An increasing number of embedded systems are being designed—whether for budgetary, philosophical or other reasons—using open source elements. For the most part, these elements are software based, although there are some open source board designs in use as well. Now, the microcontroller that empowers a PCB design is available as an open source design.

A little over a month ago, start-up SiFive announced a milestone product in the development of the RISC-V (pronounced risk-five) open source microprocessor instruction set architecture (ISA). Originally developed for research and education, the architecture began moving toward industry implementation with the creation of the RISC-V Foundation in 2015. SiFive advanced that movement by developing a microcontroller design implementing the RISC-V ISA. The company has now proven that design in silicon and donated the RTL code for the design to the open source community.

SiFive's Freedom E310 MCU is implemented in the TSMC 180G process, runs at better than 300MHz, and includes 16kB each of L1 cache and of data scratchpad memory. It also has 8kB of mask ROM and 8kB of in-circuit one-time programmable memory as well as a quad-SPI Flash controller for off-chip, execute-in-place code. The E310's IO offerings include a GPIO complex, watchdog, and a JTAG-connected debug module.

Developers interested in exploring the RISC-V architecture, and the Freedom E310 in particular, can use the company's $59 HiFive1 development board. The board is also open source, has an associated software development kit, and is Arduino compatible. Developers can thus use the Arduino IDE to create code for the E310.

HiFive board 01 (cr) Figure 1: HiFive1 Development Board (Source: SiFive)

Traditional processor designs are either proprietary, requiring developers to select among and purchase stock chips, or are based on licensed IP, as with the ARM architecture. The RISC-V, being open source, does not readily support either of those traditional business models. The whole idea of making the microcontroller open source, after all, is to allow developers to customise the design for their specific needs rather than compromising on a best-match stock model. And there is no cost to license the base design.

The business model that SiFive chose in order to turn a profit from their open-sourced design is based on the idea of building customised systems on chip (SoCs) for customers. Its goal is to establish a "chip design factory" that can handle 1,000 new chip designs a year. For under $100,000, the company claims, it can—in three to six months—create and start manufacturing a custom MCU for a customer while ensuring the SoC remains compatible with the Freedom E310 core. This compatibility means that a team can develop software on the HiFive1 board and then execute it on their custom MCU with little to no porting effort. It also means that any other developments that the open source community creates for the E310 will work with the custom MCU as well.

Behind this whole RISC-V movement is the belief that Moore's Law has reached its economic limits. No longer is the cost per transistor for an MCU design going down, the thinking goes, and the development cost for a new chip has exploded. Processor vendors are thus concentrating on winning a few big customers so that they can have production volumes sufficient to amortise the high design cost while minimising the cost of providing development support to their customer base. The small, entrepreneurial company ends up being hostage to this trend, having to deal with "black box" hardware and software that it cannot optimise for its specific needs. It has to settle for whatever it can get.

By moving processor design to open source, supporters believe, the SME (small to medium enterprise) design community will get the ability to compete more directly with the big guys. Development teams no longer have to start out as major customers to gain the benefits of customisation. Chips optimized for the application become affordable and will enjoy widespread development support. Open source MCUs could thus be a game-changer in the emerging Internet of Things and other embedded markets.