One ROM Gets a USB Stack

Our hacker [Piers Finlayson] is at it again, and this time he has added USB support to One ROM.

With this new connectivity you can attach your One ROM to your computer with a USB cable and then in a matter of seconds upload new firmware from your Chrome (or Chromium) web browser. This new connectivity will supplement but not replace the existing serial wire connectivity because the serial wire connectivity enables certain advanced use cases not supported by the USB stack, such as reprogramming a ROM in-place as it’s being served. The new USB interface will probably suit most users who just want to use One ROM to manage the ROMs for their old kit and who don’t need the extra functionality.

Addressing the question as to why he didn’t have USB connectivity from the start [Piers] claimed it was because he didn’t like soldering the USB sockets! But given this is a service he can get from his board house that is no longer his problem! [Piers] said he picked Micro USB over USB-C because the former demands less circuit board real estate than the latter. Squeezing everything on to the board remains a challenge!

[Piers] isolates the two power subsystems with Schottky diodes. This keeps the One ROM and USB power sources separate, meaning they can safely be used at the same time. The USB support also demanded the inclusion of an external 12 MHz oscillator but only needed three extra pins on the micro: VBUS, D+, and D-.

The fun thing about this video is the number of false starts and red herrings [Piers] chases down as he does his diagnoses. This is how the sausage is made! And speaking of making sausage, [Piers] has recorded an additional two hour video showing how he laid out the new USB version in KiCad: One ROM Fire USB – Laying out RP2350 + USB in 1/2 x 1 inch.

If you haven’t been keeping track with where we’re at with One ROM we first heard of it back in July with an update in September, and since then [Piers] appeared on FLOSS Weekly, so be sure to check that out! It has been fun to watch this project develop and we look forward to seeing where [Piers] takes it in future, wishing him every success.