External Memory Interface Handbook Volume 2: Design Guidelines

ID 683385
Date 5/08/2017
Document Table of Contents

9.11.3. Read Capture Timing

In Stratix III and Stratix IV devices, read capture timing may fail if the DQS phase shift selected is not optimal or if the board skew specified is large.
  • You can adjust the effective DQS phase shift implemented by the DLL to balance setup and hold margins on the read timing path. The DQS phase shift can be adjusted in coarse PVT-compensated steps of 22.5°, 30°, 36°, or 45° by changing the number of delay buffers (range 1 to 4), or in fine steps using the DQS phase offset feature that supports uncompensated delay addition and subtraction in approximately 12 ps steps.
  • To adjust the coarse phase shift selection, determine the supported DLL modes for your chosen memory interface frequency by referencing the DLL and DQS Logic Block Specifications tables in the Switching Characteristics section of the device data sheet. For example, a 400 MHz DDR2 interface on a -2 speed grade device can use DLL mode 5 (resolution 36°, range 290 – 450 MHz) to implement a 72° phase shift, or DLL mode 6 (resolution 45°, range 360–560 MHz) to implement a 90° phase shift.
Note: Ensure that you specify the appropriate board‑skew parameter when you parameterize the controllers in the parameter editor. The default board trace length mismatch used is 20 ps.

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