ContributorWhat would happen if Transceiver Calibration fails?
Hi
We use Arial10 10AX057H3F34E2SG for both PCIe and none-PCIe XCVRs. Some questions about XCVR calibrations.
What would happen if Transceiver Calibration fails?
As Intel UG-01143 chapter 7 (page575) writes, "Transceivers include both analog and digital blocks that require calibration to compensate for process, voltage, and temperature (PVT) variations".
For example, Suppose XCVR reference clock is not ready when A10 is powered up. So this XCVR is not successfully Calibrated. For now POWER-ON-Calibration Failed.
Under this situation, will XCVR cannot run at all? Or it can still run with de-rated performance (worse than it should be), such as lower speed, more bit errors, etc. I guess it should be (b), but not officially confirmed.
Suppose after a while, the XCVR reference clock is available, for None-PCIe XCVRs, user indeed could start the “re-calibration” sequence, and then XCVR could normally work. Unfortunately, for PCIe XCVRs, since Altera said "PCIe link does not allow user recalibration"(UG-01143 section 7.3), is the PCIe XCVRs DEAD now? What can we do to make it work again?
To be honest, this RE-Calibration mechanism seems to be kind of Altera-Special feature. It requires stable and free-running external CLKUSR and XCVR ref clocks BEFORE FPGA powered up. No similar requirement in Xilinx competition devices such as XCKU060, which only requires calibration resistors correctly connected for proper XCVR calibration, and this means simpler hardware design.
One of the possible reason of this is said to be: Altera XCVR’s fPLL do not have enough driving strength than Xilinx’s does. We asked our supplier Cytech (one representative of intel FPGA), but received no reply.
This is why I had to post my questions and confusions here. Anyway, bleakly hope some real expert could help on this desperated topic. Thanks, and happy weekend.
- Hie Yi Xiao, Please check my replies to your questions. Do let me know if you understand my explanation and need further clarification. But after JTAG configuration is done, what can we do to re-establish and re-enumerate the PCIe connection? You only require transceiver calibration to be successful once for every power cycle of the FPGA. Hence,if you perform configuration via JTAG and PCIe refclk is available, your power up calibration will start and complete successfully. After this you do not need the power up calibration for every re-enumeration. Hence, to re-establish and re-enumerate PCIe connection; you could use any other traditional method such as warm reboot (for windows) or lspci command (for linux). Maybe we would warm-restart PC to tigger re-enumerate, but when during PC shutdown and restart interval, the PCIe_REF_CLK will surely disappear, then FPGA XCVR might stop working as a result of lost REFCLK…..it seem to be a desperate loop….or maybe my thought is wrong somewhere. How do you think? We will appreciate your insights. Actually, lost of REFCLK is not going to cost the FPGA to behave incorrectly. As long as power up calibration is successful; ON and OFF of refclk (with same PPM and phase jitter - from same source) can be accepted by FPGA PLL without performance degradation. When windows PC triggers ON and OFF of refclk; it only changes the refclk phase for every cycle; having little impact on the phase jitter and no impact on the PPM or frequency. Hence, as long as FPGA is not power cycles, it can accept the ON and OFF of refclk from same source. If FPGA is power cycled, you need to run power-up calibration again. For windows restart, the FPGA is not power cycled; hence FPGA is not required to be re-configured via JTAG; also don't require power-up calibration. Windows restart will momentarily stop and restart REFCLK and re-trigger PRSTn. However, since FPGA not re-configured or power cycled, it does not need to be re-calibrated. FPGA can work as normal based on the last power up calibration. If windows restart is performed with FPGA re-configuration, then a new power up calibration is required. As for windows shutdown, if the FPGA is powered from PCIe slot; then the FPGA experiences a power cycle and required new configuration. This will require a new power up calibration. Hence, the whole process repeats. However, if FPGA powered up externally, then you don't need to reconfigure FPGA. When windows is restarted, the FPGA still can behave normally. Hence, this is the behavior of FPGA in respective to windows restart and shutdown. Regards, Nathan
