I'm playing with the temperature/humidity sensor HTU21D. Datasheet: https://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7FHPC199_6%7FA6%7Fpdf%7FEnglish%7FENG_DS_HPC199_6_A6.pdf%7FCAT-HSC0004

The sensor is properly wired and yields reasonable values if I use the library and example here: https://github.com/sparkfun/SparkFun_HTU21D_Breakout_Arduino_Library/blob/master/examples/SparkFun_HTU21D_Demo/SparkFun_HTU21D_Demo.ino

But I'm more interested in how I2C works, so I'm trying to write a small sketch from scratch to communicate with the sensor. Here is what I have right now:

#include <Wire.h>

#define SensorAddress 0x40
#define SoftReset 0xFE
#define TriggerTemperatureNoHold 0xF3
#define TriggerHumidityNoHold 0xF5

void setup(){

    Serial.println("HTU21D library");



    Serial.println("init finished");


void loop(){


    //Hang out while measurement is taken. datasheet says 50ms, practice may call for more
    bool validResult;
    byte counter;
    for (counter = 0, validResult = 0 ; counter < 10 && !validResult ; counter++){

        //Comes back in three bytes, data(MSB) / data(LSB) / Checksum
        validResult = (3 == Wire.requestFrom(0x40, 3));


I'm in the middle of debugging the sketch. When the sketch is uploaded, I'm reading on the serial connection, and all I can see is:

HTU21D library
init finished

It seems the program hangs on the first communication attempt, when I send the TriggerTemperatureNoHold command.

Could you give me a hand please? I don't understand why it's hanging there.

  • 2
    You forgot Wire.begin()...
    – Majenko
    Aug 14 '19 at 10:22
  • ah...Indeed....thanks
    – JPFrancoia
    Aug 14 '19 at 10:28
  • Post an answer, so I can accept it. Even if my question was slightly dumb...
    – JPFrancoia
    Aug 14 '19 at 11:04

You forgot Wire.begin();. You should add it in setup().

Wire.begin(); will configure the IO pins for I2C usage. Without that the signals don't get in or out of the MCU, so I2C functions that wait for an ACK signal never see it and don't complete.

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