Booting the RP2350 from UART | Hacker News Reader

Booting the RP2350 from UART

pfister.dev

89 pointshugolundin21 comments

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?!)

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.

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?

That requires having a flash chip in the first place. By booting via UART you don't need any flash at all.

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.

https://www.adafruit.com/product/6000 has the pads for external PSRAM you can connect to the QSPI pins there (pt @ adafruit)

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

Are you implying the SWD signals would send the RAM contents every time? If I had to do that, I would first use a logic analyzer like Saleae to capture the SWD signals of a JLink performing the necessary operations to load the image into RAM. Then figure out, from the bytes that get send and received, whatever needs to be parameterized, and where to put the image data itself, perhaps by capturing different scenarios, and seeing what changes. Maybe even look up the SWD spec. You would also need to figure out what kind of back and forth is necessary, what must block waiting for a response. From there, assuming there isn't cryptography involved, it just becomes a matter of providing bytes to a bus in the correct order or timing based on the proper events. Some of those bytes are "canned" and never change. Some of them are parameters that describe some important quantity relevant your specific image. And the rest are your firmware image, probably chunked up with some overhead wrapped around it. I allow for the possibility that SWD is far more complex than I imagine, but this approach works pretty well for figuring out whats going on with SPI or I2C or BLE.

SWD and the associated debug interfaces are all documented by ARM; there's no need to reverse-engineer anything here. See the ADIv5 documentation [1] for a starter.

[1]: https://developer.arm.com/documentation/ihi0031/a

ADIv6 for RP2350 (!important)

> I allow for the possibility that SWD is far more complex than I imagine, but this approach works pretty well for figuring out whats going on with SPI or I2C or BLE.

SWD is pretty well documented. I won't claim its simple, but, in my opinion, it's decent at what it does. The RISC-V folks haven't seemed to be able to do better (and, IMO, did quite a bit worse in a few places, actually).

The SWD description at the packet/command level: https://arm-software.github.io/CMSIS-DAP/latest/index.html

There is open source code directly from ARM for it: https://github.com/ARMmbed/DAPLink/tree/main/source/daplink/...

The documentation of the actual wire protocol is also extensive, but a little more scattered: https://developer.arm.com/documentation/ihi0031/a?lang=en https://community.nxp.com/pwmxy87654/attachments/pwmxy87654/...

The big problem with the SWD wire protocol ARM documentation (and everybody who copies it) is that they don't point out the fact that when you go from Write-to-Read the active edge of the clock changes. In SPI-speak, you switch from CPHA=1 to CPHA=0. This makes sense if you stop to think about it for a moment because during debug there is no clock. Consequently, SWD must provide the clock and you switch from "put something on DATA a half phase early->pulse clock to make chip do something with it" to "pulse clock which makes chip put something on Data->read it a half phase later". However, if it has never been pointed out to you before, it's likely to trip you up.

Sigrok (or similar) which can decode SWD properly and a digital signal analyzer (even a cheap $10 one) are your friends.

The only diagrams which seem to resemble scope traces that point this out are on obscure Chinese engineering blogs.

This is completely unnecessary since SWD is both trivial as well as well documented

Well... I wouldn't call it "trivial". But it is documented.

We (Pimoroni) actually shipped this technique in PicoVision, used to load the “GPU” firmware (an RP2040 used to offload the HDMI signal generation) at runtime-

https://github.com/pimoroni/picovision/blob/main/drivers/dv_...

What are the advantages of doing this instead booting it through UART? Speed perhaps?

I think RP2040 does not support UART booting.

In theory you wouldn’t even need to load firmware- you could just manipulate the relevant registers directly over SWD for the silliest IO expander.

In our case it was the only choice. I’d say we’d use UART now but the RP2350 can pretty much do it all in one chip.

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.

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.

this is also how some BLE controller boot.

Some wifi controllers can also boot like that. In particular ESP8089 chip that shipped with some android tablets circa 2012-2014.

Later, Espressif took that chip, modified bootrom to be able to boot from an SPI flash as well, and marketed that variant as "ESP8266". Serial bootloader was kept as a debug/programming interface, and that was inherited to ESP32 and later chips. All of which can boot directly from serial.

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