18.104.22.168. Skew Matching Guidelines for QDR-IV Configurations
Board designers must observe the following guidelines for QDR-IV skew matching:
- Perform skew matching in time (picoseconds) rather than in actual trace length, to better account for via delays when signals are routed on different layers.
- Include both package per-pin skew and PCB delay when performing skew matching.
- Skew (length) matching for the alert signal is not required.
The following table provides skew matching guidelines for QDR-IV topologies.
|QDR-IV Length Matching Rules||Length Matching in Time (ps)|
|Length Matching between DK/QK and CLK||-50 < CLK - DK/QK < 50ps|
|Length Matching between DQ and DK/QK within byte||-5ps < DQ - DK/QK < 5ps|
|Length matching between DK/QK and DK/QK#||< 1 ps|
|Length matching between DK and QK pairs||< 5 ps|
|Length matching between CLK and CLK#||< 1 ps|
|Length Matching between CMD and CTRL and Clock||-20 ps < CLK - CMD < 20 ps|
|Length matching among CMD and CTRL within each channel||< 20 ps|
|Include package length in skew matching for FPGA device with no migration||Required|
|Include package length in skew matching for FPGA device with migration when all package net length are available||It is recommended to use the final migrated package net length|
|Include package length in skew matching for FPGA device with migration when all package net length are not available||Not recommended|
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