7.1.1.2 Skew Constraints For Signal Groups

1. The address and command groups must be skew matched with respect to CK signals within ±100 ps.
2. The DQ, DM signals must be skew matched with respect to DQS within ±25 ps.
3. The read DQ/DQS training adjusts the DQS signal to optimal sampling point based on the board skew between DQ and DQS.
4. The write level training ensures that the tDQSS specification for memory is matched for each of the DQ/DQS byte groups and offset the skew introduced due to fly-by routing.

The characteristic impedance of 40Ω–50Ω must be used for single-ended signals and 90Ω–100Ω for differential signals.