20.3.5.4 Timestamp Error Margin
According to the IEEE 1588 specifications, a timestamp must be captured at the SFD of the
transmitted and received frames at the MAC interface. Because the reference timing source, MOSC,
is taken as different from MAC reference clocks, a small error margin is introduced, because of the
transfer of information across asynchronous clock domains. In the transmit path, the captured and
reported timestamp has a maximum error margin of 2 PTP (MOSC) clocks. It means that the captured
timestamp has the reference timing source (MOSC) value that is given within 2 clocks after the SFD
has been transmitted to the PHY. Similarly, in the receive path, the error margin is 3 MAC reference
clocks, plus up to 2 PTP clocks. The error margin of the three MAC reference clocks can be ignored
by assuming that this constant delay is present in the system (or link) before the SFD data reaches
the interface of MAC.
Note: The reference clock to the integrated PHY must be 25 MHz.
Note: When IEEE 1588 timestamping is enabled with internal timestamp, use a PTP clock
frequency that is greater than 5 MHz. This is because the SSINC field in the
EMACSUBSECINC register limits the PTP frequency that can be used to ~4 MHz.
20.3.5.5 IEEE 1588-2008 Advanced Timestamps
In addition to the basic timestamp features mentioned in IEEE 1588-2002 Timestamps, the Ethernet
Controller supports the following advanced timestamp features defined in the IEEE 1588-2008
standard:
■ Supports the IEEE 1588-2008 (version 2) timestamp format.
■ Provides an option to take snapshot of all frames or only PTP type frames.
■ Provides an option to take snapshot of only event messages.
■ Provides an option to take the snapshot based on the clock type: ordinary, boundary, end-to-end,
and peer-to-peer.
■ Provides an option to select the node to be a master or slave clock.
■ Identifies the PTP message type, version, and PTP payload in frames sent directly over Ethernet
and sends the status.
■ Provides an option to measure sub-second time in digital or binary format.
Peer-to-Peer Transparent Clock Message Support
The IEEE 1588-2008 version supports Peer-to-Peer PTP (Pdelay) message in addition to SYNC,
Delay Request, Follow-up, and Delay Response messages. Figure 20-10 on page 1446 shows the
method to calculate the propagation delay in clocks supporting peer-to-peer path correction.
1445June 18, 2014
Texas Instruments-Production Data
Tiva
™
TM4C1294NCPDT Microcontroller
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