SHT31.cpp

//
//    FILE: SHT31.cpp
//  AUTHOR: Rob Tillaart
// VERSION: 0.5.0
//    DATE: 2019-02-08
// PURPOSE: Arduino library for the SHT31 temperature and humidity sensor
//          https://www.adafruit.com/product/2857
//     URL: https://github.com/RobTillaart/SHT31


#include "SHT31.h"


//  SUPPORTED COMMANDS - single shot mode only
#define SHT31_READ_STATUS       0xF32D
#define SHT31_CLEAR_STATUS      0x3041

#define SHT31_SOFT_RESET        0x30A2
#define SHT31_HARD_RESET        0x0006

#define SHT31_MEASUREMENT_FAST  0x2416     //  page 10 datasheet
#define SHT31_MEASUREMENT_SLOW  0x2400     //  no clock stretching

#define SHT31_HEAT_ON           0x306D
#define SHT31_HEAT_OFF          0x3066
#define SHT31_HEATER_TIMEOUT    180000UL   //  milliseconds


SHT31::SHT31(uint8_t address, TwoWire *wire)
{
  _wire           = wire;
  _address        = address;
  _lastRead       = 0;
  _rawTemperature = 0;
  _rawHumidity    = 0;
  _heatTimeout    = 0;
  _heaterStart    = 0;
  _heaterStop     = 0;
  _heaterOn       = false;
  _error          = SHT31_OK;
}


bool SHT31::begin()
{
  if ((_address != 0x44) && (_address != 0x45))
  {
    return false;
  }
  return reset();
}


bool SHT31::isConnected()
{
  _wire->beginTransmission(_address);
  int rv = _wire->endTransmission();
  if (rv != 0) _error = SHT31_ERR_NOT_CONNECT;
  return (rv == 0);
}


uint8_t SHT31::getAddress()
{
  return _address;
}


bool SHT31::read(bool fast)
{
  if (writeCmd(fast ? SHT31_MEASUREMENT_FAST : SHT31_MEASUREMENT_SLOW) == false)
  {
    return false;
  }
  delay(fast ? 4 : 15);   //  table 4 datasheet
  return readData(fast);
}


#ifdef doc
//  bit - description
//  ==================
//  15   Alert pending status
//        '0': no pending alerts
//        '1': at least one pending alert - default
//  14  Reserved ‘0’
//  13  Heater status
//        '0’ : Heater OFF - default
//        '1’ : Heater ON
//  12  Reserved '0’
//  11  Humidity tracking alert
//        '0’ : no alert - default
//        '1’ : alert
//  10  Temp tracking alert
//        '0’ : no alert - default
//        '1’ : alert
//  9:5 Reserved '00000’
//  4   System reset detected
//        '0': no reset since last ‘clear status register’ command
//        '1': reset detected (hard or soft reset command or supply fail) - default
//  3:2 Reserved ‘00’
//  1   Command status
//        '0': last command executed successfully
//        '1': last command not processed. Invalid or failed checksum
//  0   Write data checksum status
//        '0': checksum of last write correct
//        '1': checksum of last write transfer failed
#endif


uint16_t SHT31::readStatus()
{
  uint8_t status[3] = { 0, 0, 0 };
  //  page 13 datasheet
  if (writeCmd(SHT31_READ_STATUS) == false)
  {
    return 0xFFFF;
  }
  //  16 bit status + CRC
  if (readBytes(3, (uint8_t*) &status[0]) == false)
  {
    return 0xFFFF;
  }

  if (status[2] != crc8(status, 2))
  {
    _error = SHT31_ERR_CRC_STATUS;
    return 0xFFFF;
  }

  return (uint16_t) (status[0] << 8) + status[1];
}


bool SHT31::reset(bool hard)
{
  bool b = writeCmd(hard ? SHT31_HARD_RESET : SHT31_SOFT_RESET);
  if (b == false)
  {
    return false;
  }
  delay(1);   //  table 4 datasheet
  return true;
}


void SHT31::setHeatTimeout(uint8_t seconds)
{
  _heatTimeout = seconds;
  if (_heatTimeout > 180) _heatTimeout = 180;
}


bool SHT31::heatOn()
{
  if (isHeaterOn()) return true;
  if ((_heaterStop > 0) && (millis() - _heaterStop < SHT31_HEATER_TIMEOUT))
  {
    _error = SHT31_ERR_HEATER_COOLDOWN;
    return false;
  }
  if (writeCmd(SHT31_HEAT_ON) == false)
  {
    _error = SHT31_ERR_HEATER_ON;
    return false;
  }
  _heaterStart = millis();
  _heaterOn    = true;
  return true;
}


bool SHT31::heatOff()
{
  //  always switch off the heater - ignore _heaterOn flag.
  if (writeCmd(SHT31_HEAT_OFF) == false)
  {
    _error = SHT31_ERR_HEATER_OFF;  // can be serious!
    return false;
  }
  _heaterStop = millis();
  _heaterOn   = false;
  return true;
}


bool SHT31::isHeaterOn()
{
  if (_heaterOn == false)
  {
    return false;
  }
  //  did not exceed time out
  if (millis() - _heaterStart < (_heatTimeout * 1000UL))
  {
    return true;
  }
  heatOff();
  return false;
}


bool SHT31::requestData()
{
  if (writeCmd(SHT31_MEASUREMENT_SLOW) == false)
  {
    return false;
  }
  _lastRequest = millis();
  return true;
}


bool SHT31::dataReady()
{
  return ((millis() - _lastRequest) > 15);  //  TODO MAGIC NR
}


bool SHT31::readData(bool fast)
{
  uint8_t buffer[6];
  if (readBytes(6, (uint8_t*) &buffer[0]) == false)
  {
    return false;
  }

  if (!fast)
  {
    if (buffer[2] != crc8(buffer, 2))
    {
      _error = SHT31_ERR_CRC_TEMP;
      return false;
    }
    if (buffer[5] != crc8(buffer + 3, 2))
    {
      _error = SHT31_ERR_CRC_HUM;
      return false;
    }
  }

  _rawTemperature = (buffer[0] << 8) + buffer[1];
  _rawHumidity    = (buffer[3] << 8) + buffer[4];

  _lastRead = millis();

  return true;
}


int SHT31::getError()
{
  int rv = _error;
  _error = SHT31_OK;
  return rv;
}


//////////////////////////////////////////////////////////

uint8_t SHT31::crc8(const uint8_t *data, uint8_t len)
{
  //  CRC-8 formula from page 14 of SHT spec pdf
  const uint8_t POLY(0x31);
  uint8_t crc(0xFF);

  for (uint8_t j = len; j; --j)
  {
    crc ^= *data++;

    for (uint8_t i = 8; i; --i)
    {
      crc = (crc & 0x80) ? (crc << 1) ^ POLY : (crc << 1);
    }
  }
  return crc;
}


bool SHT31::writeCmd(uint16_t cmd)
{
  _wire->beginTransmission(_address);
  _wire->write(cmd >> 8 );
  _wire->write(cmd & 0xFF);
  if (_wire->endTransmission() != 0)
  {
    _error = SHT31_ERR_WRITECMD;
    return false;
  }
  return true;
}


bool SHT31::readBytes(uint8_t n, uint8_t *val)
{
  int rv = _wire->requestFrom(_address, (uint8_t) n);
  if (rv == n)
  {
    for (uint8_t i = 0; i < n; i++)
    {
      val[i] = _wire->read();
    }
    return true;
  }
  _error = SHT31_ERR_READBYTES;
  return false;
}


//  -- END OF FILE --