RP2350 - PICOBOOT EXIT_XIP is NOT a no-op on A2 silicon

[piersfinlayson]

The RP2350 datasheet says this:

5.6.4.6. EXIT_XIP (0x06)
A no-op provided for compatibility with RP2040. An XIP exit sequence (flash_exit_xip()) is issued once before entering
the USB bootloader, which returns the external QSPI device from whatever XIP state it was in to a serial command state,
and the external QSPI device then remains in this state until reboot.

This doesn’t seem to be true on A2 silicon.

Specifically, if I issue a FLASH_ERASE to an RP2350A2 (datecode 29 24) 1in BOOTSEL mode without issuing EXIT_XIP first, the command completes “instantly", with no error reported, but has no effect.

If I perform the same action on RP2350A4 (no EXIT_XIP followed by a FLASH_ERASE), the command takes time to complete (expected), and actually erases the flash.

I assume this is due to a bug in the A2 bootrom vs the datasheet, but I couldn’t find an issue for that in the bootrom repo. I also assume (but haven’t checked) that this is worked around in picotool, by explicitly issuing EXIT_XIP before a flash erase.

It would be useful to correct the datasheet, clarifying the different behaviour between steppings/bootroms and noting that an EXIT_XIP is required on A2 silicon and is not a no-op there.

[will-v-pi]

I believe this is just a different form of the RP2350-E10 errata, rather than a new bug, and as you say it is fixed in A4 (and A3) - picotool already has this workaround for erase, but it looks like picotool erase doesn't issue EXIT_XIP for RP2040 so I will fix that (raspberrypi/picotool#302)

I will look into re-wording the E10 errata to include this - the workaround is to issue an EXIT_XIP before the first flash operation on RP2350-A2 in bootsel mode