External Memory Interface Handbook Volume 3: Reference Material

ID 683841
Date 7/24/2019
Public
Document Table of Contents

1.17.7. Stage 4: Read Calibration Part Two—Read Latency Minimization

At this stage of calibration the sequencer adjusts LFIFO latency to determine the minimum read latency that guarantees correct reads.

Read Latency Tuning

In general, DQ signals from different DQ groups may arrive at the FPGA in a staggered fashion. In a DIMM or multiple memory device system, the DQ/DQS signals from the first memory device arrive sooner, while the DQ/DQS signals from the last memory device arrive the latest at the FPGA.

LFIFO transfers data from the capture registers in IOE to the core and aligns read data to the AFI clock. Up to this point in the calibration process, the read latency has been a maximum value set initially by LFIFO; now, the sequencer progressively lowers the read latency until the data can no longer be transferred reliably. The sequencer then increases the latency by one cycle to return to a working value and adds an additional cycle of margin to assure reliable reads.

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