The 1588 timestamp capture on the TX accounts for the jitter introduced by the transmit gearbox. The gearbox takes in 66-bit timestamped frames in 34/32 bit chunks and outputs data 32 bits at a time. Because 66 bits is not a multiple of 32, the gearbox accumulates excess data that is added to the beginning of subsequent cycles of data output. When data is appended from the gearbox buffer, jitter is introduced to the timestamped frames received by the gearbox. The gearbox has a state that is called the sequence number. For each sequence number, the gearbox has a specific number of bits buffered for adding to the beginning of the output data. The amount of jitter introduced by the gearbox can be represented by the graph in This Figure .
TIP: The timestamp of a frame aligns with the control bits at the start of the 66-bit frame. As a result, timestamp jitter compensation is applied according to the arrival time of the control bits at the gearbox. The actual compensation is done by multiplying the cycle period (3.103 ns) by the n/32 fraction based on the sequence number and adding that to the timestamp already associated with the 66-bit frame.
