Functional Description
919
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Ethernet Controller
15.3.6.4 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:
When the Ethernet Controller is configured to use the MII interface to an external PHY, the
MII clock is provided by the external PHY through EN0RXCK and EN0TXCK. When the
Ethernet Controller is connected to the integrated PHY, the reference clock must be 25 MHz
because it is also the source to the PHY.
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 approximately
4 MHz.
15.3.6.5 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 subsecond time in digital or binary format.
15.3.6.5.1 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.
shows the method to calculate the
propagation delay in clocks supporting peer-to-peer path correction.