External Memory Interface Handbook Volume 3: Reference Material

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

5.5.5. Soft Memory Interface to Hard Memory Interface Migration Guidelines

The following instructions provide information on mapping your soft memory interface to a hard memory interface.

Pin Connections

  1. The hard and soft memory controllers have compatible pinouts. Assign interface pins to the hard memory interface according to the pin table.
  2. Ensure that your soft memory interface pins can fit into the hard memory interface. The hard memory interface can support a maximum of a 40-bit interface with user ECC, or a maximum of 80-bits with same-side bonding. The soft memory interface does not support bonding.
  3. Follow the recommended board layout guidelines for the hard memory interface.

Software Interface Preparation

Observe the following points in preparing your soft memory interface for migration to a hard memory interface:

  • You cannot use the hard PHY without also using the hard memory controller.
  • The hard memory interface supports only full-rate controller mode.
  • Ensure that the MPFE data port width is set according to the soft memory interface half-rate mode Avalon data width.
  • The hard memory interface uses a different Avalon port signal naming convention than the software memory interface. Ensure that you change the avl_<signal_name> signals in the soft memory interface to .avl_<signal_name>_0 signals in the hard memory interface.
  • The hard memory controller MPFE includes an additional three clocks and three reset ports (CMD port, RFIFO port, and WFIFO port) that do not exist with the soft memory controller. You should connect the user logic clock signal to the MPFE clock port, and the user logic reset signal to the MPFE reset port.
  • In the soft memory interface, the half-rate afi_clk is a user logic clock. In the hard memory interface, afi_clk is a full-rate clock, because the core fabric might not be able to achieve full-rate speed. When you migrate your soft memory interface to a hard memory interface, you need to supply an additional slower rate clock. The maximum clock rate supported by core logic is one-half of the maximum interface frequency.

Latency

Overall, you should expect to see slightly more latency when using the hard memory controller and multi-port front end, than when using the soft memory controller.

The hard memory controller typically exhibits lower latency than the soft memory controller; however, the multi-port front end does introduce additional latency cycles due to FIFO buffer stages used for synchronization. The MPFE cannot be bypassed, even if only one port is needed.

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