Temperature Sensors DS18B20 and DHT11 with Atmega8

Both are neat little devices, that can offer useful data to your hardware applications. The DS18B20 comes with better temperature interval and resolution and with more features, but the DHT11 is cheaper and also provides humidity data. Which one is better? It depends on your application. Below see how to use them both, with two small size libraries to keep the program memory to a minimum.

DS18B20 – digital temperature sensor

My previous article on the DS18B20, focused on getting only the positive temperature values out of this sensor’s Scratchpad. For a better approach the 12 bit data needs to be interpreted correctly, with the most significant bit as the sign bit. This leaves us with 11 bits for the integer temperature part , and 4 bits for the decimal.
A few bitwise operations, and a little bit level surgery, gets the job done.

// first 12bits represent the integer temperature
// the last 4bits represent the fractional temperature
// I I I I I I I I I I I I F F F F
//Store temperature integer digits and decimal digits
uint16_t integer_bits = temp >> 4,
		fractional_bits = temp & 0xF;
int negative = 0;
if (integer_bits & 0x800) negative = 1; //the 12th digit is one, representing a negative number

Here’s a direct conversion using the float data type:

float therm_read_temperature() {
	//Reset, skip ROM and start temperature conversion
	therm_reset();
	therm_write_byte(THERM_CMD_SKIPROM);
	therm_write_byte(THERM_CMD_CONVERTTEMP);
	//Wait until conversion is complete
	while(!therm_read_bit());
	//Reset, skip ROM and send command to read Scratchpad
	therm_reset();
	therm_write_byte(THERM_CMD_SKIPROM);
	therm_write_byte(THERM_CMD_RSCRATCHPAD);
	uint8_t l = therm_read_byte();
	uint8_t h = therm_read_byte();
	therm_reset();
	float temp = ( ( h << 8 ) + l )*0.0625;
	return temp;
}				

The DS18B20 can do much more than this . See the complete datasheet, here.

DHT11 - cheap digital temperature and humidity sensor

This Temperature & Humidity Sensor features a temperature & humidity sensor complex with a calibrated digital signal output. It includes a resistive-type humidity measurement component and an NTC temperature measurement component, and connects to a high-performance 8-bit microcontroller, offering excellent quality, fast response, anti-interference ability and cost-effectiveness. Datasheet here.
This sensor returns sequences of 5bytes representing the humidity integer value, the temperature, and a checksum. To get the values correctly, precise timing at the 40uS scale is required.

// READ OUTPUT - 40 BITS => 5 BYTES or TIMEOUT
for (int i=0; i<40; i++)
{
	loopCnt = 10000;
	while(!(DHT11_PIN&(1< 40) bits[idx] |= (1 << cnt);
	if (cnt == 0)   // next byte?
	{
		cnt = 7;    // restart at MSB
		idx++;      // next byte!
	}
	else cnt--;
}

// WRITE TO RIGHT VARS
// as bits[1] and bits[3] are always zero they are omitted 
*humidity    = bits[0];
*temp = bits[2];

uint8_t sum = bits[0] + bits[2];

if (bits[4] != sum) return DHTLIB_ERROR_CHECKSUM;
return DHTLIB_OK;

Schematics:

Source code:
I have designed the code for an Atmega8 microcontroller. The code is available here.