STM32 external DHT11 display temperature and humidity

This paper introduces STM32 How to connect an external temperature and humidity sensor to read the current ambient temperature and humidity , And display to OLED On the screen .

1 DTH11 Temperature and humidity sensor

DHT11 Digital temperature and humidity sensor is a temperature and humidity composite sensor with calibrated digital signal output , It includes a resistive humidity sensing element and a NTC Temperature measuring element .

1.1 Data reading protocol

Micro controller MCU And DHT11 Communication and synchronization between , Use single bus data format , Primary communication time 4ms about .

user MCU Send a start signal after ,DHT11 From low power mode to high speed mode , Wait for the end of the host start signal ,DHT11 Send a response signal , Send out 40bit The data of , And trigger a signal acquisition , Users can choose to read some data .

From mode ,DHT11 Receive the start signal to trigger a temperature and humidity acquisition , If no start signal is received from the host ,DHT11 Do not actively collect temperature and humidity . After collecting data, switch to low speed mode .

1.1.1 Start signal

The bus idle state is high ,MCU Pull the bus down and wait DHT11 Respond to ,MCU Pull down the bus must be greater than 18 millisecond , Guarantee DHT11 Can detect the start signal .

DHT11 After receiving the start signal from the host , wait for MCU The start signal is over , And then send 80us Low level response signal .

MCU After sending the start signal , Delay waiting for 20-40us after , Read DHT11 The response signal ,MCU After sending the start signal , You can switch to input mode , Or the average output high power can , The bus is pulled up by a pull-up resistor .

1.1.2 Data digital signal

The bus is low , explain DHT11 Send a response signal ,DHT11 After sending the response signal , Then pull up the bus 80us, Prepare to send data , each bit All the data are based on 50us The low level slot starts , The length of the high level determines that the data bit is 0 still 1.

Numbers 0 And number 1 It means , As shown in the figure below ：

• Numbers 0：50us After the low level starts ,26-28us High level of Express 0

• Numbers 1：50us After the low level starts ,70us High level of Express 1

If the read response signal is high , be DHT11 No response , It is necessary to check whether the line is connected normally .

When the last one bit After the data is transmitted ,DHT11 Pull down the bus 50us, Then, the bus is pulled up by the pull-up resistance and enters the idle state .

1.1.3 Temperature and humidity data format

A complete data transfer is 40bit, High first out . The data is divided into decimal part and integer part , data format :

• 8bit Humidity integer data

• 8bit Humidity decimal data

• 8bit Temperature integer data

• 8bit Temperature decimal data

• 8bit The checksum

When the data transmission is correct, the check sum data is equal to “ 8bit Humidity integer data +8bit Humidity decimal data +8bit Temperature integer data +8bit Temperature decimal data ” The end of the result 8 position .

1.2 Hardware wiring

DHT11 Only one line is needed for data reading , I'm using PB8, in addition ,OLED Used to display the value of temperature and humidity , Use IIC signal communication , It uses PB6 and PB7.

2 Programming

according to DHT11 Data reading protocol , Write the corresponding data reading function .

2.1 DHT11 Reset and detect response functions

First of all MCU towards DHT11 Start signal sent , Pull it down 20ms, Pull up again 30us.

u8 DHT11RstAndCheck(void){    u8 timer = 0;​    __set_PRIMASK(1);          // General manager Guan interrupted     DHT11_OUT = 0;             // Output low level     delay_ms(20);              // Pull down at least 18ms    DHT11_OUT = 1;             // Output high level     delay_us(30);              // pull up 20~40us    while (!DHT11_IN)          // Wait for the bus to pull down ,DHT11 Will pull down 40~80us As a response signal     {        timer++;               // Count when the bus is pulled down         delay_us(1);    }    if (timer>100 || timer<20) // Determine the response time     {        __set_PRIMASK(0);      // General interruption         return 0;    }        timer = 0;    while (DHT11_IN)           // wait for DHT11 Release the bus , The duration of the 40~80us    {        timer++;               // Count when the bus is pulled high         delay_us(1);    }    __set_PRIMASK(0);          // General interruption     if (timer>100 || timer<20) // Detect the high level after the response signal     {        return 0;    }        return 1;}

2.2 data fetch

MCU towards DHT11 After sending the start signal , You can receive DHT11 The data returned , Read the humidity and temperature at one time .

