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diff --git a/devices/SCD40.js b/devices/SCD40.js
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+++ b/devices/SCD40.js
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+/* Copyright (c) 2025 Hywel Warsop. See the file LICENSE for copying permission. */
+/*
+Module for Sensiron SCD40, SCD41 and SCD43.
+
+I2C2.setup({scl:B10,sda:B3});
+var scd = require("SCD40").connectI2C(I2C2);
+
+//single measurement
+scd.measure_single_shot();
+scd.get_data_ready_status();
+readings = scd.read_measurement();
+console.log({readings});
+
+//continous measurement (5 sec interval)
+scd.start_periodic_measurement();
+readings = scd.read_measurement();
+console.log({readings});
+
+scd.stop_periodic_measurement()
+*/
+
+var C = {
+  I2C_ADDR : 0x62,
+
+  REQUEST_SINGLE_LOOKUP : [0x21,0x9d],
+  REQUEST_SINGLE_LOOKUP_RHT : [0x21,0x96],
+  START_MEASUREMENTS : [0x21,0xb1],
+  START_LOW_POWER_MEASUREMENTS : [0x21,0xac],
+  STOP_MEASUREMENTS : [0x3f,0x86],
+  IS_DATA_READY : [0xe4,0xb8],
+  READ_DATA : [0xec,0x05],
+
+  PERFORM_FORCED_RECALIBARTION : 0x362f,
+  GET_AUTOMATIC_SELF_CALIBRATION_ENABLED : [0x23,0x13],
+  SET_AUTOMATIC_SELF_CALIBRATION_ENABLED : 0x2416,
+  GET_AUTOMATIC_SELF_CALIBRATION_TARGET : [0x23,0x3f],
+  SET_AUTOMATIC_SELF_CALIBRATION_TARGET : 0x243a,
+
+  GET_TEMP_OFFSET : [0x23,0x18],
+  SET_TEMP_OFFSET : 0x241d,
+  GET_SENSOR_ALTITUDE : [0x23,0x22],
+  SET_SENSOR_ALTITUDE : 0x2427,
+  GET_AMBIENT_PRESSURE : [0xe0,0x00],
+  SET_AMBIENT_PRESSURE : 0xe000,
+
+  PERSIST_SETTINGS : [0x36,0x15],
+  GET_SERIAL_NUMBER : [0x36,0x82],
+  PERfORM_SELF_TEST : [0x36,0x39],
+  FACTORY_RESET : [0x36,0x32],
+  RE_INIT : [0x36,0x46],
+
+  /** Chip identifier */
+  CHIP_GET_VARIANT : [0x20,0x2f],
+  CHIP_VARIANT : [0x0441,0x1440,0x5441],//scd40,scd41,scd43
+
+  POWER_DOWN : [0x36,0xe0],
+  WAKE_UP : [0x36,0xf6],
+
+  GET_CALIBRATION_INITIAL_PERIOD : [0x23,0x40],
+  SET_CALIBRATION_INITIAL_PERIOD : 0x2445,
+  GET_CALIBRATION_STANDARD_PERIOD : [0x23,0x4b],
+  SET_CALIBRATION_STANDARD_PERIOD : 0x244e
+};
+
+function SCD40(options,read,simpleRead,write) {
+  this.w = write;
+  this.r = read;
+  this.sr = simpleRead;
+  this.getValue = function(ref) {
+    resp = this.r(ref,3);
+    v = new DataView(resp.buffer);
+    return v.getInt16(0);
+  };
+  if ( ! C.CHIP_VARIANT.includes(this.getValue(C.CHIP_GET_VARIANT)) ) {
+    throw new Error("SCDXX not found");
+  }
+  this.crc = function(data) {
+    "jit";
+    bytes=[data>>8,data];
+    var crc=0xff;
+    for (j=0;j<2;++j){
+      crc ^= (bytes[j]);
+      for (var i=8;i>0;i--) {
+        if (crc & 0x80)  {
+          crc=(crc<<1) ^ 0x31;
+        } else {
+          crc=(crc<<1);
+        }
+      }
+    }
+    return crc;
+  };
+  this.updateValue = function(command,data) {
+    arr = Uint8Array(5);
+    update = new DataView(arr.buffer);
+    update.setInt16(0,command);
+    update.setInt16(2,data);
+    update.setInt8(4,this.crc(data));
+    this.w(arr);
+  };
+  this.delay_ms = function(d) {
+    var t = getTime()+d/1000; while(getTime()<t);
+  };
+}
+
+SCD40.prototype.start_periodic_measurement = function() {this.w(C.START_MEASUREMENTS);};
+SCD40.prototype.start_low_power_periodic_measurement = function() {this.w(C.START_LOW_POWER_MEASUREMENTS);};
+SCD40.prototype.get_data_ready_status = function() {return this.getValue(C.IS_DATA_READY) != 0;};
+SCD40.prototype.stop_periodic_measurement = function() {this.w(C.STOP_MEASUREMENTS);};
