Add simple UART bootloader example

[will-v-pi]

Add a UART bootloader example, which reads a binary from the partition table on the main device, and uses the UART bootloader to boot that binary on a separate device. Requires a modified separate board to run, as you need access to the QSPI pins to use them as UARTs for the bootloader (see 5.8 in the datasheet) - the main board can be a normal RP2350 board.

This sample binary flashes the LED and prints "Hello, world" back over the UART interface, which demonstrates setting up QSPI GPIOs as UARTs.

[lurch]

The baud-rate is hardcoded in uart_binary.c (because I assume that it can't be changed, as it's the baud-rate required by the bootrom) so should it also be hardcoded (rather than being a #define) in uart_boot.c too?

Although I guess that the uart-bootloader baud rate could theoretically be different to the post-boot application baud rate (IYSWIM); so I wonder if having them deliberately different might make this a more interesting example? 🤔

[will-v-pi]

Yes, it should be hardcoded in both - I've fixed that

If the post-boot application used a different baud rate, then it wouldn't be able to boot the board again if it's power cycled - this way you can power cycle the separate device and watch the bootloader run again without rebooting the main device

[lurch]

Yeah, I was assuming that in order to accommodate my suggestion, the code (in the main device) would need to dynamically reconfigure the baud rate at the start and end of the uart-boot sequence 🙂

[lurch]

I wonder if things like the "splash string", the "knock sequence", the "uart-bootloader baudrate", etc. ought to be defined in a header-file somewhere in pico-sdk? (As I assume they'll always be the same for every RP2350 chip?)

EDIT: Ah, perhaps https://github.com/raspberrypi/pico-sdk/blob/develop/src/rp2_common/boot_bootrom_headers/include/boot/bootrom_constants.h would be a good place for those 🙂

[lurch]

Any idea why this function doesn't appear at https://www.raspberrypi.com/documentation/pico-sdk/runtime.html#functions ?
(It seems to have a good chunk of Doxygen at https://github.com/raspberrypi/pico-sdk/blob/master/src/rp2_common/pico_bootrom/include/pico/bootrom.h#L627 )

[will-v-pi]

I'm not sure what's gone wrong, but it looks like none of the RP2350 functions are included in that documentation - the doxygen is all there, but maybe the #if !PICO_RP2040 halfway down the src/rp2_common/pico_bootrom/include/pico/bootrom.h file has something to do with it?

[lurch]

I'll have a fiddle later... rom_get_partition_table_info isn't appearing in https://datasheets.raspberrypi.com/pico/raspberry-pi-pico-c-sdk.pdf either! 😱

[lurch]

What does the magic value 3 indicate? 😉

[will-v-pi]

This one isn't actually a magic value - it's just the number of words written in the buffer (which should be 3 here - 1 word for the returned flags, and 2 words for the partition_location_and_flags)

[lurch]

I guess it might be useful if https://github.com/raspberrypi/pico-sdk/blob/develop/src/rp2_common/pico_bootrom/include/pico/bootrom.h#L628 documented the return value.

[will-v-pi]

It does mention it https://github.com/raspberrypi/pico-sdk/blob/develop/src/rp2_common/pico_bootrom/include/pico/bootrom.h#L634

[lurch]

Oh, I missed that 😆 My eyes just jumped straight to the bottom of the doxygen description looking for the \return ... line 😜

[lurch]

If this is intended as a "generic UART bootloader", should we increase the size of this partition to allow for bigger UF2s?

[will-v-pi]

The issue is that this simple bootloader just loads the whole partition onto the other device, so the larger the partition the longer it takes. From a quick play it takes 0.35s for this 32kiB partition, vs 5.6s for a full 512kiB partition (which would be the maximum size, as that fills the RP2350 SRAM)