About DHT11 sensor
The DHT11 sensor comes in a single row 4-pin package and operates from 3.5 to 5.5V power supply. It can measure temperature from 0-50 °C with an accuracy of ±2°C and relative humidity ranging from 20-95% 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.
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Data (40-bit) = Integer Byte of RH + Decimal Byte of RH + Integer Byte of Temp. + Decimal Byte of Temp. + Checksum Byte
For DHT11 sensor, the decimal bytes of temperature and humidity measurements are always zero. Therefore, the first and third bytes of received data actually give the numeric values of the measured relative humidity (%) and temperature (°C). The last byte is the checksum byte which is used to make sure that the data transfer has happened without any error. If all the five bytes are transferred successfully then the checksum byte must be equal to the last 8 bits of the sum of the first four bytes, i.e.,
Checksum = Last 8 bits of (Integer Byte of RH + Decimal Byte of RH + Integer Byte of Temp. + Decimal Byte of Temp.)
Now let’s talk about the most important thing, which is signalling for transmitting “0″ and “1″. In order to send a bit of data, the sensor first pulls the line low for 50 μs. Then it raises the line to high for 26-28 μs if it has to send “0″, or for 70 μs if the bit to be transmitted is “1″. So it is the width of the positive pulse that carries information about 1 and 0.
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Circuit diagram
Here is the circuit diagram showing the DHT11 sensor and a HD44780-based character LCD interfaced to the PIC16F877A microcontroller. The microcontroller runs at 20.0 MHz clock using an external resonator connected between OSC1 and OSC2 pins. The use of 20.0 MHz clock makes the timing calculation easier as 1 machine cycle becomes 0.2 μs. The timing information will be used to calculate the width of the received data pulse from the sensor so that we could identify if it is carrying a 1 or 0. The clock frequency is very important for this sensor specially using Flowcode as there are couple of inherent delays in Flowcode components which fails the sensor to run properly at lower speed.
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Writing software for DHT11 sensor is little more challenging than the hardware part because of the timing conditions for 1s and 0s. I have written sub-routines in Flowcode v5.4 for PIC for initializing the DHT11 sensor and reading the 40-bit of data in sequence. I have used Timer0 module to keep track of the width of the received data pulse, which is required to identify if the received bit is 1 or 0. When a low-to-high pulse is detected at the beginning of any data bit, TMR0 is cleared and turned ON. Since the clock frequency used here is 20.0 MHz, the TMR0 increments by 1 in every 0.8 μs (prescaler 1:4). The TMR0 is stopped whenever the data pulse is low again. The value of the TMR0 register multiplied by 0.8 gives you the width of the data pulse in μs. I am using 30 μs (38*0.8=30 μs) as the threshold for identifying 0 and 1. If the TMR0 is greater than 38 (30 μs), it means the received bit is 1, else it is 0. Here is the complete source code written in Flowcode for PIC. It can be easily adapted to any other platform, but remember that if you are using a different clock frequency you should have to modify the timer operation accordingly and carefully.
Note: I have tried to use macro read byte to re-use the macro code..but failed I guess because there are some inherent delay in calling and returning from macro in Flowcode. That's why you will see I have just read bytes one after another. Output
The accuracy of DHT11 is not as good as Sensirion’s SHT1X/7X series sensors, but it provides an easy and cheap solution to hobbyists for measuring relative humidity and temperature in parallel using a single device, which is sometime required in certain applications such as calculating the dew point.
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