External Memory Interface Handbook Volume 3: Reference Material

ID 683841
Date 7/24/2019
Document Table of Contents Centering DQ/DQS

The centering DQ/DQS stage attempts to align DQ and DQS signals on reads within a group. Each DQ signal within a DQS group might be skewed and consequently arrive at the FPGA at a different time. At this point, the sequencer sweeps each DQ signal in a DQ group to align them, by adjusting DQ input delay chains (D1).

The following figure illustrates a four DQ/DQS group per-bit-deskew and centering.

Figure 28. Per-bit Deskew

To align and center DQ and DQS, the sequencer finds the right edge of DQ signals with respect to DQS by sweeping DQ signals within a DQ group to the right until a failure occurs. In the above figure, DQ0 and DQ3 fail after six taps to the right; DQ1 and DQ2 fail after 5 taps to the right. To align the DQ signals, DQ0 and DQ3 are shifted to the right by 1 tap.

To find the center of DVW, the DQS signal is shifted to the right until a failure occurs. In the above figure, a failure occurs after 3 taps, meaning that there are 5 taps to the right edge and 3 taps to the left edge. To center-align DQ and DQS, the sequencer shifts the aligned DQ signal by 1 more tap to the right.

Note: The sequencer does not adjust DQS directly; instead, the sequencer center-aligns DQS with respect to DQ by delaying the DQ signals.

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