External Memory Interface Handbook Volume 3: Reference Material

ID 683841
Date 7/24/2019
Document Table of Contents

1.6. Read Datapath

The read datapath passes read data from memory to the PHY. The following figure shows the blocks and flow in the read datapath.

For all protocols, the DQS logic block delays the strobe by 90 degrees to center-align the rising strobe edge within the data window. For DDR2, DDR3, and LPDDR2 protocols, the logic block also performs strobe gating, holding the DQS enable signal high for the entire period that data is received. One DQS logic block exists for each data group.

One VFIFO buffer exists for each data group. For DDR2, DDR3, and LPDDR2 protocols, the VFIFO buffer generates the DQS enable signal, which is delayed (by an amount determined during calibration) to align with the incoming DQS signal. For QDR and RLDRAM protocols, the output of the VFIFO buffer serves as the write enable signal for the Read FIFO buffer, signaling when to begin capturing data.

DDIO_IN receives data from memory at double-data rate and passes data on to the Read FIFO buffer at single-data rate.

The Read FIFO buffer temporarily holds data read from memory; one Read FIFO buffer exists for each data group. For half-rate interfaces, the Read FIFO buffer converts the full-rate, single data-rate input to a half-rate, single data-rate output which is then passed to the PHY core logic. In the case of a quarter-rate interface, soft logic in the PHY performs an additional conversion from half-rate single data rate to quarter-rate single data rate.

One LFIFO buffer exists for each memory interface; the LFIFO buffer generates the read enable signal for all Read FIFO blocks in an interface. The read enable signal is asserted when the Read FIFO blocks have buffered sufficient data from the memory to be read. The timing of the read enable signal is determined during calibration.

Figure 5. Read Datapath

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