External Memory Interface Handbook Volume 3: Reference Material

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

6.6.3. MPFE SDRAM Burst Scheduling

SDRAM burst scheduling recognizes addresses that access the same row/bank combination, known as open page accesses. Operations to a page are served in the order in which they are received by the single-port controller. Selection of SDRAM operations is a two-stage process. First, each pending transaction must wait for its timers to be eligible for execution. Next, the transaction arbitrates against other transactions that are also eligible for execution.

The following rules govern transaction arbitration:

  • High-priority operations take precedence over lower-priority operations
  • If multiple operations are in arbitration, read operations have precedence over write operations
  • If multiple operations still exist, the oldest is served first

A high-priority transaction in the SDRAM burst scheduler wins arbitration for that bank immediately if the bank is idle and the high-priority transaction's chip select, row, or column fields of the address do not match an address already in the single-port controller. If the bank is not idle, other operations to that bank yield until the high-priority operation is finished. If the chip select, row, and column fields match an earlier transaction, the high-priority transaction yields until the earlier transaction is completed.

Clocking

The FPGA fabric ports of the MPFE can be clocked at different frequencies. Synchronization is maintained by clock-domain crossing logic in the MPFE. Command ports can operate on different clock domains, but the data ports associated with a given command port must be attached to the same clock as that command port.

Note: A command port paired with a read and write port to form an Avalon‑MM interface must operate at the same clock frequency as the data ports associated with it.

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