This answer record discusses how to debug Clock and Data Recovery Problems.
Note: This article is part of the Xilinx HSSIO Solution Center (Xilinx Answer 37181).
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CDR (Clock and Data Recovery)
The CDR can be tested in the following ways:
- Transmit known characters and monitor the received data.
- If the 8B10B decoder is enabled, RXDISPERR and RXNOTINTABLE errors will be asserted.
- For Virtex-5, Virtex-6 GTX, CDR status can be monitored through Loss Of Sync state machine.
Possible causes of CDR problems:
- REFCLK is not of good quality.
- CDR is not properly configured. For 7 series, make sure that the RXCDR_CFG is configured for the protocol and ppm variation used. Check for design advisories for the level of silicon used.
For example (Xilinx Answer 51884) - Design Advisory for Kintex-7 and Virtex-7 FPGA GTX Production Silicon CDR Attribute Updates.
- Incoming data does not have an adequate eye opening.
- CDR is not locked.
- CDR problems can occur if an RX cable is unplugged and replugged; a reset is required for this issue.
- During long period of idle with the same pattern sent repetitively, the CDR can sometimes have difficulty.
Setting RXCDRHOLD during this period will avoid this issue.
Important specifications to consider:
- Runlength (CID) - If the incoming data has no transitions within the Max number of UI specified in this spec, CDR can lose lock.
A DC balanced signal is required for proper function.
Basic debug steps:
- Confirm that the REFCLK meets the phase noise mask requirements of the device.
- Check if the CDR is properly configured.
- If possible, probe the incoming data eye diagram.
For 7 Series, IBERT can help with monitoring the eye diagram of the RX data.
- The Eye Scan with MicroBlaze Processor MCS Application Note (XAPP743) shows how Eyescan can be integrated into an existing design and run without interrupting normal operation.
Ports to monitor:
- RXLOSSOFSYNC (not present with 7 Series)
Note: The port RXCDRLOCK is only a coarse indicator of CDR lock so should not be used solely as a CDR lock indicator, the incoming data should be monitored.
Dynamically Programmable DRU for High-Speed Serial I/O (XAPP875) - Non Integer Data Recovery Unit:
The NI-DRU extends the lower data rate limit to 0 Mb/s and the upper limit to 1,250 Mb/s, making embedded high-speed transceivers the ideal solution for true multi-rate serial interfaces.