user66056: I am interfacing the HTU21D sensor with my nodeMCU v3. All good! I managed to get some readings etc. But when I re-upload my code the device does not respond. If I remove the power supply from the pin and hard reset it, it works again. I have exactly the same problem using the Sparkfun library, so it seems more of a hardware problem. Anyone else had the same problem?
link to the sparkfun library
https://github.com/sparkfun/SparkFun_HTU21D_Breakout_Arduino_Library
for reference, here is my code
   This program interfaces the HTU21 temp/humidity sensor via I2C
*/

#include <Wire.h>
#include <Math.h>

const int HTU21_ADDR = 0x40;
const int Trigger_Temp_Measurement = 0xE3;// hold master
const int Trigger_Hum_Measurement = 0xE5; // hold master
const int Trigger_Temp = 0xF3; //No Hold Master
const int Trigger_Hum = 0xF5; //No Hold Master

int16_t Temp, Hum;//variables to store temp and humidity
int8_t TmpCrc, HumCrC;

void setup() {// put your setup code here, to run once:
  Serial.begin(9600);
  Wire.begin();
  Wire.beginTransmission(HTU21_ADDR);
  Wire.write(0xFE);
  Wire.endTransmission(true);

}

void loop() {// put your main code here, to run repeatedly:
  TempReading();
  HumReading();

}

void TempReading() {

  Wire.beginTransmission(HTU21_ADDR);
  Wire.write(Trigger_Temp_Measurement);
  Wire.endTransmission();

  Wire.requestFrom(HTU21_ADDR, 3, true);
  if (Wire.available() <= 3) {
    Temp = (Wire.read() << 8) | (Wire.read() );
    TmpCrc = Wire.read();
  }
  float TempCalc = -46.85 + (175.72 * ((float)Temp / (pow(2, 16))));
  Serial.print("Temperature \t");
  Serial.print(TempCalc);
  //Serial.println(TmpCrc, HEX);

}

void HumReading() {
  Wire.beginTransmission(HTU21_ADDR);
  Wire.write(Trigger_Hum_Measurement);
  Wire.endTransmission();

  Wire.requestFrom(HTU21_ADDR, 3, true);
  if (Wire.available() <= 3) {
    Hum = (Wire.read() << 8) | (Wire.read());
    HumCrC = Wire.read();
  }

  float HumCalc = -6 + 125 * (Hum / (pow(2, 16)));
  Serial.print("\t Humidity \t");
  Serial.println(HumCalc);
  //Serial.println(HumCrC, HEX);


}```




user66056: Yes the setup runs completely. I found out that the bus hangs if I terminate communication by uploading new code to the board. I found a piece of code that releases the bus which works. See below:
#if defined(TWCR) && defined(TWEN)
  TWCR &= ~(_BV(TWEN)); //Disable the Atmel 2-Wire interface so we can control the SDA and SCL pins directly
#endif

  pinMode(SDA, INPUT_PULLUP); // Make SDA (data) and SCL (clock) pins Inputs with pullup.
  pinMode(SCL, INPUT_PULLUP);


  boolean SCL_LOW = (digitalRead(SCL) == LOW); // Check is SCL is Low.
  if (SCL_LOW) { //If it is held low Arduno cannot become the I2C master.
    return 1; //I2C bus error. Could not clear SCL clock line held low
  }

  boolean SDA_LOW = (digitalRead(SDA) == LOW);  // vi. Check SDA input.
  int clockCount = 20; // > 2x9 clock

