帧同步状态机主要是为了克服通信中可能出现的各种意外情况，包括信号丢失、信道误码等造成的通信中断，尽量维护正常的通信的进行。当系统启动后，同步状态机处于失步态，并且不断搜索输入信号中的有效同步模式（“10011011”），一旦当其找到一个有效的同步模式后，进入预同步态；在预同步态还不能完全确定当前找到的帧开始位置（由同步模式确定的位置）是正确的，还需要继续检查2个相隔一个帧长（256个时钟）后的位置是否仍然存在有效的同步模式，如果存在那么同步状态机将进入同步态，否则将返回失步态继续搜索下一个有效的同步模式；在同步态时，状态机不断检测每一帧的帧头位置是否存在有效的同步模式，如果一帧的开始位置没有检测到同步模式，那么将进入保持态，此时还不能确定接收信号的帧头位置已经丢失，还需要根据相邻下3帧的情况进行判定，如果相邻下3帧的帧头位置仍然没有检测到有效的同步模式，那么认为帧结构已经丢失，进入失步态重新进行搜索，如果在某一帧的帧头位置检测到了有效的同步模式，那么认为帧结构没有丢失（上一帧的无效同步模式是由于误码所致），返回同步态。-Frame synchronization state machine is mainly to overcome all kinds of unexpected situations that may occur in communications, including loss of signal, the communication channel error caused by the interruption, try to maintain a normal communicate. When the system starts, synchronous state machine is out of gait, and continues to search for a valid input signal synchronous mode (" 10011011" ), as soon as it finds a valid synchronous mode, enter the pre-synchronization state　in pre-synchronization state also can not completely determine the start position of the current frame is found (as determined by the synchronization mode position) are correct, but also the need to continue to check whether the two separated one frame length (256 clocks) after the location is still valid in the synchronous mode, if present, then synchronous state machine will enter the synchronized state, otherwise it will return loss gait continue to search for the next valid synchronization pattern　in