diff --git a/scd4x/README.md b/scd4x/README.md
new file mode 100644
index 0000000..b1b0cf4
--- /dev/null
+++ b/scd4x/README.md
@@ -0,0 +1,56 @@
+# Sensirion SCD4x CO<sub>2</sub> Sensors
+
+## Overview
+
+This package provides a driver for the Sensirion SCD4x CO<sub>2</sub> sensors. This is a 
+compact sensor that provides temperature, humidity, and CO<sub>2</sub> concentration 
+readings. The datasheet for this device is available at:
+
+https://sensirion.com/media/documents/48C4B7FB/66E05452/CD_DS_SCD4x_Datasheet_D1.pdf
+
+## Testing
+
+The unit tests can function with either a live sensor, or in playback mode. If the 
+environment variable SCD4X is set, then the self test code will use a live 
+sensor on the default I<sup>2</sup>C bus. For example:
+
+```bash
+$> SCD4X=1 go test -v
+```
+If the environment variable is not present, then unit tests will be conducted using
+playback values.
+
+## Notes
+
+### Acquisition Time
+
+The minimum acquisition time for the sensor is 5 seconds. If you call Sense() more
+frequently, it will block until a reading is ready.
+
+### Forced Calibration and Self-Test
+
+These functions are not implemented. From examining the datasheet, and 
+experimenting, it appears that these two calls require the i2c communication 
+driver to wait a specified period before initiating the read. The periph.io 
+I<sup>2</sup>C library doesn't support this functionality. This means that attempts 
+to call these functions will always fail so they're not implemented.
+
+### Acquisition Mode
+
+Only certain commands can be issued while the device is running in acquisition 
+mode. If you're working on the low-level code, be aware that attempts to send
+a non-allowed command while in acquisition mode will return an i2c remote 
+io-error.
+
+### Automatic Self Calibration
+
+When Automatic Self Calibration is enabled, and the sensor has run for the 
+required period, it will adjust itself so that the LOWEST recorded reading 
+during the period yields the value set for ASC Target. The factory default 
+target is 400PPM, but the current PPM is ~425PPM. To get a more accurate 
+value for CO2 concentration in Earth's atmosphere, refer to:
+
+https://www.co2.earth/daily-co2
+
+For more details, refer to the datasheet.
+
diff --git a/scd4x/doc.go b/scd4x/doc.go
new file mode 100644
index 0000000..e8409eb
--- /dev/null
+++ b/scd4x/doc.go
@@ -0,0 +1,12 @@
+// Copyright 2024 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+// This package provides a driver for the Sensiron SCD4x CO2 sensors.
+// The scd4x family provide a compact sensor that can be used to measure
+// Temperature, Humidity, and CO2 concentration.
+//
+// Refer to the datasheet for more information.
+//
+// https://sensirion.com/media/documents/48C4B7FB/66E05452/CD_DS_SCD4x_Datasheet_D1.pdf
+package scd4x
diff --git a/scd4x/example_test.go b/scd4x/example_test.go
new file mode 100644
index 0000000..cad5a3e
--- /dev/null
+++ b/scd4x/example_test.go
@@ -0,0 +1,63 @@
+//go:build examples
+// +build examples
+
+// Copyright 2024 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package scd4x_test
+
+import (
+	"fmt"
+	"log"
+
+	"periph.io/x/conn/v3/i2c/i2creg"
+	"periph.io/x/devices/v3/scd4x"
+	"periph.io/x/host/v3"
+)
+
+// basic example program for scd4x sensors using this library.
+//
+// To execute this as a stand-alone program:
+//
+// Copy the file example_test.go to a new directory.
+// rename the file to main.go
+// rename the Example() function to main, and the package to main
+//
+// execute:
+//
+//	go mod init mydomain.com/scd4x
+//	go mod tidy
+//	go build -o main main.go
+//	./main
+func Example() {
+	fmt.Println("scd4x example program")
+	if _, err := host.Init(); err != nil {
+		fmt.Println(err)
+	}
+	bus, err := i2creg.Open("")
+	if err != nil {
+		log.Fatal(err)
+	}
+	dev, err := scd4x.NewI2C(bus, scd4x.SensorAddress)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	env := scd4x.Env{}
+	err = dev.Sense(&env)
+	if err == nil {
+		fmt.Println(env.String())
+	} else {
+		fmt.Println(err)
+	}
+
+	cfg, err := dev.GetConfiguration()
+	if err == nil {
+		fmt.Printf("Configuration: %#v\n", cfg)
+	} else {
+		fmt.Println(err)
+	}
+	// Output: Temperature: 24.845°C Humidity: 32.3%rH CO2: 581 PPM
+	// Configuration: &scd4x.DevConfig{AmbientPressure:0, ASCEnabled:true, ASCInitialPeriod:158400000000000, ASCStandardPeriod:561600000000000, ASCTarget:400, SensorAltitude:0, SerialNumber:127207989525260, TemperatureOffset:4, SensorType:0}
+}
diff --git a/scd4x/scd4x.go b/scd4x/scd4x.go
new file mode 100644
index 0000000..14128fe
--- /dev/null
+++ b/scd4x/scd4x.go
@@ -0,0 +1,623 @@
+// Copyright 2024 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package scd4x
+
+import (
+	"errors"
+	"fmt"
+	"sync"
+	"time"
+
+	"periph.io/x/conn/v3/i2c"
+	"periph.io/x/conn/v3/physic"
+)
+
+// PPM=Parts Per Million. Units of measure for CO2 concentration.
+type PPM int
+
+// Sensor Variant type
+type Variant int
+
+const (
+	SCD40 Variant = iota
+	SCD41
+)
+
+// Type of reset to perform.
