按照协议规定，总线空闲时两线为高，当进行通信时，SDA在SCL高电平时由高变低表示通信开始（ void iic_start(void) ），以后传输数据，SDA只能在SCL为低的时候才能发生变化，在SCL上升抓取数据；而停止位是在SCL高时SDA由低变高（void iic_stop(void) ）。-Under the agreement, the two-wire bus is idle is high and when to communicate, SDA SCL high, from high to low the start of communication (void iic_start (void)), after the transmission of data, SDA only when SCL is lowwhen will change, data in SCL rising crawl　while the stop bit is in high SCL SDA from low to high (void iic_stop (void)).