V. Hunter Adams discusses the RP2040 microcontroller at the heart of the Raspberry Pi Pico (and Pico W). Specifically, what happens between applying power to an RP2040, and a loaded program starting execution.
I’ve attempted to walk through everything that happens between applying power to an RP2040 and the first line of main() being executed. Some of these steps are covered in more depth than others, but I’ve attempted to at least enumerate each one.
The steps with extensive explanations are those which are of most relevance for writing one’s own bootloader. These include the exit from boot2 process, and the interaction between the reset handler and the linker script. The parts of the boot sequence that the programmer is unlikely or unable to modify (bootROM code, and the stage 2 bootloader which configures the external flash chip) are given a more cursory treatment.
The RP2040 datasheet separates the boot sequence into the “hardware-controlled” section which happens before the processors begin executing the bootrom, and the “processor-controlled” section during which processor cores 0 and 1 begin to execute the bootrom. The second stage of the bootloader (which lives at the beginning of the user’s program) then runs after the bootrom.
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