+type ResetMode int
+
+const (
+	ResetFactory ResetMode = iota
+	// Reset to last values stored in EEPROM
+	ResetEEPROM
+)
+
+const (
+	// These devices only support this i2c address.
+	SensorAddress uint16 = 0x62
+)
+
+type cmd uint16
+
+// Structure to simplify sending commands to the device.
+type command struct {
+	// The 16-bit command words.
+	cmdWord cmd
+	// The expected number of bytes returned. 0, 3, or 9.
+	responseSize int
+	// True if this command is permitted while the sensor is running in
+	// acquisition mode.
+	whileSensing bool
+}
+
+// The various implemented commands.
+
+var cmdStartMeasurement = command{
+	cmdWord: 0x21b1,
+}
+
+var cmdReadMeasurement = command{
+	cmdWord:      0xec05,
+	responseSize: 9,
+	whileSensing: true,
+}
+
+var cmdStopMeasurement = command{
+	cmdWord:      0x3f86,
+	whileSensing: true,
+}
+var cmdGetTemperatureOffset = command{
+	cmdWord:      0x2318,
+	responseSize: 3,
+}
+var cmdSetTemperatureOffset = command{
+	cmdWord: 0x241d,
+}
+var cmdGetSensorAltitude = command{
+	cmdWord:      0x2322,
+	responseSize: 3,
+}
+var cmdSetSensorAltitude = command{
+	cmdWord: 0x2427,
+}
+var cmdGetAmbientPressure = command{
+	cmdWord:      0xe000,
+	responseSize: 3,
+	whileSensing: true,
+}
+var cmdSetAmbientPressure = command{
+	cmdWord:      0xe000,
+	whileSensing: true,
+}
+var cmdSetASCEnabled = command{
+	cmdWord: 0x2416,
+}
+var cmdGetASCEnabled = command{
+	cmdWord:      0x2313,
+	responseSize: 3,
+}
+var cmdGetASCTarget = command{
+	cmdWord:      0x233f,
+	responseSize: 3,
+}
+var cmdSetASCTarget = command{
+	cmdWord: 0x243a,
+}
+var cmdGetDataReadyStatus = command{
+	cmdWord:      0xe4b8,
+	responseSize: 3,
+	whileSensing: true,
+}
+var cmdPersistSettings = command{
+	cmdWord: 0x3615,
+}
+var cmdGetSerialNumber = command{
+	cmdWord:      0x3682,
+	responseSize: 9,
+}
+var cmdPerformFactoryReset = command{
+	cmdWord: 0x3632,
+}
+var cmdReinit = command{
+	cmdWord: 0x3646,
+}
+var cmdGetSensorVariant = command{
+	cmdWord:      0x202f,
+	responseSize: 3,
+}
+var cmdGetASCInitialPeriod = command{
+	cmdWord:      0x2340,
+	responseSize: 3,
+}
+var cmdSetASCInitialPeriod = command{
+	cmdWord: 0x2445,
+}
+var cmdGetASCStandardPeriod = command{
+	cmdWord:      0x234b,
+	responseSize: 3,
+}
+var cmdSetASCStandardPeriod = command{
+	cmdWord: 0x244e,
+}
+var cmdWakeUp = command{
+	cmdWord: 0x36f6,
+}
+
+// DevConfig is the current running configuration of the device. Values prefixed
+// with ASC refer to Auto-Self-Calibration. Use Dev.GetConfiguration() to read
+// the value, and Dev.SetConfiguration() to apply changes.
+//
+// Refer to the datasheet for more information on settings.
+type DevConfig struct {
+	// Ambient pressure value. Used to adjust operation of sensor.
+	AmbientPressure physic.Pressure
+	// Automatic-Self-Calibration enabled. True or false.
+	ASCEnabled bool
+	// Refer to datasheet for usage.
+	ASCInitialPeriod time.Duration
+	// Refer to datasheet for usage.
+	ASCStandardPeriod time.Duration
+	// Target CO2 concentration for automatic self calibration. To obtain the
+	// current value, visit:
+	//
+	// https://www.co2.earth/daily-co2
+	ASCTarget PPM
+	// Sensor altitude in metres. Alternative method to adjust ambient pressure
+	// for sensor correction.
+	SensorAltitude physic.Distance
+	// The 48 bit unique serial number of the device. Read-Only
+	SerialNumber int64
+	// Offset temperature added to reading. Refer to the datasheet for usage.
+	TemperatureOffset physic.Temperature
+	// The Type of sensor. SCD40 or SCD41. Read-Only
+	SensorType Variant
+}
+
+// Dev represents an SCD4x device.
+type Dev struct {
+	// The i2c bus device.
+	d *i2c.Dev
+	// channel to halt SenseContinuous
+	chHalt chan bool
+	mu     sync.Mutex
+	// True if the device is in continuous sense mode.
+	sensing bool
+}
+
+func (ppm *PPM) String() string {
+	return fmt.Sprintf("%d PPM", *ppm)
+}
+
+// The sensor reading. Returns CO2 PPM, Temperature, and Humidity.
+type Env struct {
+	physic.Env
+	CO2 PPM
+}
+
+// Return the sensor readings in string format.
+func (e *Env) String() string {
+	return fmt.Sprintf("Temperature: %s Humidity: %s CO2: %s", e.Temperature.String(), e.Humidity.String(), e.CO2.String())
+}
+
+// NewI2c creates a new SCD4x sensor using the supplied bus and address.
+// The constant value SensorAddress should be supplied as the value for
+// addr.
+func NewI2C(b i2c.Bus, addr uint16) (*Dev, error) {
+	d := &Dev{d: &i2c.Dev{Bus: b, Addr: addr}, chHalt: nil}
+	return d, d.start()
+}
+
+// GetConfiguration returns a structure containing all of the scd4x configuration
+// variables. You can then alter settings and call SetConfiguration with it.
