External Memory Interface Handbook Volume 2: Design Guidelines

ID 683385
Date 5/08/2017
Document Table of Contents

11.2.5. Command Queue Look-Ahead Depth

The command queue look-ahead depth value determines the number of read or write requests that the look-ahead bank management logic examines. The command queue look-ahead depth value also determines how many open pages the High-Performance Controller II (HPC II) can track.

For example, if you set the command queue look-ahead depth value to 4, the HPC II controller can track 4 open pages. In a 4-bank interleaving case, HPCII will receive repeated commands with addresses of bank A, bank B, bank C, and bank D. To receive the next set of commands, the controller issues a precharge command to exit the current page and then issues an activate command to track the new incoming page, leading to a drop in efficiency.

Figure 107. Simulation with Command Queue Look-ahead Depth of 4

With the command queue look-ahead depth set to 8, the controller can track 8 open pages and overall efficiency is much improved relative to a command queue look-ahead value of 4.

Figure 108. Simulation with Command Queue Look-ahead Depth of 8

There is a trade-off between efficiency and resource usage. Higher command queue look-ahead values are likely to increase bank management efficiency, but at the cost of higher resource usage. Smaller command queue look-ahead values may be less efficient, but also consume fewer resources. Also, a command queue look-ahead value greater than 4 may cause timing violations for interfaces approaching their maximum frequency.

Note: If you set Command Queue Look-ahead depth to a value greater than 4, you may not be able to run the interface at maximum frequency.

To achieve an optimized balance of controller efficiency versus resource usage and frequency, you must understand your traffic patterns. You should simulate your design with a variety of controller settings to observe the results of different settings.

Note: User-selectable Command Queue Look-ahead depth is available only when using the soft memory controller. For the hard memory controller, the Command Queue Look-ahead depth value is hard-coded to 8.

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