Synchronous memory read data capture

Abstract

A method of snap-shot data training to determine the optimum timing of the DQS enable signal in a single read operation is provided. This is accomplished by first writing a Gray code count sequence into the memory and then reading it back in a single burst. The controller samples the read burst at a fixed interval from the time the command was issued to determine the loop-around delay. A simple truth table lookup determines the optimum DQS enable timing for normal reads. Advantageously, during normal read operations, the first positive edge of the enabled DQS signal is used to sample a counter that is enabled every time a command is issued. If the counter sample changes, indicating timing drift has occurred, the DQS enable signal can be adjusted to compensate for the drift and maintain a position centered in the DQS preamble. This technique can also be applied to a system that uses the iterative approach to determining DQS enable timing on power up. Another embodiment of the invention is a simple, low latency clock domain crossing circuit based on the DQS latched sample of the counter.