Skip to content
Bootloader for Fomu
Branch: master
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.

Foboot: The Bootloader for Fomu

Foboot is a failsafe bootloader for Fomu. It exposes a DFU interface to the host. Foboot comes in two halves: A Software half and a Hardware half. These two halves are integrated into a single "bitstream" that is directly loaded onto an ICE40UP5k board, such as Fomu.


To build the hardware, you need:

  • Python 3.5+
  • Nextpnr
  • Icestorm
  • Yosys
  • Git

Subproject hardware dependencies will be taken care of with lxbuildenv.

To build the software, you need:

  • RISC-V toolchain

Building the project

The hardware half will take care of building the software half, if it is run with --boot-source bios (which is the default). Therefore, to build Foboot, enter the hw/ directory and run:

$ python3 --revision hacker

This will verify you have the correct dependencies installed, compile the Foboot software, then synthesize the Foboot bitstream. The resulting output will be in build/gateware/. You should write build/gateware/top-multiboot.bin to your Fomu device in order to get basic bootloader support.


You can write the bitstream to your SPI flash. If you're using fomu-flash, you would run the following:

$ fomu-flash -w build/gateware/top-multiboot.bin
Erasing @ 018000 / 01973a  Done
Programming @ 01973a / 01973a  Done
$ fomu-flash -r
resetting fpga

Fomu should now show up when you connect it to your machine:

[172294.296354] usb 1-1.3: new full-speed USB device number 33 using dwc_otg
[172294.445661] usb 1-1.3: New USB device found, idVendor=1209, idProduct=70b1
[172294.445675] usb 1-1.3: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[172294.445684] usb 1-1.3: Product: Fomu Bootloader (0)
[172294.445692] usb 1-1.3: Manufacturer: Kosagi

To load a new bitstream, use the dfu-util -D command. For example:

$ dfu-util -D blink.bin

This will reflash the SPI beginning at offset 262144.

To exit DFU and run the bitstream at offset 262144, run:

$ dfu-util -e

Note that, unlike Toboot, Foboot requires you to issue dfu-util -e in order to start the program.

Building the Software

Software is contained in the sw/ directory.

You can’t perform that action at this time.