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Zynq-7000 AP SoC Technical Reference Manual www.xilinx.com 522
UG585 (v1.11) September 27, 2016
Chapter 16: Gigabit Ethernet Controller
Note: Since the PTP message first travels through the external PHY before being time stamped
at the MII interface by the hardware, for calculating the exact time stamp for the received packet,
the delay introduced by the external PHY must be subtracted from the hardware reported time
stamp to reach at the exact time stamp.
8. Read the time stamp for the PDelay_Req frame received at the slave (peer). The master
validates the received PDelay_Resp for correct sequenceId which should be the same as that for
the PDelay_Req frame sent by the master. Similarly the master validates the PTP message body
field requestingPortIdentity for the correct value which should be same as the sourcePortIdentity
of the master. The master reads the PTP message body field requestReceiptTimestamp and stores
it. This is the timestamp when the slave (peer) received the PDelay_Req packet. Let this time
stamp be t2.
9. Process the received PDelay_Resp_Follow_Up frame. The Master receives a
PDelay_Resp_Follow_Up frame from the slave (peer) clock port. This is a general PTP message and
hence no PTP event interrupt is generated. The master validates for sequenceId and
requestingPortIdentity fields as described above. The master reads the PTP message body field
responseOriginTimestamp to get the exact timestamp when the last received PDelay_Resp
message was transmitted from the slave (peer). Let the time stamp be t3.
10. Calculate the peer delay. The master calculates the peer delay as (t4-t1) - (t3-t2). The calculated
peer delay is not used by the master; however every node on a PTP network should maintain
peer delays with other PTP peers on the network.
X-Ref Target - Figure 16-5
Figure 16-5: Link Delay Measurement
Port-1
Time
Port-2
Time
Pdelay_Req
Pdelay_Resp
Pdelay_Resp_Follow_Up
UG585_c16_08_100512
t
2
t
1
t
sm
t
ms
t
3
T
4