Soft core processors vs hard core processors

Question

I am doing a study on FPGA interfacing with microprocessors such as ARM9.

I found the concept of soft core processors and hard core processors in my study. May I know what is the comparison between these two types, the similarities and differences in the implementation?

Answer 1

Soft core is implemented in FPGA fabric while Hard is implemented the same as any integrated circuit while still connected to the FPGA fabric. The comparison generally boils down to:

Hard Core Processors - 100's of MHz up to 1GHz+ of speed

• Can achieve much faster processing speeds since they are optimized and not limited by fabric speed
• Fixed and cannot be modified (though it can take advantage of custom logic in FPGA fabric for processing

Soft Core Processors - 250MHz and less (usually less than 200MHz)

• Can be easily modified and tuned to specific requirements, more features, custom instructions, etc.
• Multiple cores can be used (at the cost of resources)
• Limited by the speed of the fabric.

In recent years, there's been a large move towards hard processor primarily due to the need for faster processing that the soft cores can't provide. When you put a hard core processor with the FPGA fabric, you can typically enable processing of a lot of data, which is needed for communication infrastructure applications (lots of GBs of data going by), for example.

Answer 2

A soft-core processor is a processor implemented using the FPGA fabric.

A hard-core processor is a processor that's actually physically implemented as a structure in the silicon.

Basically, you can add a soft-core processor to a FPGA-based system after it's already designed. However, adding a hard-core processor requires either a different FPGA, or an additional chip on the board.

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Hard-core processors are preferred when possible because the price/computing-power ratio for a hardware CPU is much better. Implementing a CPU in FPGA fabric is very resource intensive, particularly if you want a lot of computing power. The equivalent hardware CPU is likely much cheaper.

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Not only is the hardware CPU much cheaper, but it is also likely to be much more energy efficient. In battery-operated platforms, efficiency is crucial for long battery life. Unused gates in an FPGA can sometimes be turned off, but usually there are far more active circuits in a soft core processor than in a purpose-design hardware CPU. All of that potential FPGA silicon consumes power.

Answer 3

Implementing soft processor to achieve a certain functionality might be cheaper in terms of purchasing the hardware in comparison to purchasing equivalent hard processor.