+//
+// To examine the device use:
+//
+//	cfg, _ :=dev.GetConfiguration()
+//	fmt.Printf("Configuration=%#v\n", cfg)
+func (d *Dev) GetConfiguration() (*DevConfig, error) {
+
+	cfg := &DevConfig{}
+	var words []uint16
+	var err error
+
+	if words, err = d.sendCommand(cmdGetAmbientPressure, nil); err != nil {
+		return nil, err
+	}
+	cfg.AmbientPressure = physic.Pascal * 100 * physic.Pressure(words[0])
+
+	if words, err = d.sendCommand(cmdGetASCEnabled, nil); err != nil {
+		return nil, err
+	}
+	cfg.ASCEnabled = words[0] != 0
+
+	if words, err = d.sendCommand(cmdGetASCInitialPeriod, nil); err != nil {
+		return nil, err
+	}
+	cfg.ASCInitialPeriod = time.Hour * time.Duration(words[0])
+
+	if words, err = d.sendCommand(cmdGetASCStandardPeriod, nil); err != nil {
+		return nil, err
+	}
+	cfg.ASCStandardPeriod = time.Hour * time.Duration(words[0])
+
+	if words, err = d.sendCommand(cmdGetASCTarget, nil); err != nil {
+		return nil, err
+	}
+	cfg.ASCTarget = PPM(words[0])
+
+	if words, err = d.sendCommand(cmdGetSerialNumber, nil); err != nil {
+		return nil, err
+	}
+	cfg.SerialNumber = int64(words[0])<<32 | int64(words[1])<<16 | int64(words[2])
+
+	if words, err = d.sendCommand(cmdGetSensorVariant, nil); err != nil {
+		return nil, err
+	}
+	if (words[0]>>11)&0x07 == 0 {
+		cfg.SensorType = SCD40
+	} else {
+		cfg.SensorType = SCD41
+	}
+
+	if words, err = d.sendCommand(cmdGetSensorAltitude, nil); err != nil {
+		return nil, err
+	}
+	cfg.SensorAltitude = physic.Distance(words[0]) * physic.Metre
+
+	if words, err = d.sendCommand(cmdGetTemperatureOffset, nil); err != nil {
+		return nil, err
+	}
+	cfg.TemperatureOffset = countToOffset(words[0])
+
+	return cfg, nil
+}
+
+// SetConfiguration alters the configuration of the sensor. Note that this call
+// does not persist the settings to EEPROM. You need to call Persist() to
+// commit the writes to EEPROM. If you do not persist changes, then those settings
+// will be lost when the unit is power-cycled.
+func (d *Dev) SetConfiguration(newCfg *DevConfig) error {
+
+	_ = d.Halt()
+	d.mu.Lock()
+	defer d.mu.Unlock()
+
+	w := make([]uint16, 1)
+	currentConfig, err := d.GetConfiguration()
+	if err != nil {
+		return fmt.Errorf("scd4x GetConfiguration(): %w", err)
+	}
+
+	if currentConfig.AmbientPressure != newCfg.AmbientPressure {
+		w[0] = uint16(newCfg.AmbientPressure / (100 * physic.Pascal))
+		_, err := d.sendCommand(cmdSetAmbientPressure, w)
+		if err != nil {
+			return err
+		}
+	}
+
+	if currentConfig.ASCEnabled != newCfg.ASCEnabled {
+
+		if newCfg.ASCEnabled {
+			w[0] = 1
+		} else {
+			w[0] = 0
+		}
+		_, err := d.sendCommand(cmdSetASCEnabled, w)
+		if err != nil {
+			return err
+		}
+	}
+
+	if currentConfig.ASCInitialPeriod != newCfg.ASCInitialPeriod {
+		if newCfg.ASCInitialPeriod%4 != 0 {
+			return fmt.Errorf("scd4x: invalid initial period %d. must be a multiple of 4", newCfg.ASCInitialPeriod)
+		}
+		w[0] = uint16(newCfg.ASCInitialPeriod / time.Hour)
+		_, err := d.sendCommand(cmdSetASCInitialPeriod, w)
+		if err != nil {
+			return err
+		}
+	}
+
+	if currentConfig.ASCStandardPeriod != newCfg.ASCStandardPeriod {
+		if newCfg.ASCStandardPeriod%4 != 0 {
+			return fmt.Errorf("scd4x: invalid standard period %d. must be a multiple of 4", newCfg.ASCStandardPeriod)
+		}
+		w[0] = uint16(newCfg.ASCStandardPeriod / time.Hour)
+		_, err := d.sendCommand(cmdSetASCStandardPeriod, w)
+		if err != nil {
+			return err
+		}
+	}
+
+	if currentConfig.ASCTarget != newCfg.ASCTarget {
+		w[0] = uint16(newCfg.ASCTarget)
+		_, err := d.sendCommand(cmdSetASCTarget, w)
+		if err != nil {
+			return err
+		}
+	}
+
+	if currentConfig.SensorAltitude != newCfg.SensorAltitude {
+		w[0] = uint16(newCfg.SensorAltitude / physic.Metre)
+		_, err := d.sendCommand(cmdSetSensorAltitude, w)
+		if err != nil {
+			return err
+		}
+	}
+
+	if currentConfig.TemperatureOffset != newCfg.TemperatureOffset {
+		val := float64(newCfg.TemperatureOffset.Celsius()) * (float64(65535) / float64(175))
+		w[0] = uint16(val)
+		_, err := d.sendCommand(cmdSetTemperatureOffset, w)
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// Halt stops continuous sensing if enabled, and if a SenseContinuous operation
+// is in progress, it too is halted.
