HTU21D(F) I2C

Github Repo	C Header	C source	JS source
mongoose-os-libs/htu21df-i2c	mgos_htu21df.h

A Mongoose library for Measurement Specialities HTU21D(F) integrated circuit.

Sensor details

The HTU21D(F) is a new digital humidity sensor with temperature output by MEAS. Setting new standards in terms of size and intelligence, it is embedded in a reflow solderable Dual Flat No leads (DFN) package with a small 3 x 3 x 0.9 mm footprint. This sensor provides calibrated, linearized signals in digital, I2C format.

HTU21D(F) digital humidity sensors are dedicated humidity and temperature plug and play transducers for OEM applications where reliable and accurate measurements are needed. Direct interface with a micro-controller is made possible with the module for humidity and temperature digital outputs. These low power sensors are designed for high volume and cost sensitive applications with tight space constraints.

Every sensor is individually calibrated and tested. Lot identification is printed on the sensor and an electronic identification code is stored on the chip – which can be read out by command. Low battery can be detected and a checksum improves communication reliability. The resolution of these digital humidity sensors can be changed by command (8/12bit up to 12/14bit for RH/T).

With MEAS’ improvements and miniaturization of this sensor, the performance-to-price ratio has been improved – and eventually, any device should benefit from its cutting edge energy saving operation mode. Optional PTFE filter/membrane (F) protects HTU21D digital humidity sensors against dust and water immersion, as well as against contamination by particles. PTFE filter/membranes preserve a high response time. The white PTFE filter/membrane is directly stuck on the sensor housing.

See datasheet for implementation details.

A great place to buy a ready made and tested unit is at Adafruit.

Example application

An example program using a timer to read data from the sensor every 5 seconds:

#include "mgos.h"
#include "mgos_i2c.h"
#include "mgos_htu21df.h"

static struct mgos_htu21df *s_htu21df;

static void timer_cb(void *user_data) {
  float temperature, humidity;

  temperature=mgos_htu21df_getTemperature(s_htu21df);
  humidity=mgos_htu21df_getHumidity(s_htu21df);

  LOG(LL_INFO, ("htu21df temperature=%.2f humidity=%.2f", temperature, humidity));

  (void) user_data;
}

enum mgos_app_init_result mgos_app_init(void) {
  struct mgos_i2c *i2c;

  i2c=mgos_i2c_get_global();
  if (!i2c) {
    LOG(LL_ERROR, ("I2C bus missing, set i2c.enable=true in mos.yml"));
  } else {
    s_htu21df=mgos_htu21df_create(i2c, 0x40); // Default I2C address
    if (s_htu21df) {
      mgos_set_timer(5000, true, timer_cb, NULL);
    } else {
      LOG(LL_ERROR, ("Could not initialize sensor"));
    }
  }
  return MGOS_APP_INIT_SUCCESS;
}

Disclaimer

This project is not an official Google project. It is not supported by Google and Google specifically disclaims all warranties as to its quality, merchantability, or fitness for a particular purpose.

_read

uint32_t read;                 // calls to _read()
  uint32_t read_success;         // successful _read()
  uint32_t read_success_cached;  // calls to _read() which were cached
  // Note: read_errors := read - read_success - read_success_cached
  double   read_success_usecs;   // time spent in successful uncached _read()
};

value of mg_time() upon last call to _read()

mgos_htu21df_create

struct mgos_htu21df *mgos_htu21df_create(struct mgos_i2c *i2c, uint8_t i2caddr);

Initialize a HTU21DF on the I2C bus i2c at address specified in i2caddr parameter (default HTU21DF is on address 0x40). The sensor will be polled for validity, upon success a new struct mgos_htu21df is allocated and returned. If the device could not be found, NULL is returned.

mgos_htu21df_destroy

void mgos_htu21df_destroy(struct mgos_htu21df **sensor);

Destroy the data structure associated with a HTU21DF device. The reference to the pointer of the struct mgos_htu21df has to be provided, and upon successful destruction, its associated memory will be freed and the pointer set to NULL.

mgos_htu21df_read

bool mgos_htu21df_read(struct mgos_htu21df *sensor);

The sensor will be polled for its temperature and humidity data. If the poll has occured in the last MGOS_HTU21DF_READ_DELAY seconds, the cached data is used (so as not to repeatedly poll the bus upon subsequent calls).

mgos_htu21df_getTemperature

float mgos_htu21df_getTemperature(struct mgos_htu21df *sensor);

The sensor will be polled for its temperature and humidity data. If the poll has occured in the last MGOS_HTU21DF_READ_DELAY seconds, the cached data is used (so as not to repeatedly poll the bus upon subsequent calls).

The return value is the temperature of the sensor in Celsius, or NAN if no data was found.

mgos_htu21df_getHumidity

float mgos_htu21df_getHumidity(struct mgos_htu21df *sensor);

The sensor will be polled for its temperature and humidity data. If the poll has occured in the last MGOS_HTU21DF_READ_DELAY seconds, the cached data is used (so as not to repeatedly poll the bus upon subsequent calls).

The return value is the humidity of the sensor in percent relative humidity, or NAN if no data was found.

mgos_htu21df_getStats

bool mgos_htu21df_getStats(struct mgos_htu21df *sensor, struct mgos_htu21df_stats *stats);

Returns the running statistics on the sensor interaction, the user provides a pointer to a struct mgos_htu21df_stats object, which is filled in by this call.

Upon success, true is returned. Otherwise, false is returned, in which case the contents of stats is undetermined.

mgos_htu21df_i2c_init

bool mgos_htu21df_i2c_init(void);

Initialization function for MGOS -- currently a noop.

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