  while (SDA_LOW && (clockCount > 0)) { //  vii. If SDA is Low,
    clockCount--;
    // Note: I2C bus is open collector so do NOT drive SCL or SDA high.
    pinMode(SCL, INPUT); // release SCL pullup so that when made output it will be LOW
    pinMode(SCL, OUTPUT); // then clock SCL Low
    delayMicroseconds(10); //  for >5uS
    pinMode(SCL, INPUT); // release SCL LOW
    pinMode(SCL, INPUT_PULLUP); // turn on pullup resistors again
    // do not force high as slave may be holding it low for clock stretching.
    delayMicroseconds(10); //  for >5uS
    // The >5uS is so that even the slowest I2C devices are handled.
    SCL_LOW = (digitalRead(SCL) == LOW); // Check if SCL is Low.
    int counter = 20;
    while (SCL_LOW && (counter > 0)) {  //  loop waiting for SCL to become High only wait 2sec.
      counter--;
      delay(100);
      SCL_LOW = (digitalRead(SCL) == LOW);
    }
    if (SCL_LOW) { // still low after 2 sec error
      return 2; // I2C bus error. Could not clear. SCL clock line held low by slave clock stretch for >2sec
    }
    SDA_LOW = (digitalRead(SDA) == LOW); //   and check SDA input again and loop
  }
  if (SDA_LOW) { // still low
    return 3; // I2C bus error. Could not clear. SDA data line held low
  }

  // else pull SDA line low for Start or Repeated Start
  pinMode(SDA, INPUT); // remove pullup.
  pinMode(SDA, OUTPUT);  // and then make it LOW i.e. send an I2C Start or Repeated start control.
  // When there is only one I2C master a Start or Repeat Start has the same function as a Stop and clears the bus.
  /// A Repeat Start is a Start occurring after a Start with no intervening Stop.
  delayMicroseconds(10); // wait >5uS
  pinMode(SDA, INPUT); // remove output low
  pinMode(SDA, INPUT_PULLUP); // and make SDA high i.e. send I2C STOP control.
  delayMicroseconds(10); // x. wait >5uS
  pinMode(SDA, INPUT); // and reset pins as tri-state inputs which is the default state on reset
  pinMode(SCL, INPUT);
  return 0; // all ok
}

Thanks for pointing me in the right direction!

66056: <p>I am interfacing the HTU21D sensor with my nodeMCU v3. All good! I managed to get some readings etc. But when I re-upload my code the device does not respond. If I remove the power supply from the pin and hard reset it, it works again. I have exactly the same problem using the Sparkfun library, so it seems more of a hardware problem. Anyone else had the same problem?
link to the sparkfun library
<a href="https://github.com/sparkfun/SparkFun_HTU21D_Breakout_Arduino_Library" rel="nofollow noreferrer">https://github.com/sparkfun/SparkFun_HTU21D_Breakout_Arduino_Library</a>
for reference, here is my code</p>

<pre><code>   This program interfaces the HTU21 temp/humidity sensor via I2C
*/

#include &lt;Wire.h&gt;
#include &lt;Math.h&gt;

const int HTU21_ADDR = 0x40;
const int Trigger_Temp_Measurement = 0xE3;// hold master
const int Trigger_Hum_Measurement = 0xE5; // hold master
const int Trigger_Temp = 0xF3; //No Hold Master
const int Trigger_Hum = 0xF5; //No Hold Master

int16_t Temp, Hum;//variables to store temp and humidity
int8_t TmpCrc, HumCrC;

void setup() {// put your setup code here, to run once:
  Serial.begin(9600);
  Wire.begin();
  Wire.beginTransmission(HTU21_ADDR);
  Wire.write(0xFE);
  Wire.endTransmission(true);

}

void loop() {// put your main code here, to run repeatedly:
  TempReading();
  HumReading();

}

void TempReading() {

  Wire.beginTransmission(HTU21_ADDR);
  Wire.write(Trigger_Temp_Measurement);
  Wire.endTransmission();

  Wire.requestFrom(HTU21_ADDR, 3, true);
  if (Wire.available() &lt;= 3) {
    Temp = (Wire.read() &lt;&lt; 8) | (Wire.read() );
    TmpCrc = Wire.read();
  }
  float TempCalc = -46.85 + (175.72 * ((float)Temp / (pow(2, 16))));
  Serial.print("Temperature \t");
  Serial.print(TempCalc);
  //Serial.println(TmpCrc, HEX);

