说明：  DHT21使用范例,硬件连接： P2.0口为通讯口连接DHT11,DHT11的电源和地连接单片机的电源和地，单片机串口加MAX232连接电脑.
(DHT21 usage examples, hardware connection: P2.0 port as communication port connected DHT11, DHT11 the power and ground connections and power microcontroller, the microcontroller serial plus MAX232 connected computer.)