External Memory Interface Handbook Volume 2: Design Guidelines

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

1.1.2. DDR, DDR2, DDR3, and DDR4 SDRAM Clock Signals

DDR, DDR2, DDR3, and DDR4 SDRAM devices use CK and CK# signals to clock the address and command signals into the memory. Furthermore, the memory uses these clock signals to generate the DQS signal during a read through the DLL inside the memory. The SDRAM data sheet specifies the following timings:
  • tDQSCK is the skew between the CK or CK# signals and the SDRAM-generated DQS signal
  • tDSH is the DQS falling edge from CK rising edge hold time
  • tDSS is the DQS falling edge from CK rising edge setup time
  • tDQSS is the positive DQS latching edge to CK rising edge

SDRAM have a write requirement (tDQSS) that states the positive edge of the DQS signal on writes must be within ± 25% (± 90°) of the positive edge of the SDRAM clock input. Therefore, you should generate the CK and CK# signals using the DDR registers in the IOE to match with the DQS signal and reduce any variations across process, voltage, and temperature. The positive edge of the SDRAM clock, CK, is aligned with the DQS write to satisfy tDQSS.

DDR3 SDRAM can use a daisy-chained control address command (CAC) topology, in which the memory clock must arrive at each chip at a different time. To compensate for the flight-time skew between devices when using the CAC topology, you should employ write leveling.

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