Storage solutions - external SRAM based on FPGA

 

advantages

External SRAM memory to provide greater storage capacity than on chip memory, is also quite fast, but slightly less in internal memory.Commonly used external SRAM capacity of up to 128 KB to 10 MB.Special SRAM can be smaller or larger capacity.SRAM is generally very low response time and high throughput equipment, because it is through the sharing of bidirectional bus connected to the FPGA, the speed of external SRAM slightly lower than on-chip memory.SRAM interface is very simple, which connect the SRAM FPGA as a simple design work.You can also use multiple external SRAM Shared bus, or even use different kinds of external memory, such as flash or SDRAM.

 

disadvantages

Using external SRAM in embedded system based on FPGA is the main disadvantages of high cost, and take up space on the board.SRAM devices than other high capacity storage device such as SDRAM, the average price of each MB more expensive.But also than SRAM SDRAM and "zero space" of the FPGA chip memory per MB takes up more space on the board.

 

The best applications

External SRAM as the storage medium size buffer performance data module.Can use external SRAM as not suitable for on chip memory and reaction time and below the SDRAM data cache.Can also be multiple SRAM combined to increase the capacity.

SRAM is the best choice of the random addressing data.Many SRAM devices can access the disorderly address data, short reaction time and orderly access address data, the SDRAM is not doing well on this point.SRAM is an ideal storage types, large LUT for on-chip memory to store such as too large color conversion algorithms of data.

External SRAM as the implementation of memory to the cache when the CPU working performance is relatively good.When there is no CPU cache to buffer other kinds high reaction time of memory, external SRAM features low reaction time helps to improve the performance of the CPU.

 

Not suitable for applications

External SRAM is not applicable to the system include: the need to use large amount of data storage system and cost constrained system.If the system needs a larger than 10 MB of storage module, the designer will be more inclined to consider other types of memory, such as lower SDRAM.

 

The types of external SRAM

There are many kinds of SRAM devices to choose from.The most common types are as follows:

Asynchronous SRAM - because it does not rely on the clock, so a SRAM is the slowest.

Synchronous SRAM (SSRAM) - an SRAM run synchronization in a clock signal.Synchronous SRAM faster than asynchronous SRAM, but also more expensive.

Pseudo SRAM, pseudo SRAM (PSRAM) is a SSRAM interface dynamic RAM (DRAM)

Zero SRAM turnaround time bus - zero bus turnaround time SRAM (ZBT SRAM) conversion from reading to writing needs zero clock cycle, which makes it a short reaction time.ZBT SRAM usually require a dedicated controller to make it play out the advantages of low reaction time.

 

Matters needing attention

In order to make the external SRAM devices to achieve the best performance, suggest that follow the following principles:

Use and connection of the main system of the controller of the same SRAM interface data bandwidth.

If the pin is used or the limit of the space on the plate is higher than the system performance requirements, you can use the connection of the controller data bandwidth smaller SRAM devices, in order to reduce the pin number and reduces the amount of memory may be on PCB.However, this change will result in lower performance of the SRAM interface.


Key words：sram