SiFive collaborated with AB Open to create a rack-mount RISC-V cluster based around the SiFive HiFive Unmatched developer platform for compute and a SiFive HiFive1 Rev. B development board for control. In this article we take a closer look at the design of the cluster and consider some of the key use cases.


The objective was quite simply to package together a total of four SiFive HiFive Unmatched boards in a rack-mount enclosure, that would serve as a powerful and convenient way to host and use a native RISC-V development platform. From this it was decided that the solution must feature:

  • Integrated mains power supply and cooling
  • Local and remote control of board power and reset
  • Local and remote board power status indication
  • Local board console access
  • Remote power monitoring


The four SiFive HiFive Unmatched boards can be seen pictured above, secured to a 3mm thick aluminium mounting plate for extra rigidity and ease of assembly. The enclosure side panels are fitted with four high quality ebm-papst fans for cooling, which are secured by silicone anti-vibration pins.

The Control board is cabled to a 12VDC switched-mode power supply, which is pictured above bottom-left. The board provides power distribution to fans and supply sequencing for the SiFive HiFive Unmatched boards. At the heart of the Control board is a SiFive HiFive1 Rev. B, which can be seen inverted and plugged into this. Four relays are used for hard power control, with MOSFETs for toggling the power and reset lines present on the front panel headers of the SiFive HiFive Unmatched boards.

ATX power rails are provided by PicoPSUs fitted to each SiFive HiFive Unmatched, with the individual 12VDC supplies to each instrumented by an INA260 precision digital current and power monitor.

Fitted just above the Control Board is the Rear Panel board, which provides feed-through for the SiFive HiFive Unmatched Gigabit Ethernet, along with Ethernet/UART interfacing for the SiFive HiFive1 Rev. B.

The Front Panel board features an OLED display for power status indication, with a square for each SiFive HiFive Unmatched board and whereby an empty square means that it is powered off, a half-filled square means that power is supplied to the PicoPSU, and a fully filled square means that it is powered on.

There are also buttons for local power and reset control, plus USB feed-through for access to the console on each SiFive HiFive Unmatched board.

Just to the right there is a USB UART, which provides local access to the control subsystem.


The firmware is pretty straightforward and for each SiFive HiFive Unmatched board enables:

  • Set:
    • Hard power state
    • Soft power state
    • Reset
  • Get:
    • Hard and soft power status
    • 12VDC supply (to PicoPSU) voltage and current

This is supported by a simple text-based command protocol. Of course the firmware also drives the OLED display.

Typical use cases

The cooling fans are reasonably quiet and the cluster could equally be installed in an office rack or a data centre, providing a solution that is much more convenient than boards fitted into multiple off-the-shelf PC enclosures or secured directly to rack cabinet shelves.

The are many potential uses for the rack-mount cluster and these range from RISC-V software development and test, through to build farms and even scale out to enable proof-of-concept of larger designs that utilise SiFive Essential U74 processor cores.

It’s also easy to see how the RISC-V Rack Cluster could be integrated into existing operations. For example, using tools such as Ansible with a custom role for managing remote power and reset along with software configuration tasks. Not to mention of course integration with CI platforms for automated RISC-V software build and test.

RISC-V Summit 2021

If you’re at the RISC-V Summit in San Francisco 6-8 December be sure to head along to the SiFive booth at Booth D1 to see the Rack Cluster in operation. A short video is also available below that provides a tour of the hardware for those who are unable to make it to the summit. In-person and virtual registration is available until December 8th, here.