External Memory Interface Handbook Volume 2: Design Guidelines

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

1.4.2.1. Timing Impact on x36 Emulation

With ×36 emulation, the CQ/CQn signals are split on the board, so these signals see two loads (to the two FPGA pins)—the DQ signals still only have one load. The difference in loading gives some slew rate degradation, and a later CQ/CQn arrival time at the FPGA pin.

The slew rate degradation factor is taken into account during timing analysis when you indicate in the UniPHY Preset Editor that you are using ×36 emulation mode. However, you must determine the difference in CQ/CQn arrival time as it is highly dependent on your board topology.

The slew rate degradation factor for ×36 emulation assumes that CQ/CQn has a slower slew rate than a regular ×36 interface. The slew rate degradation is assumed not to be more than 500 ps (from 10% to 90% VCCIO swing). You may also modify your board termination resistor to improve the slew rate of the ×36-emulated CQ/CQn signals. If your modified board does not have any slew rate degradation, you do not need to enable the ×36 emulation timing in the UniPHY-based controller parameter editor.

For more information about how to determine the CQ/CQn arrival time skew, refer to Determining the CQ/CQn Arrival Time Skew.

Because of this effect, the maximum frequency supported using x36 emulation is lower than the maximum frequency supported using a native x36 DQS group.

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