External Memory Interface Handbook Volume 3: Reference Material

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ID 683841
Date 7/24/2019
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4.5.2. Read Datapath

One PLL output is used to capture data from the memory during read operations. The clock phase is calibrated by the sequencer before the interface is ready for use.
Figure 137. Read Datapath


For DDR3 interfaces, two PLL outputs capture data from the memory devices during a read. In a 24-bit interface—whether the top 8 bits are used by ECC or not—the supported topology is two discrete DDR3 devices of 16-bit and 8-bit DQ each. Each discrete device has a dedicated capture clock output from the PLL.

For LPDDR2 interfaces, the supported configuration is a single memory device with memory width of 16-bit DQ. The other PLL output is used for DQS tracking purposes, because the tDQSCK drift might cause data capture to fail otherwise. The tracking clock is a shifted capture clock used to sample the DQS signal. By capturing the DQS signal, the system can compensate for DQS signal drift.

Figure 138. Read Datapath Timing Diagram


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