACTIVITY_TYPE   = 0x27
+	_INTERNAL_STATUS = 0x2A
+	_ACC_CONF        = 0x40
+	_ACC_RANGE       = 0x41
+	_AUX_CONF        = 0x44
+	_FIFO_DOWNS      = 0x45
+	_FIFO_WTM_0      = 0x46
+	_FIFO_WTM_1      = 0x47
+	_FIFO_CONFIG_0   = 0x48
+	_FIFO_CONFIG_1   = 0x49
+	_AUX_DEV_ID      = 0x4B
+	_AUX_IF_CONF     = 0x4C
+	_AUX_RD_ADDR     = 0x4D
+	_AUX_WR_ADDR     = 0x4E
+	_AUX_WR_DATA     = 0x4F
+	_INT1_IO_CTRL    = 0x53
+	_INT2_IO_CTRL    = 0x54
+	_INT_LATCH       = 0x55
+	_INT1_MAP        = 0x56
+	_INT2_MAP        = 0x57
+	_INT_MAP_DATA    = 0x58
+	_INIT_CTRL       = 0x59
+	_FEATURES_IN     = 0x5E
+	_INTERNAL_ERROR  = 0x5F
+	_NVM_CONF        = 0x6A
+	_IF_CONF         = 0x6B
+	_ACC_SELF_TEST   = 0x6D
+	_NV_CONF         = 0x70
+	_OFFSET_0        = 0x71
+	_OFFSET_1        = 0x72
+	_OFFSET_2        = 0x73
+	_PWR_CONF        = 0x7C
+	_PWR_CTRL        = 0x7D
+	_CMD             = 0x7E
+
+	// Commands send to regCommand.
+	cmdSoftReset = 0xB6
+)
diff --git a/examples/bma42x/main.go b/examples/bma42x/main.go
new file mode 100644
index 000000000..8f0b4a0f3
--- /dev/null
+++ b/examples/bma42x/main.go
@@ -0,0 +1,54 @@
+package main
+
+// Smoke test for the BMA421/BMA425 sensors.
+// Warning: this code has _not been tested_. It's only here as a smoke test.
+
+import (
+	"fmt"
+	"machine"
+	"time"
+
+	"tinygo.org/x/drivers"
+	"tinygo.org/x/drivers/bma42x"
+)
+
+func main() {
+	time.Sleep(5 * time.Second)
+
+	i2cBus := machine.I2C1
+	i2cBus.Configure(machine.I2CConfig{
+		Frequency: 400 * machine.KHz,
+		SDA:       machine.SDA_PIN,
+		SCL:       machine.SCL_PIN,
+	})
+
+	sensor := bma42x.NewI2C(i2cBus, bma42x.Address)
+	err := sensor.Configure(bma42x.Config{
+		Device:   bma42x.DeviceBMA421 | bma42x.DeviceBMA425,
+		Features: bma42x.FeatureStepCounting,
+	})
+	if err != nil {
+		println("could not configure BMA421/BMA425:", err)
+		return
+	}
+
+	if !sensor.Connected() {
+		println("BMA42x not connected")
+		return
+	}
+
+	for {
+		time.Sleep(time.Second)
+
+		err := sensor.Update(drivers.Acceleration | drivers.Temperature)
+		if err != nil {
+			println("Error reading sensor", err)
+			continue
+		}
+
+		fmt.Printf("Temperature: %.2f °C\n", float32(sensor.Temperature())/1000)
+
+		accelX, accelY, accelZ := sensor.Acceleration()
+		fmt.Printf("Acceleration: %.2fg %.2fg %.2fg\n", float32(accelX)/1e6, float32(accelY)/1e6, float32(accelZ)/1e6)
+	}
+}
diff --git a/smoketest.sh b/smoketest.sh
index 38bfed6e2..d2712e5ea 100755
--- a/smoketest.sh
+++ b/smoketest.sh
@@ -13,6 +13,7 @@ tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/apa
 tinygo build -size short -o ./build/test.hex -target=microbit ./examples/at24cx/main.go
 tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/bh1750/main.go
 tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/blinkm/main.go
+tinygo build -size short -o ./build/test.hex -target=pinetime     ./examples/bma42x/main.go
 tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/bmi160/main.go
 tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/bmp180/main.go
 tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/bmp280/main.go