The AM2302 (DHT22) is a 3 wire (Vcc, data, gnd) temperature and humidity sensor manufactured by Aosong.
Pull Up Resistor
The Adafruit module (left) includes a 5.1K pull up resistor and a bypass capacitor.
The Pi does have internal pull ups that are turned on. They are about 50K. Our tests on 3 meter cables did not show any significant difference in performance or read errors whether or not the 5.1K pull up or bypass capacitor was used. However, good engineering practices would use the pull up and bypass cap.
The new DHT22 spec sheet shows the pull up value as 1K in Figure 5. The old spec sheet said: "single bus usually require an external about 5.1kΩ pull-up resistor."
You must wait at least 2 seconds before reading again or you will get a bad read (the checksum will fail).
The new spec sheet: "the interval of whole process must beyond 2 seconds."
The old spec sheet:
3.Read the sensor minimum time interval for the 2S; read interval is less than 2S, may cause the temperature and humidity are not allowed or communication is unsuccessful, etc.. 4.Temperature and humidity values are each read out the results of the last measurement For real-time data that need continuous read twice, we recommend repeatedly to read sensors, and each read sensor interval is greater than 2 seconds to obtain accurate data.
WARNING: The spec sheet refers to this a a "1-wire" device. This is not a 1-Wire bus trademarked by Dallas Semiconductor Corp. The later spec sheet has a disclaimer.