}

void HumReading() {
  Wire.beginTransmission(HTU21_ADDR);
  Wire.write(Trigger_Hum_Measurement);
  Wire.endTransmission();

  Wire.requestFrom(HTU21_ADDR, 3, true);
  if (Wire.available() &lt;= 3) {
    Hum = (Wire.read() &lt;&lt; 8) | (Wire.read());
    HumCrC = Wire.read();
  }

  float HumCalc = -6 + 125 * (Hum / (pow(2, 16)));
  Serial.print("\t Humidity \t");
  Serial.println(HumCalc);
  //Serial.println(HumCrC, HEX);


}```


</code></pre>
 66056: <p>Yes the setup runs completely. I found out that the bus hangs if I terminate communication by uploading new code to the board. I found a piece of code that releases the bus which works. See below:</p>

<pre><code>#if defined(TWCR) &amp;&amp; defined(TWEN)
  TWCR &amp;= ~(_BV(TWEN)); //Disable the Atmel 2-Wire interface so we can control the SDA and SCL pins directly
#endif

  pinMode(SDA, INPUT_PULLUP); // Make SDA (data) and SCL (clock) pins Inputs with pullup.
  pinMode(SCL, INPUT_PULLUP);


  boolean SCL_LOW = (digitalRead(SCL) == LOW); // Check is SCL is Low.
  if (SCL_LOW) { //If it is held low Arduno cannot become the I2C master.
    return 1; //I2C bus error. Could not clear SCL clock line held low
  }

  boolean SDA_LOW = (digitalRead(SDA) == LOW);  // vi. Check SDA input.
  int clockCount = 20; // &gt; 2x9 clock

  while (SDA_LOW &amp;&amp; (clockCount &gt; 0)) { //  vii. If SDA is Low,
    clockCount--;
    // Note: I2C bus is open collector so do NOT drive SCL or SDA high.
    pinMode(SCL, INPUT); // release SCL pullup so that when made output it will be LOW
    pinMode(SCL, OUTPUT); // then clock SCL Low
    delayMicroseconds(10); //  for &gt;5uS
    pinMode(SCL, INPUT); // release SCL LOW
    pinMode(SCL, INPUT_PULLUP); // turn on pullup resistors again
    // do not force high as slave may be holding it low for clock stretching.
    delayMicroseconds(10); //  for &gt;5uS
    // The &gt;5uS is so that even the slowest I2C devices are handled.
    SCL_LOW = (digitalRead(SCL) == LOW); // Check if SCL is Low.
    int counter = 20;
    while (SCL_LOW &amp;&amp; (counter &gt; 0)) {  //  loop waiting for SCL to become High only wait 2sec.
      counter--;
      delay(100);
      SCL_LOW = (digitalRead(SCL) == LOW);
    }
    if (SCL_LOW) { // still low after 2 sec error
      return 2; // I2C bus error. Could not clear. SCL clock line held low by slave clock stretch for &gt;2sec
    }
    SDA_LOW = (digitalRead(SDA) == LOW); //   and check SDA input again and loop
  }
  if (SDA_LOW) { // still low
    return 3; // I2C bus error. Could not clear. SDA data line held low
  }

  // else pull SDA line low for Start or Repeated Start
  pinMode(SDA, INPUT); // remove pullup.
  pinMode(SDA, OUTPUT);  // and then make it LOW i.e. send an I2C Start or Repeated start control.
  // When there is only one I2C master a Start or Repeat Start has the same function as a Stop and clears the bus.
  /// A Repeat Start is a Start occurring after a Start with no intervening Stop.
  delayMicroseconds(10); // wait &gt;5uS
  pinMode(SDA, INPUT); // remove output low
  pinMode(SDA, INPUT_PULLUP); // and make SDA high i.e. send I2C STOP control.
  delayMicroseconds(10); // x. wait &gt;5uS
  pinMode(SDA, INPUT); // and reset pins as tri-state inputs which is the default state on reset
  pinMode(SCL, INPUT);
  return 0; // all ok
}
</code></pre>

<p>Thanks for pointing me in the right direction!</p>
