Overview of DHT11
The DHT11 is a digital sensor that provides both humidity and temperature readings via a single-wire communication interface. It measures relative humidity in a range from 20% to 90% RH and temperature in degrees Celsius from 0 to 50°C. This sensor has four pins, with two for power supply, one unused, and one dedicated for data transmission. Communication is handled through the data pin, where the sensor sends pulses of varying durations (TON and TOFF) to represent logic levels, start pulses, and end-of-frame signals. For a comprehensive guide on the DHT11 sensor and its usage, refer to the “DHT11 Sensor” section in the Sensors and Modules topic.
DHT11 Temperature and Humidity Sensor
Connection Diagram of DHT11 to PIC18F4550
DHT11 Interfacing with PIC18F4550
The circuit diagram above illustrates how the DHT11 sensor is interfaced with the PIC18F4550 microcontroller. In this setup, the DHT11 sensor is connected to the RA0 pin on PORTA.
Programming Steps
- Begin by initializing the
LCD16x2_8-bitlibrary for display functionality. - Define the pin number to connect the DHT11 sensor; in this program, RA0 (pin 2) is specified.
- Send a start pulse to the DHT11 sensor by toggling the data pin from low to high.
- Wait to receive the response pulse from the DHT11 sensor, indicating it is ready to transmit data.
- After receiving the response, collect the 40-bit data serially from the DHT11 sensor.
- Display the received temperature and humidity data on the LCD16x2 display, along with any error indications if necessary.
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 */
}