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Timestamps
Known by Delay
Requester
t
AB
t
BA
t
1
t
4
t
2
t
3
t
1
Pdelay_Resp_Follow_Up:
t
2
, t
3
Pdelay_Resp
Pdelay_Req
t
1
, t
4
t
1
, t
2
, t
3
, t
4
P2P TCA
Delay Requester
Time
P2P TCB
Delay Requester
Time
Functional Description
920
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Ethernet Controller
Figure 15-12. Propagation Delay Calculation in Clocks Supporting Peer-to-Peer Path Correction
As shown in
, the propagation delay is calculated in the following way:
1. Port-1 issues a Pdelay_Req message and generates a timestamp, t1, for the Pdelay_Req message.
2. Port-2 receives the Pdelay_Req message and generates a timestamp, t2, for this message.
3. Port-2 returns a Pdelay_Resp message and generates a timestamp, t3, for this message.
To minimize errors because of any frequency offset between the two ports, Port-2 returns the
Pdelay_Resp message as quickly as possible after the receipt of the Pdelay_Req message. The Port-2
returns any one of the following:
a. The difference between the timestamps t2 and t3 in the Pdelay_Resp message.
b. The difference between the timestamps t2 and t3 in the Pdelay_Resp_Follow_Up message.
c. The timestamps t2 and t3 in the Pdelay_Resp and Pdelay_Resp_Follow_Up messages
respectively.
4. Port-1 generates a timestamp, t4, on receiving the Pdelay_Resp message.
5. Port-1 uses all four timestamps to compute the mean link delay.
15.3.6.5.2 Advanced Timestamp Supported Clock Types
The Advance Timestamp Module supports an ordinary clock as defined by the IEEE 1588-2008 standard.
The characteristics of this clock is as follows:
•
Sending and receiving of PTP messages. The timestamp snapshot can be controlled through the
Ethernet MAC Timestamp Control (EMACTIMSTCTRL) register, offset 0x700. Timestamp snapshots
can be for Sync messages.
•
Maintaining the data sets such as timestamp values.
For an ordinary clock, snapshots can be taken of either version 1 or version 2 PTP types but not both.
Selecting between the two is controlled by the PTPVER2 bit of the EMACTIMSTCTRL register.