+SCD40.prototype.measure_single_shot = function(ref) {resp = this.w(C.REQUEST_SINGLE_LOOKUP);};
+SCD40.prototype.measure_single_shot_rht_only = function(ref) {resp = this.w(C.REQUEST_SINGLE_LOOKUP_RHT);};
+SCD40.prototype.read_measurement = function() {
+  resp = this.r(C.READ_DATA,9);
+  a = new DataView(resp.buffer);
+  return {
+     ppm : a.getUint16(0),
+     t : -45+175*(a.getUint16(3))/65535,
+     rh : 100*(a.getUint16(6))/65535
+  };
+};
+
+SCD40.prototype.perform_forced_recalibration = function(target) {
+  this.updateValue(C.PERFORM_FORCED_RECALIBARTION,target);
+  this.delay_ms(400);
+  return this.sr(3).toString();
+};
+SCD40.prototype.get_automatic_self_calibration_enabled = function() {
+  return this.getValue(C.GET_AUTOMATIC_SELF_CALIBRATION_ENABLED);
+};
+SCD40.prototype.set_automatic_self_calibration_enabled = function(enabled) {
+  this.updateValue(C.SET_AUTOMATIC_SELF_CALIBRATION_ENABLED,enabled);
+};
+SCD40.prototype.get_automatic_self_calibration_target = function() {
+  return this.getValue(C.GET_AUTOMATIC_SELF_CALIBRATION_TARGET);
+};
+SCD40.prototype.set_automatic_self_calibration_target = function(target) {
+  this.updateValue(C.SET_AUTOMATIC_SELF_CALIBRATION_TARGET,target);
+};
+
+SCD40.prototype.persist_settings = function() {this.w(C.PERSIST_SETTINGS);};
+SCD40.prototype.get_serial_number = function() {return this.getValue(C.GET_SERIAL_NUMBER,9);};
+
+SCD40.prototype.get_sensor_variant = function() {return this.getValue(C.CHIP_GET_VARIANT);};
+SCD40.prototype.perform_self_test = function() {this.w(C.PERfORM_SELF_TEST);};
+SCD40.prototype.perform_factory_reset = function() {this.w(C.FACTORY_RESET);};
+SCD40.prototype.reinit = function() {this.w(C.RE_INIT);};
+
+SCD40.prototype.get_temperature_offset = function() {
+  return this.getValue(C.GET_TEMP_OFFSET)*(175/(Math.pow(2,16)-1));
+};
+
+SCD40.prototype.set_temperature_offset = function(offset) {
+  newOffset = offset * ((Math.pow(2,16)-1)/175);
+  this.updateValue(C.SET_TEMP_OFFSET,newOffset);
+};
+
+SCD40.prototype.get_sensor_altitude  = function() { return this.getValue(C.GET_SENSOR_ALTITUDE);};
+SCD40.prototype.set_sensor_altitude  = function(val) { return this.updateValue(C.SET_SENSOR_ALTITUDE,val);};
+SCD40.prototype.get_ambient_pressure = function() { return this.getValue(C.GET_AMBIENT_PRESSURE);};
+SCD40.prototype.set_ambient_pressure = function(val) { return this.updateValue(C.SET_AMBIENT_PRESSURE,val);};
+
+SCD40.prototype.power_down = function() {this.w(C.POWER_DOWN);};
+SCD40.prototype.wake_up = function() {this.w(C.WAKE_UP);};
+
+SCD40.prototype.get_automatic_self_calibration_initial_period = function() {
+  return this.getValue(C.GET_CALIBRATION_INITIAL_PERIOD);
+};
+SCD40.prototype.set_automatic_self_calibration_initial_period = function(period) {
+  this.updateValue(C.SET_CALIBRATION_INITIAL_PERIOD,period);
+};
+SCD40.prototype.get_automatic_self_calibration_standard_period = function() {
+  return this.getValue(C.GET_CALIBRATION_STANDARD_PERIOD);
+};
+SCD40.prototype.set_automatic_self_calibration_standard_period = function(period) {
+  this.updateValue(C.SET_CALIBRATION_STANDARD_PERIOD,period);
+};
+
+/* 
+The following code will calculate the crc-8 checksum used for the chip using C code. The JavaScript version above runs in around 4.5 seconds, where as the code below takes around 1.2ms. If performance is an issue this can be used instead.