+func (d *Dev) Halt() error {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	if d.sensing {
+		if d.chHalt != nil {
+			close(d.chHalt)
+		}
+		d.sensing = false
+		_, err := d.sendCommand(cmdStopMeasurement, nil)
+		time.Sleep(550 * time.Millisecond)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// Persist writes the current running configuration to the sensor EEPROM for
+// use on the next power-up.
+func (d *Dev) Persist() error {
+	_, err := d.sendCommand(cmdPersistSettings, nil)
+	return err
+}
+
+// Reset performs either a factory reset, or a re-load of settings from EEPROM
+// depending on the value of mode. During development, it was noticed that
+// ResetFactory DOES NOT reset AmbientPressure to 0.
+func (d *Dev) Reset(mode ResetMode) error {
+	var err error
+	if mode == ResetFactory {
+		_, err = d.sendCommand(cmdPerformFactoryReset, nil)
+	} else if mode == ResetEEPROM {
+		_, err = d.sendCommand(cmdReinit, nil)
+	} else {
+		err = fmt.Errorf("scd4x: invalid reset mode 0x%x", mode)
+	}
+	return err
+}
+
+func calcCRC(bytes []byte) byte {
+	polynomial := byte(0x31)
+	crc := byte(0xff)
+	for ix := range len(bytes) {
+		crc ^= bytes[ix]
+		for crc_bit := byte(8); crc_bit > 0; crc_bit-- {
+			if (crc & 0x80) == 0x80 {
+				crc = (crc << 1) ^ polynomial
+			} else {
+				crc = (crc << 1)
+			}
+		}
+	}
+	return crc
+}
+
+// makeWriteData converts the slice of word values into byte values with the
+// CRC following.
+func makeWriteData(data []uint16) []byte {
+	bytes := make([]byte, len(data)*3)
+	for ix, val := range data {
+		bytes[ix*3] = byte((val >> 8) & 0xff)
+		bytes[ix*3+1] = byte(val & 0xff)
+		bytes[ix*3+2] = calcCRC(bytes[ix*3 : ix*3+2])
+	}
+	return bytes
+}
+
+// All commands to read or write to the sensor go through this function.
+func (d *Dev) sendCommand(cmd command, writeData []uint16) ([]uint16, error) {
+
+	if d.sensing && !cmd.whileSensing {
+		// We're in sense mode and this command isn't compatible. Stop sensing.
+		if err := d.Halt(); err != nil {
+			return nil, err
+		}
+	}
+
+	w := make([]byte, 2)
+	w[0] = byte((cmd.cmdWord >> 8) & 0xff)
+	w[1] = byte(cmd.cmdWord & 0xff)
+	if writeData != nil {
+		writeBytes := makeWriteData(writeData)
+		w = append(w, writeBytes...)
+	}
+	var r []byte
+	if cmd.responseSize > 0 {
+		r = make([]byte, cmd.responseSize)
+	}
+
+	err := d.d.Tx(w, r)
+	if err != nil {
+		return nil, fmt.Errorf("scd4x cmd 0x%x: %w", cmd.cmdWord, err)
+	}
+	if cmd.responseSize == 0 {
+		return nil, nil
+	}
+
+	// OK, we need to convert the bytes into a slice of words and
+	// verify the CRC as we go.
+	result := make([]uint16, cmd.responseSize/3)
+	for ix := range len(result) {
+		crc := calcCRC(r[ix*3 : ix*3+2])
+		if r[ix*3+2] != crc {
+			return nil, fmt.Errorf("scd4x cmd 0x%x: invalid crc", cmd.cmdWord)
+		}
+
+		word := uint16(r[ix*3])<<8 | uint16(r[ix*3+1])
+
+		result[ix] = word
+	}
+
+	return result, nil
+}
+
+// start continuous sensing.
+func (d *Dev) start() error {
+	if d.sensing {
+		return nil
+	}
+	d.mu.Lock()
+	defer d.mu.Unlock()
+
+	_, err := d.sendCommand(cmdWakeUp, nil)
+	if err != nil {
+		// If an SCD4x is in measurement mode, then any non-measurement mode
+		// command will return an error. In that case, send a stop measurement
+		// command, wait the specified time and try sending a re-init.
+		_, _ = d.sendCommand(cmdStopMeasurement, nil)
+		time.Sleep(550 * time.Millisecond)
+	}
+	time.Sleep(50 * time.Millisecond)
+
+	_, err = d.sendCommand(cmdStartMeasurement, nil)
+	if err == nil {
+		d.sensing = true
+	}
+	return err
+}
+
+// Formula used for temperature offset calculation.
+func countToOffset(count uint16) physic.Temperature {
+	frac := 175.0 / 65535.0
+	return physic.Temperature(frac * float64(count))
+}
+
+// countToTemp converts a device count to Temperature
+func countToTemp(count uint16) physic.Temperature {
+	frac := float64(count) / 65535.0
+	result := -45 + 175*frac
+	return physic.ZeroCelsius + physic.Temperature(float64(physic.Celsius)*result)
+}
+
+func countToHumidity(count uint16) physic.RelativeHumidity {
+	frac := float64(count) / 65535.0
+	return physic.RelativeHumidity(frac * 100.0 * float64(physic.PercentRH))
+}
+
+// Sense returns readings (Temperature, Humidity, and CO2 concentration in PPM)
+// from the device. Note that in normal acquisition mode, the minimum reading
+// period is 5 seconds. If you call this function more frequently than this,
+// it will block until data is ready.
