SNLA465 January   2025 DP83TC817S-Q1 , DP83TC818S-Q1 , DP83TG721S-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1The Role of Time Sensitive Networking in Automotive Applications
  5. 2Generalized Precision Time Protocol Algorithm Overview
    1. 2.1 gPTP Timetamping Handshake Process
  6. 3Methods of Implementing gPTP: Timestamping Location
  7. 4Fixed Latency and Recovered Clock Modes
  8. 5Event Triggers and Monitors
  9. 6Simplified gPTP Integration
  10. 7Conclusion
  11. 8References

3 Methods of Implementing gPTP: Timestamping Location

For gPTP to work, the systems that are synced with each other must each have a common wall clock source and a way to timestamp egress and ingress packets. The point in the system where these packets are timestamped directly impacts the synchronization accuracy.

Different Kinds of Timestamping in a System Figure 3-1 Different Kinds of Timestamping in a System

The closer the packet is timestamped to the cable, the more indeterministic latencies are accounted for and the more accurate the synchronization is.

For applications that only require millisecond level synchronization such as lighting and trunks, packets can be timestamped in the software stack. This method is simple but introduces invariability due to software processing delays.

Applications requiring microsecond level precision, such as audio and road noise cancellation, can use hardware level timestamping on the MAC level. This approach reduces latency variability compared to software timestamping, providing improved accuracy.

The most accurate way to timestamp packets is directly at the Ethernet PHY level, which can have yield nanosecond to sub nanosecond sync accuracy for ADAS sensor applications. Texas Instrument’s DP83TG721 and DP83TC817/8 PHYs with IEEE 802.1AS support offer PHY level timestamping.