The operational mode of the egress timestamping function is determined by the settings on the 1588 command port, which can be considered part of the AXI4-Stream interface as the signaling is co-incident with frame signaling. The information contained within the command port indicates one of the following:
- No Operation
- The frame is not a PTP frame and no timestamp action should be taken.
- 1-Step Operation
- A Correction Field offset value is provided as part of the command port. The frame is timestamped and the captured timestamp is summed with the existing Correction Field contained within the frame. The summed result is overwritten back into the original Correction Field of the frame. Following the frame modification, the frame FCS value is also updated/recalculated. For UDP IPv4 and IPv6 PTP formatted frames, the checksum value in the header of the frame is updated/recalculated upon request.
- 2-Step Operation
- A tag value (user-sequence ID) is provided as part of the command field. The frame is timestamped and the timestamp made available to the client logic, along with the provided tag value for the frame.