+func (d *Dev) Sense(env *Env) error {
+	env.Temperature = 0
+	env.Humidity = 0
+	env.CO2 = 0
+	env.Pressure = 0
+
+	if !d.sensing {
+		err := d.start()
+		if err != nil {
+			return err
+		}
+		time.Sleep(5 * time.Second)
+	}
+	d.mu.Lock()
+	defer d.mu.Unlock()
+
+	ready := false
+	mask := uint16(1<<11 - 1)
+	tCutoff := time.Now().Unix() + 6
+	for !ready && time.Now().Unix() < tCutoff {
+		words, err := d.sendCommand(cmdGetDataReadyStatus, nil)
+		ready = err == nil && (words[0]&mask) > 0
+		if !ready {
+			time.Sleep(time.Second)
+		}
+	}
+	if !ready {
+		return errors.New("scd4x: timeout waiting for data ready status")
+	}
+	words, err := d.sendCommand(cmdReadMeasurement, nil)
+	if err != nil {
+		return err
+	}
+	env.CO2 = PPM(words[0])
+	env.Temperature = countToTemp(words[1])
+	env.Humidity = countToHumidity(words[2])
+	return nil
+}
+
+// SenseContinuous continuously reads the sensor on the specified duration, and
+// writes readings to the returned channel. The sense time for the scd4x device
+// is 5 seconds in normal acquisition mode. If you specify a shorter period than
+// that, the routine will spin until the device indicates a reading is ready. To
+// terminate a continuous sense, call Halt().
+func (d *Dev) SenseContinuous(interval time.Duration) (<-chan Env, error) {
+	if d.chHalt != nil {
+		return nil, errors.New("scd4x: SenseContinuous() running already")
+	}
+	if !d.sensing {
+		if err := d.start(); err != nil {
+			return nil, err
+		}
+	}
+	channelSize := 16
+	channel := make(chan Env, channelSize)
+
+	go func() {
+		ticker := time.NewTicker(interval)
+		defer ticker.Stop()
+		defer close(channel)
+		if d.chHalt == nil {
+			d.chHalt = make(chan bool)
+		}
+
+		defer func() { d.chHalt = nil }()
+
+		for {
+			select {
+			case <-d.chHalt:
+				return
+			case <-ticker.C:
+				// do the reading and write to the channel.
+				e := Env{}
+				err := d.Sense(&e)
+				if err == nil && len(channel) < channelSize {
+					channel <- e
+				}
+			}
+		}
+	}()
+	return channel, nil
+}
+
+// Precision returns the sensor's resolution, or minimum value between steps the
+// device can make. The specified precision is 1 PPM for CO2, 1/65535 for temperature
+// and humidity.
+func (d *Dev) Precision(env *Env) {
+	countIncrement := float64(1.0) / float64((1<<16)-1)
+	env.Temperature = physic.Temperature(countIncrement * float64(physic.Celsius))
+	env.Pressure = 0
+	env.Humidity = physic.RelativeHumidity(float64(physic.PercentRH) * countIncrement)
+	env.CO2 = 1
+}
+
+func (d *Dev) String() string {
+	return fmt.Sprintf("scd4x: %s", d.d.String())
+}
diff --git a/scd4x/scd4x_test.go b/scd4x/scd4x_test.go
new file mode 100644
index 0000000..76bc555
--- /dev/null
+++ b/scd4x/scd4x_test.go
@@ -0,0 +1,439 @@
+// Copyright 2024 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+//
+// Unit tests for the package. Note that this supports running on a live
+// sensor, or using playback mode to simulate a live device.
+//
+// To use a live device, define the environment variable SCD4X and run go test.
+
+package scd4x
+
+import (
+	"fmt"
+	"os"
+	"testing"
+	"time"
+
+	"periph.io/x/conn/v3/i2c"
+	"periph.io/x/conn/v3/i2c/i2creg"
+	"periph.io/x/conn/v3/i2c/i2ctest"
+	"periph.io/x/conn/v3/physic"
+	"periph.io/x/host/v3"
+)
+
+var bus i2c.Bus
+var liveDevice bool = false
+
+// playback values for TestSense
+var sensePlayback = []i2ctest.IO{
+	{Addr: SensorAddress, W: []uint8{0x36, 0xf6}},
+	{Addr: SensorAddress, W: []uint8{0x21, 0xb1}},
+	{Addr: SensorAddress, W: []uint8{0xe4, 0xb8}, R: []uint8{0x80, 0x0, 0xa2}},
+	{Addr: SensorAddress, W: []uint8{0xe4, 0xb8}, R: []uint8{0x80, 0x0, 0xa2}},
+	{Addr: SensorAddress, W: []uint8{0xe4, 0xb8}, R: []uint8{0x80, 0x6, 0x4}},
+	{Addr: SensorAddress, W: []uint8{0xec, 0x5}, R: []uint8{0x2, 0x2c, 0xa3, 0x67, 0xd, 0x36, 0x4d, 0x8, 0xf1}}}
+
+var senseContinuousPlayback = []i2ctest.IO{
+	{Addr: SensorAddress, W: []uint8{0x36, 0xf6}},
+	{Addr: SensorAddress, W: []uint8{0x21, 0xb1}},
+	{Addr: SensorAddress, W: []uint8{0xe4, 0xb8}, R: []uint8{0x80, 0x6, 0x4}},
+	{Addr: SensorAddress, W: []uint8{0xec, 0x5}, R: []uint8{0x2, 0x1f, 0x35, 0x65, 0x82, 0xbb, 0x53, 0x5e, 0x2a}},
+	{Addr: SensorAddress, W: []uint8{0xe4, 0xb8}, R: []uint8{0x80, 0x6, 0x4}},
