You may see excessive receiver latency when using the Low Latency or Native PHY, 10G PCS Basic mode on Stratix® V GX or Arria® V GZ devices under the following conditions:
Bit Slip is selected as the Word Alignment mode
The gearbox ratios are configured for 66:40, 64:32, or 50:40
For the transceiver PHY configurations above, the round-trip loopback latency may increase by 1-23 additional parallel clock cycles if the rx_bitslip port is toggled more than FPGA fabric interface width -1 times.
To work around this problem, you should not toggle the rx_bitslip port more than FPGA fabric interface width -1 times for the transceiver PHY configurations above.
Altera recommends separating rx_bitslip pulses by at least 20 parallel clock cycles to account for transceiver PCS pipeline latency.
An alternative workaround is to use the rx_clkslip function on the Native PHY.