+
+var crc = E.compiledC(`
+//int gencrc(int)
+
+int gencrc(int val) {
+    unsigned char data[2] = {val>>8,val};
+    int i,j;
+    unsigned char crc = 0xFF;
+    for (i = 0; i < 2; ++i) {
+        crc ^= (data[i]);
+        for (j = 8; j > 0; --j) {
+            if (crc & 0x80)
+                crc = (crc << 1) ^ 0x31;
+            else
+                crc = (crc << 1);
+        }
+    }
+    return crc;
+}
+`);
+
+*/
+
+exports.connectI2C = function(i2c, options) {
+  var a = (options&&options.addr)||C.I2C_ADDR;
+  return (new SCD40(options, function(reg, len) { // read
+    i2c.writeTo(a, reg);
+//    var t = getTime()+1; while(getTime()<t);
+    return i2c.readFrom(a, len);
+  }, function(len) { // simpleread
+    return i2c.readFrom(a, len);
+  }, function(data) { // write
+    i2c.writeTo(a, data);
+  }));
+};
+
+
diff --git a/devices/SCD40.md b/devices/SCD40.md
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+++ b/devices/SCD40.md
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+<!--- Copyright (c) 2014 Hywel Warsop. See the file LICENSE for copying permission. -->
+SCD40
+=====================
+
+* KEYWORDS: Module,SCD40,SCD41,SCD43,Temperature,Humidity,C02
+
+This module should work with Sensirons SDC40, SCD41 and SCD43 sensors, although it has only been tested on the SDC40. Use the [SCD40](/modules/SCD40.js) ([About Modules](/Modules)) module for it.
+
+You can wire this up as follows:
+
+| Device Pin | Espruino |
+| ---------- | -------- |
+| 1 (GND)    | GND      |
+| 2 (VCC)    | 3.3      |
+| 3 (SCL)    | B10       |
+| 4 (SDA)    | B3       |
+
+How to use my module:
+
+To Set up the module
+ 
+```JavaScript
+  I2C2.setup({scl:B10,sda:B3});
+  var scd = require("SCD40").connectI2C(I2C2);
+```
+
+To make a single measurement
+
+```JavaScript
+  scd.measure_single_shot();
+  scd.get_data_ready_status();
+  readings = scd.read_measurement();
+  console.log({readings});
+```
+
+To make a continous measurements
+
+```JavaScript
+  scd.start_periodic_measurement();
+  readings = scd.read_measurement();
+  console.log({readings});
+
+  scd.stop_periodic_measurement();
+```
+
+The other calls as defined in the datasheet have all been set up in the module.
+
+The module also contains a commented out version of the C code to calculate the crc-8 checksum used for these chips. It runs in around 1.2 ms on the pico, where as the Javascript version takes around 4.5 ms. If performance is an issue this can be used instead.
+
+  Reference
+  ---------
+
+  * APPEND_JSDOC: MOD123.js
+
+  Using
+  -----
+
+  * APPEND_USES: MOD123
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