DP83640_10 Datasheet, PDF (1/4 Pages) National Semiconductor (TI) – Synchronizing a DP83640 PTP Master to a GPS Receiver

Synchronizing a DP83640
PTP Master to a GPS
Receiver
National Semiconductor
Application Note 2006
Patrick O'Farrell
May 27, 2010
1.0 Introduction
The IEEE 1588 Precision Time Protocol (PTP) provides a
means of synchronizing the time between multiple nodes us-
ing standard Ethernet connections. In many applications,
telecommunications in particular, synchronization of the local
time base is essential to system performance. In these appli-
cations, a GPS receiver is typically used to synchronize re-
mote sites. This method, however, is expensive due to the
cost of the providing a GPS receiver at each remote site. An
alternate method is to utilize a single GPS receiver as the time
base for an IEEE 1588 Master and synchronize the remote
sites using the Ethernet and the IEEE 1588 Precision Time
Protocol.
This application note discusses methods of synchronizing the
PTP clock to a GPS receiver using the DP83640 Precision
PHYTER®.
2.0 Synchronization Methods
Synchronization can be accomplished using a DP83640 clock
reference that is independent of the GPS receiver clock or
that is dependent upon the GPS receiver clock source. The
DP83640 reference clock for the Pulse Per Second Synchro-
nization described below is independent of the GPS reciever.
The Direct Reference Connection simplifies the control loop
by taking advantage of the GPS receiver output clock.
2.1 Pulse Per Second Synchronization
The Pulse Per Second Synchronization method, shown in
Figure 1, assumes that the GPS receiver is operating as a
valid time reference.
A general outline of the steps required to synchronize the
Precision Time Protocol clock to the GPS clock is as follows:
1. Initialize the Precision Time Protocol (PTP) clock in the
DP83640.
2. Configure a GPIO as an event monitor to detect the rising
edge of the PPS output from the GPS receiver.
3. Upon detection of the first PPS event,
A. Capture the timestamp of the event.
B. Read the time from the GPS unit.
C. Calculate the difference between the GPS time and
the PTP time and use a step adjustment to correct
the PTP time.
4. Since the PPS output of the GPS receiver will occur at 1-
second intervals, it should not be necessary to poll the
GPS receiver for the time with each captured timestamp.
However, to ensure continued accuracy of the system,
the GPS and PTP times should be checked and verified
periodically.
NOTE: The initial PPS event captured will most likely result
in a large (> 1 second) delta between the GPS time and the
local time of the DP83640. After the initial time adjustment,
subsequent timestamps should be within a 1 second error and
allow for temporary rate adjustments to further adjust the time
of the PTP clock.
With this configuration, the 25 MHz reference used for the
DP83640 will not track the reference of the GPS source ex-
actly. As a result, the PTP clock will require rate corrections
to compensate for differences in the reference clock rates.
The rate correction can be calculated based on the difference
in time stamps between sequential PPS edges.
In the case where the 25 MHz reference to the DP83640 is a
voltage-controlled oscillator, it is possible to adjust the oscil-
lator frequency based on the needed rate correction as an
alternative to performing the rate correction to the PTP clock.
In this instance, care should be taken in selecting a D/A con-
verter that will provide sufficient voltage resolution to meet the
system clock accuracy requirements.
30106501
FIGURE 1. Block diagram using 25 MHz reference not connected to the GPS source
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