/* Read one byte of data , Return value - Data read */u8 DHT11ReadByte(void){    u8 i;    u8 byt = 0;​    __set_PRIMASK(1);      // General manager Guan interrupted     for (i=0; i<8; i++)    {        while (DHT11_IN);  // Wait for low level , Before the data bits 50us Low level slot                 while (!DHT11_IN); // Wait for high level , Start transmitting data bits                 delay_us(40);        byt <<= 1;         // Because of the high position , So move left byt, The lowest point is made up of 0        if (DHT11_IN)      // Read the bus level value to byt Lowest middle         {            byt |= 0x01;        }    }    __set_PRIMASK(0);      // General interruption ​    return byt;}​/* Read data once , Returns the parameter ：Humi- humidity ,Temp- temperature ; Return value : 0- success ,1- Failure */u8 DHT11ReadData(float *Humi, float *Temp){    s8 sta = 0;    u8 i;    u8 buf[5];​    if (DHT11RstAndCheck())         // Detect the response signal     {        for(i=0;i<5;i++)            // Read 40 Bit data         {            buf[i]=DHT11ReadByte(); // Read 1 Bytes of data         }        if(buf[0]+buf[1]+buf[2]+buf[3] == buf[4]) // Check success         {            u8 H_inte = buf[0]; // Humidity integer part data             u8 H_frac = buf[1]; // Humidity fractional data             u8 T_inte = buf[2]; // Temperature integer part data             u8 T_frac = buf[3]; // Temperature fractional data                         char tmp1[8], tmp2[8];            sprintf(tmp1, "%d.%d",H_inte,H_frac);            sscanf(tmp1, "%f", Humi);            sprintf(tmp2, "%d.%d",T_inte,T_frac);            sscanf(tmp2, "%f", Temp);        }        sta = 0;    }    else // The response failed and returned -1    {        *Humi = 88; // The response failed and returned -1        *Temp = 88; // The response failed and returned -1        sta = 1;    }        return sta;    }

2.3 initialization

Use DHT11 Before , Conduct pin initialization and device initialization .

/*DHT11 Initialization function */u8 DHT11Init(void){    GPIO_InitTypeDef GPIO_InitStructure;​    RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOC,ENABLE);// Can make GPIOC Port clock     GPIO_SetBits(GPIOB,GPIO_Pin_8);                     // Set up PC13 Output high level ,( Setting the pin level first can avoid IO Possible burrs during initialization )    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_8;           // Set up DHT11 Data pins ->PC13    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_Out_OD;    // Set to open drain output mode     GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;   // Set the output rate to 50MHz    GPIO_Init(GPIOB, &GPIO_InitStructure);              // initialization GPIOC port ​    return DHT11RstAndCheck();                          // return DHT11 state }

3 test

After transplantation U8g2 Library hello_world Make modifications on the routine , Display temperature and humidity on the screen . Notice the small circle in degrees Celsius , I don't know how to show it directly in the form of symbols , I drew a small hollow circle here alone .

int main(void){       delay_init();       // Delay function initialization          LED_Init();         // Initialization and LED Connected hardware interface     IIC_Init();         u8g2_t u8g2;    u8g2Init(&u8g2);    u8g2_SetFontMode(&u8g2, 1);    u8g2_SetFont(&u8g2, u8g2_font_unifont_t_symbols);        DHT11Init();    float Temp = 0;    float Humi = 0;    char strTemp[32];    char strHumi[32];    while(1)    {        u8g2_FirstPage(&u8g2);        do        {            //draw(&u8g2);            DHT11ReadData(&Humi, &Temp);            sprintf(strTemp, "Temp: %.1f C", Temp);            sprintf(strHumi, "Humi: %.1f %%", Humi);                        u8g2_ClearBuffer(&u8g2);            u8g2_DrawStr(&u8g2, 0, 30, strTemp);            u8g2_DrawCircle(&u8g2, 84, 22, 2, U8G2_DRAW_ALL);            u8g2_DrawStr(&u8g2, 0, 60, strHumi);            u8g2_SendBuffer(&u8g2);                     delay_ms(3000);        } while (u8g2_NextPage(&u8g2));    }}​

The test results are as follows ：

4 summary

This article introduces how to use STM32 External temperature and humidity DHT11 Realize the reading of temperature and humidity data , And pass OLED Data display .