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?
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.