[codebeaker]

The efuses are still part of hardware initialization, before one gets into the bootrom, so it's feasible to assume that this 500us is still "hardware" init, during which time it's typical for an MCU to be reading input from pins to know about voltage, clock, mode selector jumpers, etc.

And, the way all of those things work is by setting registers so that they're visible in the software either _still_ in a register, or mapped into the address space.

Edit: I checked your profile and see that you're an embedded engineer, so I must have missed some nuance in your question, because power glitching the boot sequence to mess with hardware init it a really popular vector for attacking embedded devices. Please feel free to disregard my reply.