External Memory Interface Handbook Volume 2: Design Guidelines

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ID 683385
Date 5/08/2017
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Document Table of Contents

7.3.4.3. Board Skews for UniPHY IP

PCB traces can have skews between them that can reduce timing margins. Furthermore, skews between different chip selects can further reduce the timing margin in multiple chip-select topologies.

The Board Skews section of the parameter editor allows you to enter parameters to compensate for these variations.

Note: You must ensure the timing margin reported in TimeQuest Report DDR is positive when the board skew parameters are correct for the PCB.

The following tables list the board skew parameters. For parameter equations containing delay values, delays should be measured as follows:

  • Non-fly-by topology (Balanced Tree)
    • For discrete devices–all the delay (CK, Addr/Cmd, DQ and DQS) from the FPGA are right to every memory device
    • For UDIMMs–all the delay (CK, Addr/Cmd, DQ and DQS) from the FPGA to UDIMM connector for every memory device on the UDIMM. If UDIMM delay information is available, calculate delays to every memory device on the UDIMM.
    • For RDIMMs–the Addr/Cmd and CK delay are from the FPGA to the register on the RDIMM. The DQ and DQS delay are from FPGA to RDIMM connector for every memory device on the RDIMM.
    • For LRDIMMS–the delay from the FPGA to the register on the LRDIMM.
  • Fly-by topology
    • For discrete devices–the Addr/Cmd and CK delay are from the FPGA to the first memory device. The DQ and DQS delay are from FPGA to every memory device.
    • For UDIMMs–the Addr/Cmd and CK delay are from the FPGA to the UDIMM connector. The DQ and DQS delay are from the FPGA to UDIMM connector for every memory device on the UDIMM.
    • For RDIMMs–the Addr/Cmd and CK delay are from the FPGA to the register on the RDIMM. The DQ and DQS delay are from FPGA to RDIMM connector for every memory device on the RDIMM.
    • For LRDIMMS–the delay from the FPGA to the buffer on the LRDIMM.

Equations apply to any given memory device, except when marked by the board or group qualifiers (_b or _g), where they apply to the particular device or group being iterated over.

Use the Board Skew Parameter Tool to help you calculate the board skews.

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