TRM的30章讲述了pru核心的gpio功能，其中28bit 串行输入的介绍如下
SB (Start Bit detection) is mapped to pru<n>_r31_status[29] and is set when the first 1 is captured on
PRU<n>_DATAIN. The SB flag in pru<n>_r31_status[29] is cleared in software through the
CFG_GPCFGn registor of the PRU-ICSS CFG register space。
上述表示，pru在采集到sb为1时才开始接收数据
想问该gpio是否可以配置为start bit是低电平（0）开始接收数据，空闲信号为高电平？
还有就是pru的Peripheral Interface中ENDAT<m>_OUT配置，tx,rx例程。
