This is a summary of the operations that are done to use the DHT11.

The DHT11 uses a single wire for transmitting and receiving data.  This bus
requires an external pull-up resistor of about 5kOhms to operate.  All writing
of bits over this bus is done by changing the DDR bit, not the PORT bit.

To send a 0, make the DDR bit an 1 to make it an output and make the PORT bit
is 0.  This will put a 0 on the bus.

To send a 1, make the DDR bit a 0 to make it an input and make the PORT bit a
0.  This will cause the bus to float and the pull-up will make it a 1.

To let the DHT11 control the bus for reading data from it, make the DDR and
PORT bits both 0.

The PORT bit can be made 0 at the start and doesn't have to be changed after
that.  Just change the DDR bit to send 0's or 1's, and make DDR a 0 when
reading.


To start the measurement, the microcontroller sends a low pulse and then waits
for the DHT11 to respond with a low pulse.

   Make the bus zero and wait 18ms
   Make the bus one and wait 30us

Now enter a loop reading the bus state.  Stay in it as long as the bus is a 1.
Once the bus becomes a 0, leave that loop and enter another loop reading the
bus state and stay in it until the bus becomes a 1 again.  When that happens
exit the loop.

The device will now start sending a stream of 40 bits (5 bytes).  The bits
arrive MSB first for each byte.

For each bit do the following:
1. Enter a loop reading the bus state and stay as long as the bus is a 1.
2. Once the bus is a 0, enter a loop reading the bus state and stay as long as
the bus is a 0.
3. Delay 50us and then read the value of the bus.
4. If the bus is a 1, the bit is 1.  If the bus is a 0, the bit is 0.
5. Store the bit value somewhere and repeat this for each of the 40 bits.    

Read five bytes of data (40 bits) from the device.

	Byte 1: Humidity high byte (bits 15:8)
	Byte 2: Humidity low byte (bits 7:0)
	Byte 3: Temperature integer value
	Byte 4: Temperature fractional value (0 to 9)
	Byte 5: Checksum

The temperature in degrees Celcius is returned in the 3rd and 4th bytes.  The
3rd byte is the integer part, and bits 3:0 in the 4th byte is the single digit
to the right of the decimal point.  For example, a temperature of 27.5C bytes 3
and 4 would by 0x1B, 0x05.

The humidity is returned in the 1st and 2nd bytes as a 16-bit integer
representing the humidity percentage times 10.  The 1st byte contains the most
significant 8 bits, the 2nd byte contains the least significant bits.  For
example a humidity of 83.7% would be represented as 837 base 10 = 0x0345.  So 
bytes 1 and 2 would contain 0x03 and 0x45.