avr-dhtxx

avr-dhtxx is an lightweight (~550b) AVR library for reading data from DHT11 and DHT22 sensors. It can be easily compiled for every micro-controller supported by avr-gcc.

Alright, here's some code:

#include <avr/io.h>
#include <util/delay.h>
#include "../include/dhtxx.h"

int main( )
{
	unsigned char ec; //Exit code
	int temp, humid; //Temperature and humidity

	while( 1 )
	{
		//Request DHT sensor to give it time to prepare data
		dhtxxconvert( DHTXX_DHT22, &PORTC, &DDRC, &PINC, ( 1 << 0 ) );

		_delay_ms( 1000 );

		//Read data from sensor to variables `temp` and `humid` (`ec` is exit code)
		ec = dhtxxread( DHTXX_DHT22, &PORTC, &DDRC, &PINC, ( 1 << 0 ), &temp, &humid );
	}

	return 0;
}

As you can see, code is very simple. Library itself consists of two functions:

• dhtxxconvert( ... ) - sends request pulse to sensor in order to start data conversion.
• dhtxxread( ... ) - reads humidity and temperature data from sensor and triggers data conversion (data from previous conversion is returned).

Data received by dhtxxread is stored in two int variables specified by pointers passed as arguments to the function. Temperature and humidity values returned are multiplied by 10. For instance, temperature value 230 actually means 23.0°C. Returned values come from last conversion, so if you want fresh data, dhtxxconvert should be called >1s before reading data with dhtxxread. It's important to keep in mind that enabling interrupts before calling dhtxxread will cause interrupts to be enabled between receiving single bytes, so please disable them manually if interrupt code lasts longer than ~50µs. Please always remember to take a look at datasheet.

Each function takes following arguments:

• sensor type (DHTXX_DHT11 or DHTXX_DTH22)
• port output register address (e.g. &PORTA or &PORTC)
• port direction register address (e.g. &DDRA or &DDRD)
• port input register address (e.g. &PINB or &PINC)
• a bit mask (e.g. ( 1 << 3 ) when sensor is connected to the pin 3)

And then, in case of dhtxxread:

• pointer to an int variable for storing temperature data
• pointer to an int variable for storing humidity data

It is worth mentioning, that both functions disable interrupts in timing-crucial code fragments, but before they exit SREG value is restored.

Every function also returns an error code (uint8_t):

Value	Macro	Description
0	DHTXX_ERROR_OK	Everything is fine - no error
1	DHTXX_ERROR_COMM	Communication error occurred - sensor did not respond or transmission had been interrupted
2	DHTXX_ERROR_CHECKSUM	Received checksum is invalid
3	DHTXX_ERROR_OTHER	Some other error occurred - e.g. user tried to specify device other than DHTXX_DHT11 or DHTXX_DTH22

Library can be easily compiled using included makefile. F_CPU and MCU values should be specified when calling make:

make F_CPU=16000000U MCU=atmega328p

DHTXX_TIMEOUT value can be set too. It describes communication timeout in more or less microseconds. Default value is 60.

To understand how library works better, take a look at dhtxx.c and dhtxx.h.