顶部端口（USB）接口，这个接口的调试是PL的使用
代码说明：
TOP PORT (USB) 调试接口说明 本接口是利用PL-2303HX的USB转RS232的接口芯片设计实现了一个TTL(5V)的RS232串口，并且利用了PL-2303HX的2个GPIO口软件模拟了一个低速的I2C总线.该接口由USB接口提供5V电压，可自带2Kbit(24C02)数据备份EEPROM (单独读写通道,不占用I2C总线地址)。并可对外提供小于250mA的5V电压。尤其适合WIN-XP以后的没有并口和串口笔记本电脑。整机成本不到￥20RMB。 -TOP PORT (USB) interface debugging of this interface is the use of PL-2303HX the USB to RS232 interface chip design to achieve a TTL (5V) of the RS232 serial port, and use of PL-2303HX the two GPIO port software simulation of a low-speed I2C bus. The interface from the USB interface to provide 5V voltage, can bring their own 2Kbit (24C02) data backup EEPROM (separate read and write access, do not occupy I2C bus address). And may provide less than 250mA of 5V voltage. WIN-XP is particularly suitable for subsequent parallel and serial no notebooks. Machine costs less than ￥ 20RMB.
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