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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