Timestamp Accuracy for NI-XNET Automotive Ethernet Modules

Updated Feb 10, 2021

Reported In

Hardware

  • PXIe-8521
  • PXIe-8522
  • PXIe-8523

Driver

  • NI-XNET

Issue Details

What is the timestamp accuracy for NI-XNET Automotive Ethernet modules?

Solution

There are two timestamps for NI Automotive Ethernet modules: local and network.

Local Timestamp represents the time at which the frame was received by the XNET interface. It is similar to the timestamp in a CAN frame. 
NI Automotive Ethernet modules use PXI_Clk10, a 10 MHz PXI backplane clock provided by the chassis, to drive the local time keeper and to synchronize with other modules in the PXI chassis. If the PXI backplane clock is not available, the module uses its own internal oscillator. 
PXI_Clk10 provides frequency but not date/time information. When an NI-XNET session is created, XNET initializes the date/time information for the local clock using host time and increment from there off of the 100MHz hardware oscillator. 
All 4 ports on the same NI Automotive Ethernet modules are initialized simultaneously. If you need to sync hardware timestamps across multiple NI Automotive Ethernet modules, you need to use Future Time Events to get the host times synced up. 
The local 100MHz oscillator is disciplined by hardware to the PXI_Clk10 backplane clock. The bottom line is that you have 10ns precision for two Ethernet ports on the same NI Automotive Ethernet modules, but you have 100ns for two Ethernet ports on different NI Automotive Ethernet modules (or Ethernet to a CAN card).

Network timestamp is timestamp in relation to the 802.1AS grandmaster clock when the 802.1AS network synchronization protocol is used. As stated in the specification for PXIe-852x , the network synchronization accuracy is < 1 µs for 100BASE-T1 and < 3 µs for 1000BASE-T1.

Additional Information

  • The location of the timestamp point depends on the Port Mode of the session interface. When Port Mode is Direct, the location of the timestamp point corresponds to time synchronization protocols, using the reference plane marking the boundary between the port's connector (copper wire) and PHY. When Port Mode is Tap, the location of the timestamp point is the midpoint between the connector/PHY reference plane of this session's interface and the connector/PHY reference plane of the tap partner.
  • As of NI-XNET 20.5, setting timestamp for XNET Write is not supported. You cannot specify a hardware timestamp when transmitting an Ethernet frame. 
  • User Datagram Protocol (UDP) and Transmission Control Protocol (TCP), regardless of LabVIEW Socket API or XNET IP Stack are implemented in the operating system and do not support timestamping.