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172 www.xilinx.com Ethernet AVB Endpoint User Guide
UG492 July 23, 2010
Appendix A: RTC Time Stamp Accuracy
♦ If the flip-flop samples the new value, then Timing Case 1 results. The RTC is
sampled as 200 (resulting in an error of 39 ns which is entirely due to the “RTC
Real Time Instantaneous Error”).
♦ If the flip-flop samples the old value, then Timing Case 2 results. The RTC is
sampled 1 RTC reference clock period later as 240 (resulting in an error of only 1
ns).
Hopefully these examples have illustrated that the timing uncertainty in the asynchronous
sampling circuit has not resulted in any additional error.
The maximum inaccuracy, per time stamp sample, is still equal to the period of the RTC
reference clock (in this example 40 ns). By using a high frequency RTC reference clock, a
high degree of accuracy can be obtained. For example, when using a 125 MHz clock source
for the RTC, the maximum time stamp error will be 8 ns or less.