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

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 foboot-bitstream.py --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, like Toboot, the program will auto-boot once it has finished loading.

Software is contained in the sw/ directory.