STM32 HAL Implementation of DHT11 Temperature and Humidity Collection

To interface a DHT11 temperature and humidity sensor with an STM32 using HAL (Hardware Abstraction Layer), you need to implement 1-wire timing using a GPIO pin and precise microsecond delays.

Overview

MCU: STM32 (any series, e.g., STM32F103 or STM32F407)
Sensor: DHT11
Interface: Single-wire digital (requires bidirectional GPIO)
Communication: Start signal from MCU, response and data (40 bits) from DHT11
Development Tool: STM32CubeMX + Keil / STM32CubeIDE

Step-by-Step Implementation
1. Hardware Connection

2. Microsecond Delay Function
STM32 HAL does not provide a built-in HAL_Delay_us(). You can use a hardware timer (e.g., TIM2) to create microsecond delay.

delay_us.c

c

#include "stm32f1xx_hal.h"

extern TIM_HandleTypeDef htim2;

void delay_us(uint16_t us) {
    __HAL_TIM_SET_COUNTER(&htim2, 0);  // reset the counter
    HAL_TIM_Base_Start(&htim2);
    while (__HAL_TIM_GET_COUNTER(&htim2) < us);
    HAL_TIM_Base_Stop(&htim2);
}

Use CubeMX to configure TIM2 as a basic timer with 1 MHz clock (1 tick = 1 µs).

3. GPIO Helper Macros for Bidirectional I/O

c

#define DHT11_PORT GPIOA
#define DHT11_PIN GPIO_PIN_1

void DHT11_SetPinOutput(void) {
    GPIO_InitTypeDef GPIO_InitStruct = {0};
    GPIO_InitStruct.Pin = DHT11_PIN;
    GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
    GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
    HAL_GPIO_Init(DHT11_PORT, &GPIO_InitStruct);
}

void DHT11_SetPinInput(void) {
    GPIO_InitTypeDef GPIO_InitStruct = {0};
    GPIO_InitStruct.Pin = DHT11_PIN;
    GPIO_InitStruct.Mode = GPIO_MODE_INPUT;
    GPIO_InitStruct.Pull = GPIO_NOPULL;
    HAL_GPIO_Init(DHT11_PORT, &GPIO_InitStruct);
}

4. DHT11 Protocol Implementation
dht11.c

c

#include "dht11.h"
#include "delay_us.h"

uint8_t Rh_byte1, Rh_byte2, Temp_byte1, Temp_byte2;
uint16_t SUM;
uint8_t DHT11_Data[5];

uint8_t DHT11_Start(void) {
    DHT11_SetPinOutput();      // Set as output
    HAL_GPIO_WritePin(DHT11_PORT, DHT11_PIN, GPIO_PIN_RESET);
    HAL_Delay(18);             // At least 18ms
    HAL_GPIO_WritePin(DHT11_PORT, DHT11_PIN, GPIO_PIN_SET);
    delay_us(20);              // Wait 20–40 µs
    DHT11_SetPinInput();       // Set as input

    delay_us(80);
    if (!(HAL_GPIO_ReadPin(DHT11_PORT, DHT11_PIN))) {
        delay_us(80);
        if ((HAL_GPIO_ReadPin(DHT11_PORT, DHT11_PIN))) return 1;
    }
    return 0;
}

uint8_t DHT11_ReadBit(void) {
    while (!HAL_GPIO_ReadPin(DHT11_PORT, DHT11_PIN));  // Wait for high
    delay_us(40);  // After 40us, read the pin
    if (HAL_GPIO_ReadPin(DHT11_PORT, DHT11_PIN))
        return 1;
    else
        return 0;
}

uint8_t DHT11_ReadByte(void) {
    uint8_t i, byte = 0;
    for (i = 0; i < 8; i++) {
        byte <<= 1;
        byte |= DHT11_ReadBit();
        while (HAL_GPIO_ReadPin(DHT11_PORT, DHT11_PIN)); // Wait for low
    }
    return byte;
}

uint8_t DHT11_ReadData(uint8_t *temp, uint8_t *humidity) {
    if (DHT11_Start()) {
        Rh_byte1 = DHT11_ReadByte();
        Rh_byte2 = DHT11_ReadByte();
        Temp_byte1 = DHT11_ReadByte();
        Temp_byte2 = DHT11_ReadByte();
        SUM = DHT11_ReadByte();

        if (SUM == (Rh_byte1 + Rh_byte2 + Temp_byte1 + Temp_byte2)) {
            *humidity = Rh_byte1;
            *temp = Temp_byte1;
            return 1;
        }
    }
    return 0;
}

5. Usage in main.c

c

#include "dht11.h"

uint8_t temperature, humidity;

int main(void) {
    HAL_Init();
    SystemClock_Config();
    MX_GPIO_Init();
    MX_TIM2_Init(); // Ensure timer is initialized

    while (1) {
        if (DHT11_ReadData(&temperature, &humidity)) {
            // Use temperature and humidity
        } else {
            // Error handling
        }
        HAL_Delay(2000); // DHT11 reads every 2 seconds
    }
}

Summary of Required Files

Hedy @carolineee

STM32 HAL Implementation of DHT11 Temperature and Humidity Collection

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