+	{Addr: SensorAddress, W: []uint8{0xec, 0x5}, R: []uint8{0x2, 0x22, 0xbc, 0x65, 0x39, 0xee, 0x55, 0x4b, 0xc6}},
+	{Addr: SensorAddress, W: []uint8{0xe4, 0xb8}, R: []uint8{0x80, 0x6, 0x4}},
+	{Addr: SensorAddress, W: []uint8{0xec, 0x5}, R: []uint8{0x2, 0x1c, 0x66, 0x64, 0xeb, 0x7c, 0x56, 0xd1, 0x9}},
+	{Addr: SensorAddress, W: []uint8{0xe4, 0xb8}, R: []uint8{0x80, 0x6, 0x4}},
+	{Addr: SensorAddress, W: []uint8{0xec, 0x5}, R: []uint8{0x2, 0x1f, 0x35, 0x64, 0xad, 0xe7, 0x58, 0x2f, 0xf9}},
+	{Addr: SensorAddress, W: []uint8{0xe4, 0xb8}, R: []uint8{0x80, 0x6, 0x4}},
+	{Addr: SensorAddress, W: []uint8{0xec, 0x5}, R: []uint8{0x2, 0x1f, 0x35, 0x64, 0x79, 0x27, 0x59, 0x71, 0x6c}},
+	{Addr: SensorAddress, W: []uint8{0xe4, 0xb8}, R: []uint8{0x80, 0x6, 0x4}},
+	{Addr: SensorAddress, W: []uint8{0xec, 0x5}, R: []uint8{0x2, 0x1f, 0x35, 0x64, 0x46, 0xcc, 0x5a, 0x8d, 0xbe}}}
+
+var getSetTestPlayback = []i2ctest.IO{
+	{Addr: SensorAddress, W: []uint8{0x36, 0xf6}},
+	{Addr: SensorAddress, W: []uint8{0x21, 0xb1}},
+	{Addr: SensorAddress, W: []uint8{0x3f, 0x86}},
+	{Addr: SensorAddress, W: []uint8{0x36, 0x46}},
+	{Addr: SensorAddress, W: []uint8{0xe0, 0x0}, R: []uint8{0x0, 0x5, 0x74}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x13}, R: []uint8{0x0, 0x1, 0xb0}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x40}, R: []uint8{0x0, 0x2c, 0x7a}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x4b}, R: []uint8{0x0, 0x9c, 0xc5}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x3f}, R: []uint8{0x1, 0x90, 0x4c}},
+	{Addr: SensorAddress, W: []uint8{0x36, 0x82}, R: []uint8{0x73, 0xb1, 0x19, 0xeb, 0x7, 0x7a, 0x3b, 0xc, 0x54}},
+	{Addr: SensorAddress, W: []uint8{0x20, 0x2f}, R: []uint8{0x4, 0x41, 0xe}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x22}, R: []uint8{0x0, 0x0, 0x81}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x18}, R: []uint8{0x5, 0xda, 0x29}},
+	{Addr: SensorAddress, W: []uint8{0xe0, 0x0}, R: []uint8{0x0, 0x5, 0x74}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x13}, R: []uint8{0x0, 0x1, 0xb0}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x40}, R: []uint8{0x0, 0x2c, 0x7a}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x4b}, R: []uint8{0x0, 0x9c, 0xc5}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x3f}, R: []uint8{0x1, 0x90, 0x4c}},
+	{Addr: SensorAddress, W: []uint8{0x36, 0x82}, R: []uint8{0x73, 0xb1, 0x19, 0xeb, 0x7, 0x7a, 0x3b, 0xc, 0x54}},
+	{Addr: SensorAddress, W: []uint8{0x20, 0x2f}, R: []uint8{0x4, 0x41, 0xe}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x22}, R: []uint8{0x0, 0x0, 0x81}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x18}, R: []uint8{0x5, 0xda, 0x29}},
+	{Addr: SensorAddress, W: []uint8{0xe0, 0x0, 0x0, 0xa, 0x5a}},
+	{Addr: SensorAddress, W: []uint8{0x24, 0x16, 0x0, 0x0, 0x81}},
+	{Addr: SensorAddress, W: []uint8{0x24, 0x45, 0x0, 0x30, 0x44}},
+	{Addr: SensorAddress, W: []uint8{0x24, 0x4e, 0x0, 0xa0, 0x7d}},
+	{Addr: SensorAddress, W: []uint8{0x24, 0x3a, 0x1, 0xa4, 0x4d}},
+	{Addr: SensorAddress, W: []uint8{0x24, 0x27, 0x6, 0x44, 0x22}},
+	{Addr: SensorAddress, W: []uint8{0xe0, 0x0}, R: []uint8{0x0, 0xa, 0x5a}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x13}, R: []uint8{0x0, 0x0, 0x81}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x40}, R: []uint8{0x0, 0x30, 0x44}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x4b}, R: []uint8{0x0, 0xa0, 0x7d}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x3f}, R: []uint8{0x1, 0xa4, 0x4d}},
+	{Addr: SensorAddress, W: []uint8{0x36, 0x82}, R: []uint8{0x73, 0xb1, 0x19, 0xeb, 0x7, 0x7a, 0x3b, 0xc, 0x54}},
+	{Addr: SensorAddress, W: []uint8{0x20, 0x2f}, R: []uint8{0x4, 0x41, 0xe}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x22}, R: []uint8{0x6, 0x44, 0x22}},
+	{Addr: SensorAddress, W: []uint8{0x23, 0x18}, R: []uint8{0x5, 0xda, 0x29}},
+	{Addr: SensorAddress, W: []uint8{0x36, 0x46}}}
+
+var basicStartup = []i2ctest.IO{
+	{Addr: SensorAddress, W: []uint8{0x36, 0xf6}},
+	{Addr: SensorAddress, W: []uint8{0x21, 0xb1}}}
+
+func init() {
+	var err error
+	// If the environment variable is set, assume we have a live device on
+	// the default i2c bus and use it for testing. If the variable is not
+	// present, then use the playback/read values.
