18.104.22.168. Skew Matching Guidelines for DDR4 Discrete Configurations
Observe the following rules when skew matching DDR4 discrete configurations:
- 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 down-memory topologies.
|DDR4 Device-down Length Matching Rules||Length Matching in Time (ps)|
|Skew matching between DQS and CLK||-85ps < CLK - DQS < 935ps|
|Skew matching between DQ and DQS within byte||-3.5ps < DQ - DQS < 3.5ps|
|Skew matching between DQS and DQS#||< 1ps|
|Skew matching between CLK and CLK#||< 1ps|
|Skew matching between CMD/ADDR/CTRL and Clock||-20ps < CLK - CMD/ADDR/CTRL < 20ps|
|Skew 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 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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