Interfacing PIC18F4550 with DHT11 humidity and temperature sensor

 PIC18F4550 + DHT11 (RHT01) Proteus simulation

This topic shows how to interface DHT11 (RHT01) digital relative humidity and temperature sensor with PIC18F4550 microcontroller, and how to simulate this interfacing using Proteus.
Note that for the simulation Proteus version should be 8.1 or higher. With these versions there is no need to install Proteus DHT11 library, it is included with the software, so don’t waste your time searching for dht11 Proteus library or dhtxx.mdf or dht11 module for Proteus, just use Proteus version 8.1 or higher.

About DHT11 (RHT01) relative humidity and temperature sensor:
The DHT11 sensor comes in a single row 4-pin package and operates from 3.3 to 5.5V power supply. It can measure temperature from 0-50 °C with an accuracy of ±2°C and relative humidity ranging from 20-90% with an accuracy of  ±5%. The sensor provides fully calibrated digital outputs for the two measurements. It has got its own proprietary 1-wire protocol, and therefore, the communication between the sensor and a microcontroller is not possible through a direct interface with any of its peripherals. The protocol must be implemented in the firmware of the MCU with precise timing required by the sensor.

DHT11 pinsThe following timing diagrams describe the data transfer protocol between a MCU and the DHT11 sensor. The MCU initiates data transmission by issuing a “Start” signal. The MCU pin must be configured as output for this purpose. The MCU first pulls the data line low for at least 18 ms and then pulls it high for next 20-40 us before it releases it. Next, the sensor responds to the MCU “Start“  signal by pulling the line low for 80 us followed by a logic high signal that also lasts for 80 us. Remember that the MCU pin must be configured to input after finishing the “Start“ signal. Once detecting the response signal from the sensor, the MCU should be ready to receive data from the sensor. The sensor then sends 40 bits (5 bytes) of data continuously in the data line. Note that while transmitting bytes, the sensor sends the most significant bit first.DHT11 sensor timing diagramData consists of decimal and integral parts. A complete data transmission is 40bit, and the sensor sends higher data bit first.
Data format: 8bit integral RH data + 8bit decimal RH data + 8bit integral T data + 8bit decimal T data + 8bit check sum. If the data transmission is right, the check-sum should be the last 8bit of “8bit integral RH data + 8bit decimal RH data + 8bit integral T data + 8bit decimal T data”.

The DHT11 is a digital sensor so it sends 1’s and 0’s, but it is very important to know how it sends the digital data. The figure below shows how the sensor sends its information. 

DHT11 sensor timing diagram 2

Interfacing PIC18F4550 with DHT11 (RHT01) sensor circuit:
The following circuit schematic shows complete project circuit.
PIC18F4550 DHT11 sensor interfacing Proteus simulation
The circuit is simple, there is the microcontroller PIC18F4550, DHT11 sensor and 1602 LCD to display humidity and temperature results.
PIC18F4550 uses its internal oscillator @ 8MHz and mclr pin function is disabled.

Interfacing PIC18F4550 with DHT11(RHT01) sensor CCS C code:
The interfacing code is written with CCS PIC C compiler PCWHD version 5.051.
If you want to understand the code please read the DHT11 datasheet.
Variables Time_out and k are used to test reading time to avoid wrong data reception or microcontroller hanging.

Interfacing PIC18F4550 with DHT11 (RHT01) sensor Proteus simulation video:
The following video shows Proteus simulation of this interfacing.

DHT11 Proteus simulation file download:


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