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The DHT11<\/h3>\n\n\n\n
The DHT11 is a basic, low cost digital temperature and humidity sensor. It uses a capacitive humidity sensor and a thermistor to measure the surrounding air, outputting a digital signal on its data pin [1]<\/a>. As it is not an analogue sensor, the readings are encoded in binary and sent one bit at a time, so a driver is required to decode this data.<\/p>\n\n\n The DHT11 has four pins, although only three are used. Pin The DHT11 is rated for I’ll be writing this driver for the STM32G4 microcontroller using the STM32CubeIDE in C.<\/p>\n\n\n\n To get started, I found some resources online outlining the communication protocol for the DHT11. This article from engineersgarage.com<\/a> was immensely helpful [2]<\/a>.<\/p>\n\n\n\n From this article, I was able to deduce this set of operations to read from the DHT11 sensor:<\/p>\n\n\n\n With an oscilloscope probe on the data pin of the DHT11, we can see this behaviour (images taken utilising the completed driver<\/em>):<\/p>\n\n\n\n As such, it is clear that microsecond timing is a key part in communicating with the DHT. To work on such a time scale, we can’t used the As such, we’ll have the utilise the STM32’s Data Watchpoint and Trace (DWT) unit which allows us to get the CPU’s current cycle count, which we can use to calculate the required number of CPU cycles to surpass a certain microsecond time period. With this, we can delay for a certain number of microseconds, and also time signals in microseconds.<\/p>\n\n\n\n I found this really useful article which outlined how to implement a DWT microsecond delay function: kbiva.wordpress.com<\/a> [3]<\/a>.<\/p>\n\n\n\n![\"\"](\"https:\/\/darcyjprojects.xyz\/wp-content\/uploads\/2025\/04\/386-02.jpg\")
1<\/code> is VCC (+)<\/code> which is rated for 3.3V<\/code> to 5V DC<\/code>, Pin 2 <\/code>is the bidirectional data pin, Pin 3<\/code> is not connected, and Pin 4<\/code> is GND (-)<\/code>. You can source the DHT11 in its bare form, or in the PCB module form which often includes a pull up resistor. The PCB module may not have the NC<\/code> pin, so in that case, Pin 3<\/code> is often GND<\/code> instead.<\/p>\n\n\n\n20-80%<\/code> humidity readings with 5%<\/code> accuracy and 0-50\u00b0C<\/code> temperature readings \u00b12\u00b0C<\/code> accuracy.<\/p>\n\n\n\nCommunication Protocol<\/h3>\n\n\n\n
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LOW<\/strong><\/code> for 18ms<\/code><\/strong>, and then pull HIGH<\/strong><\/code>.<\/li>\n\n\n\nLOW<\/code> <\/strong>(for 54\u00b5s<\/code><\/strong>) followed by a logical HIGH<\/code> <\/strong>(for 80\u00b5s<\/code><\/strong>).<\/li>\n\n\n\n\n
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LOW<\/code><\/strong> for 54 \u00b5s<\/code><\/strong>, followed by a logical HIGH<\/strong><\/code>. The length of the logical HIGH<\/code> <\/strong>determines whether the bit is a 1<\/strong><\/code> or a 0<\/strong><\/code>.\n\n
0<\/code> Bit –> HIGH<\/code> for 24\u00b5s<\/code><\/li>\n\n\n\n1<\/code> Bit –> HIGH<\/code> for 70\u00b5s<\/code><\/li>\n<\/ul>\n<\/li>\n\n\n\n![\"\"](\"https:\/\/www.darcyjprojects.xyz\/wp-content\/uploads\/2025\/04\/dht11-zoomed-out.png\")
![\"\"](\"https:\/\/www.darcyjprojects.xyz\/wp-content\/uploads\/2025\/04\/dht11-zoomed-in.png\")
Microsecond Accuracy<\/h3>\n\n\n\n
HAL_Delay<\/code> function as it only offers pausing at a minimum of 1<\/code> millisecond (1000<\/code> microseconds).<\/p>\n\n\n\n