What is a Soft-Core Processor?

Oct 26, 2023

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A soft-core processor is a microprocessor core that can be implemented entirely using digital logic synthesis. It is typically created on a programmable hardware like a field programmable gate array (FPGA). The key feature of a FPGA is that it is reconfigurable with different digital designs allowing functionality to be changed unlimited number of times by downloading a new file without physically changing the chip.

Development Cycle of Digital Designs with FPGAs

First, a digital design is encoded via schematic capture or a hardware description language (HDL). Then, the design is synthesized and transformed into a form that can be used to configure the FPGA. This form is normally called the configuration file. The configuration file is then used to configure the FPGA, which transforms the FPGA into the required digital solution.

Figure 1.0 FPGA Design Flow

For example, if the required digital design for the first input file is that of a microprocessor, then the resulting transformation of the FPGA yields a microprocessor. This type of microprocessor is termed to as a soft-core processor.

Figure 1.1 Soft-core processor design flow for FPGA

Once the FPGA has been transformed into a microprocessor, its usage patterns mirror those of a conventional (discrete) microprocessor. The processor must still be programmed using traditional programming languages and methods.

Figure 1.2 Programming of soft-core processor

Several traditional devices that are interfaced to discrete microprocessors like memory, buttons, switches, LEDs, etc., can also be connected to soft-core processors. The interface logic that is typically produced in the form of discrete components can be realized as logic instantiated in the FPGA along with the soft-core processor. Interface logic may include: Universal Asynchronous Receiver/Transmitter (UART), Serial Peripheral Interface (SPI), Inter-Integrated Circuit (I²C), etc.