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181 lines
4.6 KiB
Markdown
181 lines
4.6 KiB
Markdown
# Introduction
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This repository guides you to implement and use the HTU21D temperature and humidity sensor by using I2C communication. The repository contains the following files:
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- `htu21d.h` library header files
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- `htu21d.c` implemenation methods file
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- `main.c` an example file to test I2C communication using the library `htu21d`
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- `example` output file precompiled in a raspberry pi zero that returns temperature and humidity
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# Setup the raspberry
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## Enable I2C on the Raspberry Pi:
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- Run sudo raspi-config.
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- Navigate to Interfacing Options → I2C and enable it.
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- Reboot the Pi.
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## Install I2C Tools and Development Libraries:
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```
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sudo apt-get update
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sudo apt-get install i2c-tools libi2c-dev
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```
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## Check if the sensor is detected:
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```
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sudo i2cdetect -y 1
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```
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This command should show an address for the HTU21D, typically 0x40.
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```
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0 1 2 3 4 5 6 7 8 9 a b c d e f
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00: -- -- -- -- -- -- -- --
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10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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40: 40 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
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70: -- -- -- -- -- -- -- --
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```
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# My own header files for HTU21D sensor
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This is a detailed guide to configure and create your own files to comunicate with each sensor used by the air-quality sensor.
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## Header
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Create the Header File `htu21d.h` with Header Guard and Includes:
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```c
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#ifndef HTU21D_H
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#define HTU21D_H
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// I2C Address
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#define HTU21D_I2C_ADDR 0x40
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// Commands
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#define HTU21D_TEMP 0xE3
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#define HTU21D_HUMID 0xE5
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#define HTU21D_RESET 0xFE
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// Function declarations:
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// Temp
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int getTemperature(int fd, double *temperature);
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// Humidity
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int getHumidity(int fd, double *humidity);
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#endif // HTU21D_H
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```
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## Implement the Sensor Communication `htu21d.c`
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```c
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#include <unistd.h> //to send commands to and receive from I2C device
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#include <sys/ioctl.h>//setting up and controlling the I2C device settings
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#include <linux/i2c-dev.h>//definitions for system calls and structures specific to I2C
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#include <i2c/smbus.h>//SMBus commands in a more standardized way for I2C
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#include <stdio.h>//perror
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#include "htu21d.h" // my own header file
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// Reset function:
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int reset(int fd)
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{
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if(0 > ioctl(fd, I2C_SLAVE, HTU21D_I2C_ADDR))
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{
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perror("Failed to open the bus");
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return -1;
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}
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i2c_smbus_write_byte(fd, HTU21D_RESET);
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return 0;
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}
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// Get temperature:
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int getTemperature(int fd, double *temperature)
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{
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reset(fd);
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char buf[3];
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__s32 res = i2c_smbus_read_i2c_block_data(fd, HTU21D_TEMP,3,buf);
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if(res<0)
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{
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perror("Failed to read from the device");
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return -1;
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}
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*temperature = -46.85 + 175.72 * (buf[0]*256 + buf[1]) / 65536.0;
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return 0;
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}
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// Get humidity:
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int getHumidity(int fd, double *humidity)
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{
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reset(fd);
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char buf[3];
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__s32 res = i2c_smbus_read_i2c_block_data(fd, HTU21D_HUMID, 3, buf);
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if(res<0)
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{
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perror("Failed to read from the device");
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return -1;
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}
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*humidity = -6 + 125 * (buf[0]*256 + buf[1]) / 65536.0;
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return 0;
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}
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```
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### Using the library
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```c
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#include <stdio.h>
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#include <errno.h>
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#include <stdlib.h>
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#include <string.h>
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#include <fcntl.h>
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#include "htu21d.h"
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int main()
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{
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char filename[20];
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snprintf(filename, 19, "/dev/i2c-%d", 1);
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int fd = open(filename, O_RDWR);
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if (0 > fd)
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{
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fprintf(stderr, "ERROR: Unable to access HTU21D sensor module: %s\n", strerror (errno));
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exit(-1);
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}
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// Retrieve temperature and humidity
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double temperature = 0;
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double humidity = 0;
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if ( (0 > getHumidity(fd, &humidity)) || (0 > getTemperature(fd, &temperature)) )
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{
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fprintf(stderr, "ERROR: HTU21D sensor module not found\n");
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exit(-1);
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}
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// Print temperature and humidity on the screen
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printf("HTU21D Sensor Module\n");
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printf("%5.2fC\n", temperature);
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printf("%5.2f%%rh\n", humidity);
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return 0;
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}
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```
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# Compiling and testing
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Then to properly compile whitout a make file:
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```sh
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gcc -o example main.c htu21d.c -li2c
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```
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or
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```sh
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gcc -o example main.c htu21d.c -I. -li2c
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```
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### Wiring htu21d to Rasp-zero
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Htu -> Rasp-Zero
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VIN -> GPIO 1
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GND -> GPIO 9 or (6)
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SCL -> GPIO 5
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SDA -> GPIO 3
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