+	if os.Getenv("SCD4X") != "" {
+		liveDevice = true
+	}
+	if _, err = host.Init(); err != nil {
+		fmt.Println(err)
+	}
+
+	if liveDevice {
+		bus, err = i2creg.Open("")
+		if err != nil {
+			fmt.Println(err)
+		}
+		// Add the recorder to dump the data stream when we're using a live device.
+		bus = &i2ctest.Record{Bus: bus}
+	} else {
+		bus = &i2ctest.Playback{DontPanic: true}
+	}
+
+}
+
+// getDev returns an scd4x device for testing connected to either a live
+// bus, or a playback bus. playbackOps is a slice of i2ctest.IO
+// operations to be used for playback mode. Ignored for live device
+// testing.
+func getDev(t *testing.T, playbackOps ...[]i2ctest.IO) (*Dev, error) {
+	if liveDevice {
+		if recorder, ok := bus.(*i2ctest.Record); ok {
+			// Clear the operations buffer.
+			recorder.Ops = make([]i2ctest.IO, 0, 32)
+		}
+	} else {
+		if len(playbackOps) == 1 {
+			pb := bus.(*i2ctest.Playback)
+			pb.Ops = playbackOps[0]
+			pb.Count = 0
+		}
+	}
+	dev, err := NewI2C(bus, SensorAddress)
+
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	return dev, err
+}
+
+// shutdown dumps the recorder values if we we're running a live device.
+func shutdown(t *testing.T) {
+	if recorder, ok := bus.(*i2ctest.Record); ok {
+		t.Logf("%#v", recorder.Ops)
+	}
+}
+
+func TestCRC(t *testing.T) {
+	tests := []struct {
+		bytes []byte
+		crc   byte
+	}{
+		{bytes: []byte{0xbe, 0xef}, crc: 0x92},
+		{bytes: []byte{0x01, 0xa4}, crc: 0x4d},
+	}
+	for _, test := range tests {
+		res := calcCRC(test.bytes)
+		if res != test.crc {
+			t.Error(fmt.Errorf("crc calculation error bytes: %#v, result: 0x%x expected: 0x%x", test.bytes, res, test.crc))
+		}
+	}
+}
+
+func TestCountToTemperature(t *testing.T) {
+	tests := []struct {
+		count    uint16
+		expected physic.Temperature
+	}{
+		{count: 0x6667, expected: physic.ZeroCelsius + 25*physic.Celsius},
+	}
+	for _, test := range tests {
+		result := countToTemp(test.count)
+		// round to 2 sig figs for the floating point comparison.
+		result -= result % (10 * physic.MilliKelvin)
+		if result != test.expected {
+			t.Errorf("received: %.8f expected %.8f", result.Celsius(), test.expected.Celsius())
+		}
+	}
+}
+
+func TestCountToHumidity(t *testing.T) {
+	result := countToHumidity(0x5eb9) // from the datasheet
+	// Truncate to 2 decimals for comparison.
+	result -= result % physic.MilliRH
+	expected := physic.RelativeHumidity(37 * physic.PercentRH)
+	if result != expected {
+		t.Errorf("unexpected value: %d expected %d", result, expected)
+	}
+}
+
+// Non-device basic functionality.
+func TestBasic(t *testing.T) {
+	dev, err := getDev(t, basicStartup)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer func() { _ = dev.Halt() }()
+	defer shutdown(t)
+
+	env := Env{}
+	dev.Precision(&env)
+	t.Logf("scd4x.Precision()=%#v\n", env)
+	if env.CO2 != 1 || env.Humidity != physic.TenthMicroRH || env.Temperature != (15259*physic.NanoKelvin) {
+		t.Error(fmt.Errorf("incorrect value for Precision(): %#v", env))
+	}
+
+	s := dev.String()
+	t.Logf("dev.String()=%s", s)
+	if len(s) == 0 {
+		t.Error("Dev.String() returned empty value.")
