Overview of DHT11 

• DHT11 is a single wire digital humidity and temperature sensor, which provides humidity and temperature values serially.
• It can measure the relative humidity in percentage (20 to 90% RH) and temperature in degree Celsius in the range of 0 to 50°C.
• It has 4 pins of which 2 pins are used for supply, 1 is not used and the last one is used for data.
• The data is the only pin used for communication. Pulses of different TON and TOFF are decoded as logic 1 or logic 0 or start pulse or end of a frame.
• For more information about the DHT11 sensor and how to use it, refer to the topic DHT11 sensor in the sensors and modules topic.

Connection Diagram of DHT11 to PIC18F4550

• The above circuit diagram shows the interfacing of PIC18F4550 with the DHT11 sensor.
• In that, a DHT11 sensor is connected to RA0 (PORTA).

Programming Steps

• First, initialize the LCD16x2_8-bit library.
• Define pin no. to interface DHT11 sensor, in our program we define RA0 (Pin no.2)
• Send the start pulse to the DHT11 sensor by making low to high on the data pin.
• Receive the response pulse from the DHT11 sensor.
• After receiving the response, receive 40-bit data serially from the DHT11 sensor.
• Display this received data on LCD16x2 along with error indication.

DHT11 Code for PIC18F4550

/*
 * DHT11 Interfacing with PIC18F4550
 * http://www.electronicwings.com
 */


#include <pic18f4550.h>
#include <xc.h>
#include <stdio.h>
#include "Configuration_Header_File.h"
#include "LCD_16x2_8-bit_Header_File.h"

#define Data_Out LATA0         /* assign Port pin for data*/
#define Data_In PORTAbits.RA0  /* read data from Port pin*/
#define Data_Dir TRISAbits.RA0 /* Port direction */
#define _XTAL_FREQ 8000000     /* define _XTAL_FREQ for using internal delay */

void DHT11_Start();
void DHT11_CheckResponse();
char DHT11_ReadData();

void main() 
{
    char RH_Decimal,RH_Integral,T_Decimal,T_Integral;
    char Checksum;
    char value[10];    
    OSCCON = 0x72;  /* set internal oscillator with frequency 8 MHz*/

    LCD_Init();     /* initialize LCD16x2 */
    ADCON1=0x0F;    /* this makes all pins as a digital I/O pins */   
     
    while(1)
	{   
    	DHT11_Start();  			/* send start pulse to DHT11 module */
    	DHT11_CheckResponse();  	/* wait for response from DHT11 module */
    	
    	/* read 40-bit data from DHT11 module */
    	RH_Integral = DHT11_ReadData();  /* read Relative Humidity's integral value */
    	RH_Decimal = DHT11_ReadData();   /* read Relative Humidity's decimal value */
    	T_Integral = DHT11_ReadData();   /* read Temperature's integral value */
    	T_Decimal = DHT11_ReadData();    /* read Relative Temperature's decimal value */
    	Checksum = DHT11_ReadData();     /* read 8-bit checksum value */
    	
    	/* convert humidity value to ascii and send it to display*/    
    	sprintf(value,"%d",RH_Integral);
    	LCD_String_xy(0,0,value);
    	sprintf(value,".%d ",RH_Decimal);
    	LCD_String(value);
    	LCD_Char('%');
    	
    	/* convert temperature value to ascii and send it to display*/
    	sprintf(value,"%d",T_Integral);
    	LCD_String_xy(1,0,value);
    	sprintf(value,".%d",T_Decimal);
    	LCD_String(value);
    	LCD_Char(0xdf);
    	LCD_Char('C');
    	
    	sprintf(value,"%d  ",Checksum);
    	LCD_String_xy(1,8,value);
    	
    	/* check addition of humidity and temperature value equals to checksum */
    	if(Checksum != (RH_Integral + RH_Decimal + T_Integral + T_Decimal))
        	LCD_String_xy(0,8,"Error");
    	else
        	LCD_String_xy(0,8,"No Error");
    	
    	MSdelay(500);
	}   
}

char DHT11_ReadData()
{
  char i,data = 0;  
    for(i=0;i<8;i++)
    {
        while(!(Data_In & 1));  /* wait till 0 pulse, this is start of data pulse */
        __delay_us(30);         
        if(Data_In & 1)  /* check whether data is 1 or 0 */    
          data = ((data<<1) | 1); 
        else
          data = (data<<1);  
        while(Data_In & 1);
    }
  return data;
}

void DHT11_Start()
{    
    Data_Dir = 0;  /* set as output port */
    Data_Out = 0;  /* send low pulse of min. 18 ms width */
    __delay_ms(18);
    Data_Out = 1;  /* pull data bus high */
    __delay_us(20);
    Data_Dir = 1;  /* set as input port */    
}

void DHT11_CheckResponse()
{
    while(Data_In & 1);  /* wait till bus is High */     
    while(!(Data_In & 1));  /* wait till bus is Low */
    while(Data_In & 1);  /* wait till bus is High */
}

Video of Measuring Temperature and Humidity using DHT11 and PIC18F4550