====== I2C Communication with a temperature/humidity sensor : SI7021 ======

----

tags: flipper, arduino, si7021, I2C


**This is an example code of I2C communication between a temperature/humidity sensor and the flipper board.**

The temperature/humidity sensor in the example is a [[https://www.silabs.com/documents/public/data-sheets/Si7021-A20.pdf|SI7021]] .

The sensor is wired on I2C1 communication bus so we will use Wire1 instance (if we were using I2C0 it would have been Wire instance, see [[products:flipper:arduino_bus_references|flipper arduino bus references]] ). The i2C address of the SI7021 is 0x40 (see {{ :products:flipper:si7021-a20.pdf |datasheet}} ).

Following code has been modified from this [[https://github.com/ThomasBarris/Si7021/blob/master/example.ino|source]].

===== - Arduino Code =====

<file cpp flipper_si7021.ino>
/* Si7021 sensor reading <strong>without</strong> library
 *  Datasheet:      https://www.silabs.com/documents/public/data-sheets/Si7021-A20.pdf
 */
 /******************************I2C COMMAND TABLE*****************************
  * Measure Relative Humidity, Hold Master Mode         0xE5
  * Measure Relative Humidity, No Hold Master Mode      0xF5 
  * Measure Temperature, Hold Master Mode               0xE3
  * Measure Temperature, No Hold Master Mode            0xF3
  * Read Temperature Value from Previous RH Measurement 0xE0
  * Reset                                               0xFE  
  * Write RH/T User Register 1                          0xE6
  * Read RH/T User Register 1                           0xE7
  * Write Heater Control Register                       0x51
  * Read Heater Control Register                        0x11
  * Read Electronic ID 1st Byte                         0xFA 0x0F
  * Read Electronic ID 2nd Byte                         0xFC 0xC9
  * Read Firmware Revision                              0x84 0xB8
  * 
  * source datasheet
  */

// This library allows you to communicate with I2C / TWI devices.

# include <Wire.h>

// SI7021 I2C address is 0x40(64)

# define si7021Addr 0x40

// function for reading values into a array of unsigned int data[2]
// from the Si7021 with a specific i2c address si7021Addr=0x40
// with one of the commands listed above. Function body at the end of this sketch
void getSiData(unsigned int *_ret_data, byte _i2c_command);

void setup()
{

 //enabling VDD_SW power rail to give 5V source to the daughter board
 pinMode(SW_VDD_EN, OUTPUT);
 digitalWrite(SW_VDD_EN,HIGH);
 //enabling 5V_sensor power rail to switch on the sensor
 pinMode(EXP_36, OUTPUT);
 digitalWrite(EXP_36,HIGH);
 //enabling 3V3_SW power rail to pull the I2C bus up via the 10K resistors
 pinMode(SW_3V3_EN, OUTPUT);
 digitalWrite(SW_3V3_EN,HIGH);
 //wait for the power sources to be turned on
 delay(1000);
  // Initiate Wire1 and join the I2C1 bus
  Wire1.begin();
  Serial.begin(115200);
  //reset sensor by sending 0xFE command to the Si7021 address
  Wire1.beginTransmission(si7021Addr);
  Wire1.write(0xFE); // Write reset command
  Wire1.endTransmission();
  delay(15); // Default = 15ms

}

void loop(){

  //sensor returns 2 bytes via I2C. It will be converted to temperature or humidity later
  unsigned int data[2];
  //Send humidity measurement command and get response into the array 'data'
  getSiData(data, 0xE5);
  // Convert the data
  float humidity  = ((data[0] * 256.0) + data[1]);
  humidity = ((125 * humidity) / 65536.0) - 6;
  // Send temperature measurement command
  getSiData(data, 0xE3);
  /* re-use temperature already measured by humidity measurement some milliseconds before command -> E0
    *getSiData(data, 0xE0); 
    *
    *However, does not report valid results
    *
    *Datasheet: 
    *Each time a relative humidity measurement is made a temperature measurement is also made for the purposes of
    *temperature  compensation  of  the  relative  humidity  measurement.  If  the  temperature  value  is  required,  
    *it  can  be read  using  command  0xE0;  this  avoids  having  to  perform  a  second  temperature  measurement.  
    *The  measure temperature  commands 0xE3  and  0xF3  will  perform  a  temperature measurement  and  return  the  
    *measurement value, command 0xE0 does not perform a measurement but returns the temperature value measured during the
    *relative humidity measurement. 
  */ 
  // Convert the data
  float temp  = ((data[0] * 256.0) + data[1]);
  float celsTemp = ((175.72 * temp) / 65536.0) - 46.85;
  float fahrTemp = celsTemp * 1.8 + 32;
  // Output data to serial monitor
  Serial.print("Humidity : ");
  Serial.print(humidity);
  Serial.println(" % RH");
  Serial.print("Celsius : ");
  Serial.print(celsTemp);
  Serial.println(" C");
  delay(10000);
}



void getSiData(unsigned int *_ret_data, byte _i2c_command)
{

  // start i2c communication 
  Wire1.beginTransmission(si7021Addr);
  //send i2c command to sensor
  Wire1.write(_i2c_command);
  // we are done with our transmission...close i2c communication
  Wire1.endTransmission();
  delay(85);
  // Request 2 bytes of data
  Wire1.requestFrom(si7021Addr, 2);
  // Read 2 bytes of data and save it to _ret_data which points to 'data[2]'
  if(Wire1.available() == 2)
  {
    _ret_data[0] = Wire1.read();
    _ret_data[1] = Wire1.read();
  }
</file>