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Ethernet AVB Endpoint User Guide www.xilinx.com 171
UG492 July 23, 2010
Time Stamp Accuracy
Accuracy Resulting from the Combined Errors
The section “RTC Real Time Instantaneous Error” describes how a maximum error of one
RTC reference clock period can result as a consequence of the RTC itself. The section “RTC
Sampling Error” describes how the position of the time stamp request, as observed in the
RTC reference clock domain, can result in one RTC reference clock period of uncertainty.
Figure A-4 attempts to illustrate the result of the combination of these two types of error.
Again, the worst case clock period of 40 ns is illustrated.
In Figure A-4, two time stamps of the RTC are sampled. The figure shows that the accuracy
is variable. For example:
• The request for the 1st time stamp is made at 60 ns. Because the time to the next RTC
reference clock is 20 ns, this will not violate the setup time for the 1st synchronization
flip-flop in Figure A-2. Therefore, on the next RTC reference clock, the sample will be
taken as 40 ns (resulting in an error of 20 ns which is entirely due to the “RTC Real
Time Instantaneous Error”).
• The request for the 2nd time stamp is made at 239 ns. This is very close to the rising
edge of the 1st synchronization flip-flop in Figure A-2, so the situation is
unpredictable:
X-Ref Target - Figure A-4
Figure A-4: Overall Time Stamp Accuracy
Time (ns)
0
RTC Error (ns)
-40
40 80 120 160 200 240
0 40 80 120 160 200 240RTC
Sampling
uncertainty
sample RTC value as 240
sample RTC value as 200
request sample at time 239
re
quest sample at time 60
sample RTC value as 40
Timing Case 1 Timing Case 2