External Memory Interface Handbook Volume 2: Design Guidelines

ID 683385
Date 5/08/2017
Document Table of Contents Read Timing Margin

Assess the read timing margin by using sources and probes to manually control the DLL phase offset feature.

Open the autogenerated DLL using ALT_DLL and add the additionally required offset control ports. This action allows control and observation of the following signals:

dll_delayctrlout[5:0], // Phase output control from DLL to DQS pins (Gray Coded)
dll_offset_ctrl_a_addnsub, // Input add or subtract the phase offset value
dll_offset_ctrl_a_offset[5:0], // User Input controlled DLL offset value (Gray Coded)
dll_aload, // User Input DLL load command
dll_dqsupdate, // DLL Output update required signal.

In examples where the applied offset applied results in the maximum or minimum dll_delayctrlout[5:0] setting without reaching the end of the read capture window, regenerate the DLL in the next available phase setting, so that the full capture window is assessed.

Modify the example driver to constantly perform reads (mask writes). Observe the pnf_per_byte signals while the DLL capture phase is manually modified to see when failures begin, which indicates the edge of the window.

A resynchronization timing failure can indicate failure at that capture phase, and not a capture failure. You should recalibrate the PHY with the calculated phase offset to ensure that you are using the true read-capture margin.

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