Brot FPGA Board

Brot is an FPGA development board designed to be plugged into a solderless breadboard.

This repo contains schematics, pinouts and example gateware.

Find more information on the Brot product page.

Programming

There are three ways to program Brot:

1. Use a USB DFU bootloader to write to the MMOD flash module. (see below)
2. Remove the MMOD flash module, connect the Brot MMOD socket to a device supported by ldprog and program the configuration SRAM.
3. Remove the MMOD flash module and program it with a GPIO or SPI-capable device.

See the blinky example below for specific instructions.

Bootloader

The MMOD can be programmed over the USB-C port with a USB DFU bootloader. You can use the bootloader to write custom gateware and other data to the MMOD. There is a fork of the Nitro Bootloader that works with Brot. To build it:

$ git clone --recursive https://github.com/machdyne/no2bootloader
$ cd no2bootloader/gateware/ice40-stub
$ make BOARD=brot bootloader

Then write the bootloader to the MMOD (using a Werkzeug in this example):

$ ldprog -wf build-tmp/bootloader.bin

The bootlooader will stay active for about 8 seconds after power is applied, giving you time to update your custom gateware. See an example for blinky below.

Blinky

Building the blinky example requires Yosys, nextpnr-ice40 and IceStorm.

Assuming they are installed, you can simply type make to build the gateware, which will be written to output/blinky.bin. You can then use ldprog to write the gateware to the device.

Writing blinky to the MMOD using the USB DFU bootloader

$ dfu-util -a 0 -D output/blinky.bin

This will write the blinky after the bootloader on the MMOD. When you reboot the device the DFU gateware start again and about 8 seconds later blinky will start.

Writing blinky to SRAM

Connect the MMOD socket to the appropriate pins of an ldprog-supported device.

You will need to use the manual reset option (-m) with ldprog and press the Brot reset button when prompted to do so. Alternatively, you can solder a 2-pin header to the unpopulated header and connect it to the programmer device to automate this step.

$ ldprog -m -s output/blinky.bin

Writing blinky to the flash MMOD

It should be possible to connect and program the MMOD using any device capable of GPIO. If using a Müsli or Raspberry Pi [Pico] you can use the ldprog tool:

$ ldprog -f output/blinky.bin

If using a Werkzeug with the Müsli firmware you can plug the MMOD (oriented so that you can read the text) into the top of the Werkzeug PMOD socket and:

$ ldprog -wf output/blinky.bin

GPIO Header

Pin	Signal
1	GPIO0
2	GPIO1
3	GPIO2
4	GPIO3
5	GPIO4
6	GPIO5
7	GPIO6
8	GPIO7
9	GND
10	PWR3V3 (out)

MMOD Socket

The MMOD socket can be used to program the FPGA SRAM or can be populated with an MMOD containing a single bitstream or a multiboot image.

Pin	Signal
6	PWR3V3
5	GND
4	CSPI_SCK
3	CSPI_SI (MISO)
2	CSPI_SO (MOSI)
1	CSPI_SS