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SI7021 library for STM32 HAL

Just essentials, nothing else! :)

$ arm-none-eabi-gcc -c -mcpu=cortex-m0plus -mthumb <other options stripped> si7021.o
$ size si7021.o
   text    data     bss     dec     hex filename
    450       0       0     450     1c2 si7021.o

That's only 450b :)

Quickstart

Copy / add as submodule files under Src directory to your STM32 HAL project, then:

  1. Include header:
/* Private includes ----------------------------------------------------------*/
/* USER CODE BEGIN Includes */
#include "si7021.h"
// ...
/* USER CODE END Includes */
  1. Get your measurements! :)
  // Somewhere in your main():

  // Start humidity measurement
  uint32_t humidity = si7021_measure_humidity(&hi2c1);
  if (humidity == SI7021_MEASURE_FAILED) {
    // Error happened :(
  }

  // As a side effect si7021 measures temperate together with humidity
  // so you can just read it without additional measurement
  int32_t tempC = si7021_read_previous_temperature(&hi2c1);
  if (humidity == SI7021_MEASURE_FAILED) {
    // Error happened :(
  }

API

Humidity measurement

  • uint32_t si7021_measure_humidity(I2C_HandleTypeDef *hi2c) Starts humidity measurement. This blocking call takes around 20ms to complete. As a side effect temperature will be measure as well. Returns current humidity, e.g. 55 In case of error returns SI7021_MEASURE_FAILED

Temperature

  • int32_t si7021_measure_temperature(I2C_HandleTypeDef *hi2c) Starts temperature measurement. This blocking call takes ~20ms to complete. Returns current temperature in Celsius multiplied by 100 (to avoid float integers), e.g. +23.12C -> 2312 In case of error returns SI7021_MEASURE_FAILED

  • int32_t si7021_read_previous_temperature(I2C_HandleTypeDef *hi2c) Reads previous temperature measurement done by prior call of si7021_measure_humidity() Returns current temperature in Celsius multiplied by 100 (to avoid float integers) e.g. +23.12C -> 2312 In case of error returns SI7021_MEASURE_FAILED

Configuration

  • uint64_t si7021_read_id(I2C_HandleTypeDef *hi2c) The si7021 chip provides a serial number individualized for each device. This function reads this number. Returns chip id or 0 in case of error.

  • uint32_t si7021_set_config(I2C_HandleTypeDef *hi2c, uint8_t heater, uint8_t resolution) Turns on/off build-in heater / changes measurement resolution.

    • heater - build in heater mode. One of SI7021_HEATER_ON or SI7021_HEATER_OFF
    • resolution - measurement resolution. Possible values:
      • SI7021_RESOLUTION_RH12_TEMP14 - 12bit humidity, 14bit temperature.
      • SI7021_RESOLUTION_RH8_TEMP12
      • SI7021_RESOLUTION_RH10_TEMP13
      • SI7021_RESOLUTION_RH11_TEMP11 Returns 0 when new configuration successfully applied, otherwise - error.

  • uint32_t si7021_set_heater_power(I2C_HandleTypeDef *hi2c, uint8_t power) Sets build in heater power current. Possible values:

    • SI7021_HEATER_POWER_3MA
    • SI7021_HEATER_POWER_9MA
    • SI7021_HEATER_POWER_15MA
    • SI7021_HEATER_POWER_27MA
    • SI7021_HEATER_POWER_51MA
    • SI7021_HEATER_POWER_94MA