what's the relationship between F28335 Mcbsp's Frame-Synchronization Period and Frame Length

Mcbsp's Frame-Synchronization Period is set  in  FPER of SRGR2, allowing up to 4096 bits /frame,  frame length is set in register XFRLEN1,  max words number is 128(129 * 32 bits = 4096),

If word bit size (e.g.,16 bits /word)is specified and  frame length is set(e.g.,2 words/frame), then bit number in a frame is decided accordingly(16*2 = 32bits/frame),    thus, the setting of  frame-Synchronization Period seems redundant,is

it necessary to set FPER to 31?  what happens if set FPER to 20 or 40?

can someone help me to understand this ? many thanks.

gaoyang9992006：

出现错误了。