4.12.1.2 PTP and 10BASE-T1S

Effective time synchronization can be run over 10BASE-T1S networks. There are two issues that need to be considered during implementation.

• When using delay calculations, timestamps need to be taken where the delay is constant. The timestamp for packet transmission normally occurs at the end of the SFD on the MII/RMII interface. When the MAC delivers the packet to a 10BASE‑T1S PHY, however, there is a variable delay through the PHY when PLCA is enabled. PLCA requires the PHY to delay transmission of a packet until the next PLCA transmit opportunity. To have a constant delay, the transmit timestamp needs to be taken after the PLCA. Considering the receive timestamp, the MAC will only be able to create a timestamp to the resolution of the clock for the inbound data signal from the PHY, which is 400 ns for MII. The solution to both of these problems is to use the signal on the MDI to determine when to timestamp. The remainder of this section will discuss features of the LAN8670/1/2 which enable this type of timestamping.
• While 802.1AS clearly defines the Sync and PDelay methods for full-duplex Ethernet links, there is not yet a clear definition for a shared medium, like 10BASE-T1S when used with PLCA in multidrop mode. All of the message types above are currently defined as multicast. Software workarounds to existing PTP processing are required until the standards are adapted for multidrop segments. These workarounds are beyond the scope of this document.