This section details the packet timestamping function of the 40G/50G Ethernet subsystem when the MAC layer is included. The timestamping option must be specified at the time of generating the subsystem from the IP catalog or ordering the IP core asynchronously. This feature presently supports one-step and two-step IEEE PTP 1588 functionality. One-step operation is described in this appendix for reference.
Ethernet frames are timestamped at both ingress and egress. The option can be used for implementing all kinds of IEEE 1588 clocks: Ordinary, Transparent, and Boundary. It can also be used for the generic timestamping of packets at the ingress and egress ports of a system. While this feature can be used for a variety of packet timestamping applications, the rest of this appendix assumes that you are also implementing the IEEE 1588 Precision Time Protocol (PTP).
IEEE 1588 defines a protocol for performing timing synchronization across a network. A 1588 network has a single master clock timing reference, usually selected through a best master clock algorithm. Periodically, this master samples its system timer reference counter, and transmits this sampled time value across the network using defined packet formats. This timer should be sampled (a timestamp) when the start of a 1588 timing packet is transmitted. Therefore, to achieve high synchronization accuracy over the network, accurate timestamps are required. If this sampled timer value (the timestamp) is placed into the packet that triggered the timestamp, this is known as 1-step operation. Alternatively, the timestamp value can be placed into a follow up packet; this is known as 2-step operation.
Other timing slave devices on the network receive these timing reference packets from the network timing master and attempt to synchronize their own local timer references to it. This mechanism relies on these Ethernet ports also taking timestamps (samples of their own local timer) when the 1588 timing packets are received. Further explanation of the operation of 1588 is out of the scope of this document. It is assumed that the reader is familiar with the IEEE 1588 specification for the rest of this section.
The 1588 timer provided to the subsystem and the consequential timestamping taken from it are available in one of two formats which are selected during subsystem generation.
- Time-of-Day (ToD) format
- IEEE 1588-2008 format consisting of an unsigned 48-bit second field and a 32-bit nanosecond field.
- Correction Field format
- IEEE 1588-2008 numerical format consisting of a 64-bit signed field representing nanoseconds multiplied by 216 (see IEEE 1588 clause 13.3.2.7). This timer should count from 0 through the full range up to 264 -1 before wrapping around.