something like

    pac::FLASH.acr().modify(|w| {
        w.set_dcen(true);
        w.set_icen(true);
        w.set_prften(true);
    });

the tldr is:

• the hardware has a few clock sources (HSI = on-chip oscillator, typically 8mhz or 16mhz. HSE = external quartz crystal, frequency is whatever you mount on the PCB)
• it can multiply/divide using some black magic (the PLL) to get faster clocks
• it can then choose one of these for several clocks (system clock, periphral clocks...)

Thanks, this worked. But upon going back to my nrf52840 repo (in a different directory), I suddenly get tons of compiler errors about std, even though my main includes #![no_std] as the first line (I also have not changed it since compiling another app for x86).

error[E0463]: can't find crate for `std`
  |
  = note: the `thumbv7em-none-eabihf` target may not support the standard library
  = note: `std` is required by `critical_section` because it does not declare `#![no_std]`
  = help: consider building the standard library from source with `cargo build -Zbuild-std`

I feel like an idiot. But this is really weird.