DHT11/DHT22 are pretty well known sensors for sensing relative humidity and Temperature. These are digital sensors are based on a custom protocol which use a single wire/bus for communication. The primary difference between DHT11 and DHT22 is that, DHT22 has higher resolution and is more accurate as compared to DHT11. Other than that, the pinout and programming is same for both. In this tutorial we will go through basics of protocol/communication process for DHTxx sensors. Also, do note that the protocol used by DHT11/DHT22 is NOT same as 1-wire bus by Dallas Semiconductor.
Specifications & DHT11 Vs DHT22:
| Parameter | DHT11 | DHT22 |
| Temperature measurement range | 0 to 50 Deg.C | -40 to 80 Deg.C |
| Temperature Accuracy | +- 1 to 2 Deg.C | +- 0.5 Deg.C |
| Relative Humidity Range | 30% to 90% | 0% to 100% |
| Relative Humidity Accuracy | +- 4% to 5% | +-2 to 5% |
| Operating Voltage | 3.3V to 5V | 3.3V to 5V |
| Resolution | 8 bits | 16 bits |
| Sampling period | >=1 sec | >=2 sec |
DHT22 & DHT11 Pinout:
The DATA wire used for communication between microcontroller and DHT11/DHT22 is pulled HIGH using a 4.7K or 10K pull-up resistor. This is to bring the bus in an IDLE state when there is no communication taking place. A continuous HIGH on the line denotes an IDLE state. The microcontroller acts as the bus master and hence is responsible for initiating communication (i.e. Read). DHT11/DHT22 Humidity and Temperature sensor always remain as slave and responds with data when MCU asks for it. The protocol used for communication is simple and can be summarized as follows:
- When the Line is IDLE, the microcontroller pulls it to LOW for ~18ms.
- After this MCU releases the line and it is again pulled HIGH for around ~20 to ~40us.
- DHT11 will detect it as a START from the MCU and responds by pulling the line LOW for ~80us.
- Next, DHT11 will release the line and it comes back to HIGH for ~80us which indicates that it is ready to send data or “get ready”.
- Next it will send 40 bits of Data. Each bit starts with a ~50us LOW followed by ~26-28us for a “0” or ~70us for a “1”.
- After communication ends, the Line released and pulled HIGH and enters IDLE state.
DHT11 Data Format
When Humidity and Temperature sensor sends data, it sends the MSB first. The 40bits of data is divided into 5 bytes. For DHT11 sensor 2nd and 4th byte is always Zero. The significance of these bytes is as follows:
- 1st Byte: Relative Humidity Integral Data in % (Integer Part)
- 2nd Byte: Relative Humidity Decimal Data in % (Fractional Part) – Zero for DHT11
- 3rd Byte: Temperature Integral in Degree Celsius (Integer Part)
- 4th Byte: Temperature in Decimal Data in % (Fractional Part) – Zero for DHT11
- 5th Byte: Checksum (Last 8 bits of {1st Byte + 2nd Byte + 3rd Byte+ 4th Byte})
EDIT: Thanks to @Ribo for pointing out the error. Its been update now. As far as I know DHT22 is manufactured by ASAIR (Guangzhou Aosong Electronic Co., Ltd.). Datasheets attached below for both DHT11 and DHT22.
Datasheet Links: DHT11 , DHT22
