Decoding DHT11 sensor output data

 

The DHT11 sensor uses a proprietary single-wire protocol to communicate with a microcontroller. This protocol involves specific timing sequences for data transmission. Here's a detailed explanation of the DHT11 communication protocol:

Communication Sequence

Start Signal:

• The microcontroller sends a start signal to the DHT11 sensor to indicate it is ready to receive data.
• The microcontroller pulls the data line low for at least 18 milliseconds to ensure the DHT11 detects the start signal.
• The microcontroller then pulls the data line high for 20-40 microseconds.

Sensor Response:

• The DHT11 responds to the start signal by pulling the data line low for 80 microseconds.
• It then pulls the data line high for another 80 microseconds, indicating it is ready to send data.

Data Transmission:

• The DHT11 sends 40 bits of data, divided into 5 bytes. Each byte consists of 8 bits, representing:
• Integral part of the humidity.
• Decimal part of the humidity (always 0 for DHT11).
• Integral part of the temperature.
• Decimal part of the temperature (always 0 for DHT11).
• Checksum (used for error detection).

Bit Timing

Each bit starts with a 50-microsecond low signal, followed by a high signal. The length of the high signal determines whether the bit is a '0' or a '1':

• 0 Bit: 26-28 microseconds high.
• 1 Bit: 70 microseconds high.

Detailed Protocol Steps

Initialization:

Microcontroller sends a start signal:

• Pulls the data line low for at least 18 milliseconds.
• Pulls the data line high for 20-40 microseconds.

DHT11 Response:

• Pulls the data line low for 80 microseconds.
• Pulls the data line high for 80 microseconds.

Data Transmission:

For each bit (total 40 bits):

• Starts with a 50-microsecond low signal.

Followed by a high signal:

• 26-28 microseconds for a '0'.
• 70 microseconds for a '1'.

Error Checking

Checksum Calculation:

The checksum is calculated by adding the first four bytes (humidity integer, humidity decimal, temperature integer, temperature decimal) and taking the last 8 bits of the result.

The transmitted checksum must match the calculated checksum to ensure data integrity.