184.108.40.206. Skew Matching Guidelines for DIMM Configurations
Board designers must observe the following guidelines for DDR4 DIMM 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 DDR4 DIMM topologies.
|DIMM Skew Matching Rule||Length in Time (ps)|
|Length matching between DQS and CLK||-255ps < CLK - DQS < 425ps|
|Length matching between DQ and DQS within byte||-3.5ps < DQ - DQS < 3.5ps|
|Length matching between DQS and DQS#||< 1ps|
|Length matching between CLK and CLK#||< 1ps|
|Length matching between CLK0 and CLK1||< 8ps|
|Length matching between CMD/ADDR/CTRL and CLK||-20ps < CLK - CMD/ADDR/CTRL < 20ps|
|Length matching among CMD/ADDR/CTRL within each channel||< 20ps|
|Include package length in skew matching for FPGA device with no migration||Required|
|Include package length in skew matching for FPGA device with migration||Not Recommended|
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