External Memory Interface Handbook Volume 2: Design Guidelines

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

11.2.14. Bandwidth

Bandwidth depends on the efficiency of the memory controller controlling the data transfer to and from the memory device.

You can express bandwidth as follows:

Bandwidth = data width (bits) × data transfer rate (1/s) × efficiency

Data rate transfer (1/s) = 2 × frequency of operation (4 × for QDR SRAM interfaces)

The following example shows the bandwidth calculation for a 16-bit interface that has 70% efficiency and runs at 200 MHz frequency:

Bandwidth = 16 bits × 2 clock edges × 200 MHz × 70% = 4.48 Gbps.

DRAM typically has an efficiency of around 70%, but when you use the memory controller, efficiency can vary from 10 to 92%.

In QDR II+ or QDR II SRAM the IP implements two separate unidirectional write and read data buses, so the data transfer rate is four times the clock rate. The data transfer rate for a 400-MHz interface is 1, 600 Mbps. The efficiency is the percentage of time the data bus is transferring data. It is dependent on the type of memory. For example, in a QDR II+ or QDR II SRAM interface with separate write and read ports, the efficiency is 100% when there is an equal number of read and write operations on these memory interfaces.

For information on best-case and worst-case efficiency scenarios, refer to the white paper, The Efficiency of the DDR & DDR2 SDRAM Controller Compiler.

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