+	}
+}
+
+func TestSense(t *testing.T) {
+	dev, err := getDev(t, sensePlayback)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer func() { _ = dev.Halt() }()
+	defer shutdown(t)
+	env := Env{}
+	err = dev.Sense(&env)
+	if err != nil {
+		t.Error(err)
+	} else {
+		t.Log(env.String())
+	}
+}
+
+func TestSenseContinuous(t *testing.T) {
+	readings := 6
+	timeBase := time.Second
+	if liveDevice {
+		timeBase *= 10
+	}
+	dev, err := getDev(t, senseContinuousPlayback)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer func() { _ = dev.Halt() }()
+	defer shutdown(t)
+	t.Log("dev.sensing=", dev.sensing)
+	ch, err := dev.SenseContinuous(timeBase)
+	if err != nil {
+		t.Error(err)
+	}
+
+	go func() {
+		time.Sleep(time.Duration(readings) * timeBase)
+		_ = dev.Halt()
+	}()
+	received := 0
+	for env := range ch {
+		t.Log(env.String())
+		received += 1
+	}
+	if received < (readings-1) || received > readings {
+		t.Errorf("SenseContinuous() expected at least %d readings, got %d", readings-1, received)
+	}
+
+}
+
+func TestGetSetConfiguration(t *testing.T) {
+	dev, err := getDev(t, getSetTestPlayback)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = dev.Halt()
+	if err != nil {
+		t.Fatal(err)
+	}
+	time.Sleep(time.Second)
+	// Baseline our settings
+	err = dev.Reset(ResetEEPROM)
+	if err != nil {
+		t.Fatal(err)
+	}
+	time.Sleep(100 * time.Millisecond)
+	defer shutdown(t)
+	cfg, err := dev.GetConfiguration()
+
+	if err != nil {
+		t.Error(err)
+	}
+	t.Logf("existing configuration: %#v", cfg)
+	cfg.AmbientPressure += 500 * physic.Pascal
+	cfg.ASCEnabled = !cfg.ASCEnabled
+	cfg.ASCInitialPeriod += 4 * time.Hour
+	cfg.ASCStandardPeriod += 4 * time.Hour
+	cfg.ASCTarget += 20
+	cfg.SensorAltitude = 1604 * physic.Metre
+
+	err = dev.SetConfiguration(cfg)
+	if err != nil {
+		t.Error(err)
+	}
+	read, err := dev.GetConfiguration()
+	if err != nil {
+		t.Error(err)
+	}
+	t.Logf("new configuration: %#v", read)
+
+	if read.AmbientPressure != cfg.AmbientPressure {
+		t.Errorf("scd4x: error setting ambient pressure. found: %s (%d) expected: %s (%d)", read.AmbientPressure.String(), read.AmbientPressure, cfg.AmbientPressure.String(), cfg.AmbientPressure)
+	}
+	if read.ASCEnabled != cfg.ASCEnabled {
+		t.Errorf("scd4x: error setting asc enabled. Found %t expected %t", read.ASCEnabled, cfg.ASCEnabled)
+	}
+	if read.ASCInitialPeriod != cfg.ASCInitialPeriod {
+		t.Errorf("scd4x: error setting initial period. found: %d expected %d", read.ASCInitialPeriod, cfg.ASCInitialPeriod)
+	}
+	if read.ASCStandardPeriod != cfg.ASCStandardPeriod {
+		t.Errorf("scd4x: error setting standard period. found: %d expected %d", read.ASCStandardPeriod, cfg.ASCStandardPeriod)
+	}
+	if read.ASCTarget != cfg.ASCTarget {
+		t.Errorf("scd4x: error setting asc target. found %d expected %d", read.ASCTarget, cfg.ASCTarget)
+	}
+	if read.SensorAltitude != cfg.SensorAltitude {
+		t.Errorf("scd4x: error setting sensor altitude. found %d expected %d", read.SensorAltitude/physic.Metre, cfg.SensorAltitude/physic.Metre)
+	}
+
+	_ = dev.Reset(ResetEEPROM) // and go back to our known state.
+}
+
+// Since there are limited read/write cycles, by default DO NOT test persist
+// and reset factory. To perform the tests, define the environment variable
+// SCDRESET. Running this test will destructively clear customized values
+// previously programmed into the device.
+func TestPersistAndResetFactory(t *testing.T) {
+	if !liveDevice || os.Getenv("SCDRESET") == "" {
+		t.Skip("using live device and SCDRESET not defined. skipping")
+	}
+	dev, err := getDev(t)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = dev.Halt()
+	if err != nil {
+		t.Fatal(err)
+	}
+	time.Sleep(time.Second)
+
+	// Read the current running configuration.
+	cfg, err := dev.GetConfiguration()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer shutdown(t)
+
+	// Set the altitude to the current Altitude+1000M and write it to the device.
+	if cfg.SensorAltitude < (2000 * physic.Metre) {
+		cfg.SensorAltitude += 1000 * physic.Metre
+	} else {
+		cfg.SensorAltitude -= (500 * physic.Metre)
+	}
+	t.Logf("updating sensor altitude to %s", cfg.SensorAltitude)
+
+	err = dev.SetConfiguration(cfg)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Now, re-read the configuration to verify the write worked.
+	updatedCfg, err := dev.GetConfiguration()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if updatedCfg.SensorAltitude != cfg.SensorAltitude {
+		t.Fatalf("scd41x: Change sensor altitude failed. Read: %s Expected: %s", updatedCfg.SensorAltitude.String(), cfg.SensorAltitude)
+	}
+
+	// OK, now Persist()
+	err = dev.Persist()
+	if err != nil {
+		t.Error(err)
+	}
+	_ = dev.Reset(ResetEEPROM)
+	time.Sleep(time.Second)
+
+	// OK, now write 0
+	cfg.SensorAltitude = 0
+	err = dev.SetConfiguration(cfg)
+	if err != nil {
+		t.Error(err)
+	}
+	// Reset Settings to EEPROM
+	err = dev.Reset(ResetEEPROM)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Sometimes you have to wait for it to come to the party...
+	for range 5 {
+		_ = dev.Halt()
+		// Now, re-read the configuration
+		cfg, err = dev.GetConfiguration()
+		if err != nil {
+			t.Logf("GetConfiguration Failed: %s Sleeping before retry.", err)
+			time.Sleep(time.Second)
+		} else {
+			break
+		}
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// The expected value is the original value +1000M
+	if cfg.SensorAltitude != updatedCfg.SensorAltitude {
+		t.Errorf("Error using reset to eeprom. Expected SensorAltitude: %s Found: %s", updatedCfg.SensorAltitude, cfg.SensorAltitude)
+	}
+
+	t.Logf("current configuration: %#v", cfg)
+	// Almost there. Now, reset to factory and read sensor-altitude.
+	t.Logf("calling reset factory")
+	err = dev.Reset(ResetFactory)
+	if err != nil {
+		t.Error(err)
+	}
+	time.Sleep(time.Second)
+
+	cfg, err = dev.GetConfiguration()
+
+	if err != nil {
+		t.Error(err)
+	}
+	t.Logf("Reset to factory configuration is now: %#v", cfg)
+
+	if cfg.SensorAltitude != 0 {
+		t.Errorf("Error resetting to factory. Sensor Altitude: %s expected 0m", cfg.SensorAltitude)
+	}
+}