Visible to Intel only — GUID: jqb1481130299286
Ixiasoft
Visible to Intel only — GUID: jqb1481130299286
Ixiasoft
17.6.4.6. Frequency Range of Reference Timing Clock
The timestamp information is transferred across asynchronous clock domains, from the EMAC clock domain to the FPGA clock domain. Therefore, a minimum delay is required between two consecutive timestamp captures. This delay is four PHY interface clock cycles and three PTP clock cycles. If the delay between two timestamp captures is less than this amount, the MAC does not take a timestamp snapshot for the second frame.
The maximum PTP clock frequency is limited by the maximum resolution of the reference time (20 ns resulting in 50 MHz) and the timing constraints achievable for logic operating on the PTP clock. In addition, the resolution, or granularity, of the reference time source determines the accuracy of the synchronization. Therefore, a higher PTP clock frequency gives better system performance.†
The minimum PTP clock frequency depends on the time required between two consecutive SFD bytes. Because the PHY interface clock frequency is fixed by the IEEE 1588 specification, the minimum PTP clock frequency required for proper operation depends on the operating mode and operating speed of the MAC.†
Mode |
Minimum Gap Between Two SFDs |
Minimum PTP Frequency |
---|---|---|
100‑Mbps full-duplex operation |
168 MII clocks (128 clocks for a 64-byte frame + 24 clocks of min IFG + 16 clocks of preamble) |
(3 * PTP) + (4 * MII) <= 168 * MII, that is, ~0.5 MHz (168 – 4) * 40 ns ÷ 3 = 2180 ns period |
1000-Mbps half duplex operation |
24 GMII clocks (4 for a jam pattern sent just after SFD because of collision + 12 IFG + 8 preamble) |
(3 * PTP) + 4 * GMII <= 24 * GMII, that is, 18.75 MHz |