Electronic – What do terms half-rate and quarter-rate mean for DRAM memory controllers

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DDR3 Memory controller are often presented as being half rate or quarter rate, but what does this mean? e.g if I have a DDR3 SDRAM that is x16, what does the half or quarter rate imply in that case and why do we need to create such memory controllers?

Best Answer

From Altera's Using High-Performance DDR, DDR2, and DDR3 SDRAM With SOPC Builder:

Full- or half-rate SDRAM high-performance controllers have the following definitions:

  • Full-rate controllers present data to the local interface at twice the width of the actual SDRAM interface at the full SDRAM clock rate.
  • Half-rate controllers present data to the local interface at four times the width of the actual SDRAM interface at half the SDRAM clock rate.

Implementing the SDRAM high-performance controllers in half-rate mode gives the highest possible SDRAM clock frequency while allowing the more complex core logic to operate at half this frequency.

Basically, half-rate (and by extension, quarter-rate) application interfaces allow the SDRAM clock to be faster relative to your internal system clock. This improves both bandwidth and latency.

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