GTS Transceiver PHY User Guide: Agilex™ 3 FPGAs and SoCs
2.6.4. Datapath Clock Cadences
- Standard cadence: Use if the read and write frequencies of the PMA FIFO interface are the same with 0 ppm frequency delta.
- Custom cadence: Use if the read and write frequencies of the PMA FIFO interface have different frequencies or have the same frequency but with a frequency delta of greater than 0 ppm.
Datapath Clocking Mode | Configuration | Datapath Clock Frequency | Cadence |
---|---|---|---|
PMA clocking mode (maximum 1 GHz) |
PMA Direct | Datapath clock frequency = PMA clock frequency PMA clock frequency = line rate/PMA width |
Use the standard cadence on the TX and RX (data is valid at every clock edge). 10 |
System PLL clocking mode (maximum 1 GHz) |
PMA Direct | Use Case A: Chip-to-chip applications where the PMA channel and link partner share the same reference clock Datapath clock frequency ≥ (system PLL output frequency)min where (system PLL output frequency)min = PMA clock frequency |
If (system PLL output frequency = PMA clock frequency and ∆ppm = 0), use the standard cadence on the TX and RX (data is valid at every clock edge). Otherwise, use custom cadence. 11 , 12 |
Use Case B: Applications where the PMA channel and link partner do not share the same reference clock Datapath clock frequency ≥ (system PLL output frequency)min where (system PLL output frequency)min = (maximum ppm 13 ÷ 1000000 + 1) × PMA clock frequency |
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System PLL clocking mode (maximum 1 GHz) |
Other configurations with FEC, PCS, and MAC | Datapath clock frequency ≥ (system PLL output frequency)min where (system PLL output frequency)min = PMA clock frequency For example, for 10GbE-1, use ≥ 322.265625 MHz. |
If (system PLL output frequency = PMA clock frequency), use the standard cadence on the TX and RX (data is valid at every 32 of 33 or 34 clock cycles). Otherwise, use custom cadence. 14 |
Refer to Supported PMA Data Widths and Date Rates for supported data rates.
Example of 10 Gbps Ethernet with MAC and PCS Blocks Using Overclocked System Clocking Mode
- The blocks of the core interface FIFO, Ethernet hard IP MAC and PCS, and the PMA interface FIFO are clocked by the system PLL.
- On the transmitter, the TX PMA interface FIFO performs a clock transfer from the system PLL domain to the TX PMA clock domain.
- On the receiver, the RX PMA interface FIFO performs a clock transfer from the RX recovered clock domain to the system PLL domain.
- Because the system PLL clock frequency is faster than the PMA clock frequency, datapath clocking is overclocked. Therefore, you must use custom cadence.
maximum ppm = maximum ∆ppm ÷ 2
maximum ∆ppm = max(∆ppm between the link partner TX (the recovered clock on the local RX) and system PLL, ∆ppm between the system PLL and TX PMA)