from Hacker News

Booting the RP2350 from UART

by hugolundin on 5/11/25, 2:26 PM with 21 comments

• by vardump on 5/11/25, 7:47 PM

  One could also send a binary stub that sets up fast CPU clock speed and decompresses the rest of the firmware at the RP2350 side. Should be even faster.

  Just like old C64 decrunchers and Amiga PowerPacker. Or Fabrice Bellard's LZEXE. (Is there anything that guy did NOT write?!)

• by devdri on 5/12/25, 1:04 PM

  This is one of the tricks to enable using both QSPI slots for PSRAM instead of the typical FLASH+PSRAM.

  This is great for making audio modules, where the firmware is be small and operates on a big audio buffer. Since the biggest available PSRAM chips are 8MB, this combined 16 MB could hold around 3 minutes of mono 16-bit audio, which allows for a very nice multi track looper.

  Another way (in case there's no other MCU to help with uart bootstrap) would be to add a logic chip to multiplex the CS line between Flash and the first PSRAM - copy firmware to flash and then switch to using ram.

• by bluehex on 5/12/25, 3:26 AM

  This is awesome. I've had similar ideas but wasn't able to do any prototyping yet as I only have Pico 2 boards that don't expose the CSn pin in the pinout.

  Rather than UART booting every time I thought it might be nice to use UART Boot just as a way to deliver the firmware update to the sub chip - so the UART image you load would just be a program that accepts a larger image (over UART again) and would write to the flash for subsequent boots. I think that would get around the SRAM and boot time downsides the author mentioned. Is there a reason this might not work?

• by ThrowawayR2 on 5/12/25, 2:29 PM

  Are there any off-the-shelf hobbyist boards that expose QSPI CSn (pin 75 on the RP2350B?) and QPI_SD1-3 signals to a header or pin? Doesn't seem like the official Pico 2 or the Adafruit or Pimoroni versions of the Pico 2 expose access to these signals without modifying the board, which most people won't be able to do.

• by duskwuff on 5/11/25, 8:25 PM

  In principle, you could boot the RP2040 over SWD. It'd be much more difficult to code, but the possibility is there...

• by zoobab on 5/12/25, 9:21 AM

  The CH32V003 has also a UART bootloader, but for some reason there is no open source command line client to do something with it. WCH has a Windows GUI though.

• by mschuster91 on 5/11/25, 10:51 PM

  There's nothing speaking "version 1.0" more than a bunch of stuff just manually soldered as piggyback over other components of the board :D

  Thanks for the writeup.

• by mrheosuper on 5/12/25, 2:08 AM

  this is also how some BLE controller boot.