Multiple gating modes and half-frequency dynamic calibration for ddr memory controllers

Abstract

Circuits and methods are described for a DDR memory controller where two different DQS gating modes are utilized. These gating modes together ensure that the DQS signal, driven by a DDR memory to the memory controller, is only available when read data is valid. Two types of gating logic are used: Initial DQS gating logic, and Functional DQS gating logic. The Initial gating logic has additional timing margin in the Initial DQS gating value to allow for the unknown round trip timing during initial bit levelling calibration. DQS functional gating is then optimized during further calibration to gate DQS precisely as latency and phase calibration are performed, resulting in a precise gating value for Functional DQS gating. Providing dual gating modes is especially useful when data capture is performed at half the DQS frequency in view of rising clock rates for DDR memories.