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RTC Time Stamp Accuracy
Time Stamp Accuracy
The accuracy of the time stamps, taken by sampling the
whenever PTP frames are transmitted or received, is essential to the Precise Timing
Protocol across the network link. For this reason, the time stamps are performed in
hardware. Despite this, time stamp inaccuracies can be introduced from two sources:
•
•
Following this discussion, we then consider the
Errors."
RTC Real Time Instantaneous Error
Figure A-1
source (this is worst case). Therefore, the controlled frequency RTC will only be updated
every 40ns. Because the concept of a RTC is a continuous measurement of time, the
implementation of the RTC illustrated in
update. During the 40 ns update cycle, the error accrues linearly to a maximum of 40 ns.
This behavior is periodic as illustrated.
In
is variable. For example:
•
•
Ethernet AVB Endpoint User Guide
UG492 September 21, 2010
"RTC Real Time Instantaneous Error"
"RTC Sampling Error"
illustrates a RTC implementation which uses a 40 ns clock period as its clock
Figure
A-1, two time stamps of the RTC are sampled. The figure shows that the accuracy
The 1st time stamp is requested at 119 ns. However, the RTC has yet to update and so
the sample taken will be of 80 ns. This has an inaccuracy of 39 ns.
The 2nd time stamp is requested at 201 ns. The RTC has recently updated and so the
sample taken will be of 200. This has an inaccuracy of 1 ns.
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Appendix A
"Real Time Clock"
"Accuracy Resulting from the Combined
Figure A-1
is only accurate immediately after an
(RTC)
167
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