IWR1642: IWR1642 Question about the SYNC_IN

There are some details about the experiment:
We use the Oscilloscope to observe the SYNC_IN level, there are no other signal on Sync_in line.
the frame duration is 23ms,and the rising edge period is 25ms.when we input the rising edge,the frame will work in 25ms.when we use continuous input high level，the frame will work in 23ms period。