Quad Data Rate (QDR) SDRAM is a type of computer memory, more specifically a type of synchronous dynamic random-access memory, that can transfer four words of data in each clock cycle. Like Double Data-Rate (DDR) SDRAM, QDR SDRAM transfers data on both rising and falling edges of the clock signal. Unlike DDR SDRAM, which has a single bidirectional read-write port, QDR has separate read and write ports that can operate simultaneously. This requires a larger number of wires from the memory device to the memory controller, but doubles the theoretical maximum data transfer rate. QDR SDRAM uses two clocks, one for read data and one for write data.

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Quad Data Rate SRAM Clocking Scheme Cypress
Quad Data Rate Description Electronic design Artical