原创 how to improve the efficiency of External Memory

 2014-8-27 20:05  1065 10 11 分类: FPGA/CPLD

Nowadays, External Memorys(DDR2/DDR3) are widely used for data storage in communication system/video processing systems.

Even though the technolofy is developed very fast,it still can't satisfy the high data throughout requirement.

In this artical, we are trying to analysis how to improve the efficiency of the memory。

1)The bandwidth:

Bandwidth = DDR SDRAM bus width × 2 × frequency of operation × efficiency.

For example, running a 16-bit DDR SDRAM interface at a frequency of 100 MHz:

Bandwidth = 16 bits × 2 clock edges × 100 MHz × efficiency

2)The efficiency:

Efficiency = number of active cycles of data transfer/total number of cycles.

Factors affecting efficiency:

The interface standard specified by the memory vendor
The way that you transfer data.

interface standard:

Activate:Reading or writing to a closed row

has negative impact on the efficiency as the controller has to first activate that row and then wait until

tRCD time to perform a read or write.

Pre-charge:Switching a row

has a negative impact on the efficiency as you must first precharge the open row, then activate the next row and wait tRCD time to perform any read or write operation to the row.

Device CAS Latency:The higher the CAS latency, the less efficient an individual access.

Refresh

Bank Management efficiency

The following methods of data trasfer affect the efficiency:

Performing individual read or write accesses is less efficient.
Switching between read and write operation has a negative impact on the efficiency of the controller.
Performing read or write operations from different rows within a bank or in a different bank

Ways to Improve Efficiency

To improve the efficiency of your controller, you can use the following tools and methods:

• DDR2 SDRAM Controller

• Auto-Precharge Commands

• Additive Latency

• Bank Interleaving

• Command Queue Look-Ahead Depth

• Additive Latency and Bank Interleaving

• User-Controlled Refresh

• Frequency of Operation

• Burst Length

• Series of Reads or Writes