UF2 works for data too.

The USB Flashing Format (UF2) was developed as a way to update firmware on microcontrollers, but it works equally well for data. Let’s look at how and why you might want to do that using the real world application of loading an FPGA.

The great thing about UF2 is that it emulates a USB thumb drive so images can be transferred by dragging and dropping the properly formatted file using the drivers that are standard in every operating system. The key is the specially formulated packets that you can read about in this blog:
https://makecode.com/blog/one-chip-to-flash-them-all
I also spoke about it at Crowd Supply Teardown 2023:
https://www.youtube.com/watch?v=AdoWZkSSaoA

While primarily created for firmware updates, it can transfer data just as easily. In the case of loading an FPGA, the firmware that loads the image into the FPGA does not need to change for each new image. One way to transfer the data is to convert the image to a C array and build it with the firmware, but that would require rebuilding the firmware for every new image which is not necessary. Instead of converting the image to a C array, you can convert it to UF2 packets so that it can be loaded directly into flash without rebuilding.

I did just this when I ported Altera’s JBC player code to the Raspberry Pi Pico board. The RP2040 implements a UF2 bootloader in its ROM, and the SDK produces a UF2 file when building the code. The same bootloader can be used to load an FPGA JBC image into the flash by converting the file to UF2 packets with the uf2conv utility.

The JBC player firmware needs to know where the JBC FPGA image is located, and the size. To make sure the firmware is compatible with the image, and make it easier to use, I decided to combine the firmware UF2 image with the JBC UF2 image. I created a custom UF2 converter that would take the firmware output from the RP2040 SDK, add a 256byte header with the JBC image size, and append the JBC image.

The header is 256 bytes to fill one UF2 block, so I added some additional information such as a description of the image. The output is a combined file similar to if you had included the JBC file as a C array, but in this case, to replace the JBC image, you do not need to have the RP2040 SDK installed, you only need to run a Python script.

You can learn more about the process and some quirks with the RP2040 ROM UF2 implementation in this Crowd Supply Teardown Session:
https://www.youtube.com/watch?v=Ns5RiT4F2Mw

The code is available on GitHub:
https://github.com/steieio/jbcuf2
And I used it in a workshop at Crowd Supply Teardown 2024:
https://github.com/gsteiert/pico-dirty-blaster-workshop