IEEE 1588 Implementation

IEEE 1588 is a standard for precision time synchronization in local area networks. It works with the exchange of special PTP frames. The PTP messages can be transported over IEEE 802.3/Ethernet, over Internet Protocol Version 4 (IPv4) or over Internet Protocol Version 6 (IPv6). GEM detects when the PTP event messages: sync, delay_req, pdelay_req, and pdelay_resp are transmitted and received. GEM asserts various strobe signals for different PTP event messages.

GEM supports the following functionalities:

• Identifying PTP frames
• Extracting timestamp information out of received PTP frames
• Inserting timestamp information into received data frames, before passing to buffer memory
• Inserting timestamp information into transmitted data frames
• Allowing control of TSU either through MSS or FPGA fabric

GEM samples the TSU timer value when the TX or RX SOF event of the frame passes the MII/GMII boundary. This event is an existing signal synchronous to MAC TX/RX clock domains. The MAC uses the sampled timestamp to insert the timestamp into transmitted PTP sync frames (if one step sync feature is enabled) or to pass to the register block to capture the timestamp in APB accessible registers, or to pass to the DMA to insert into TX or RX descriptors. For each of these, the SOF event, which is captured in the tx_clk and rx_clk domains respectively, is synchronized to the tsu_clk domain and the resulting signal is used to sample the TSU count value.

There is a difference between IEEE 802.1 AS and IEEE 1588. The difference is, IEEE 802.1AS uses the Ethernet multi-cast address 0180C200000E for sync frame recognition whereas IEEE 1588 does not. GEM is designed to recognize sync frames with both 802.1AS and 1588 addresses and so can support both 1588 and 802.1AS frame recognition simultaneously.