* I changed the code to use `modify`, and whatever else I was trashing, it's still incrementing at 1Mhz...

Take 2:

```rust
#[pre_init]
unsafe fn pre_init() {
    // SAFETY: `#[pre_init] fn` runs before stage 1 bootloader, still in privileged thread mode.
    //      Any access of memory, including any static variables, will result in undefined behavior.
    //      (see https://docs.rs/cortex-m-rt/latest/cortex_m_rt/attr.pre_init.html for more details)
    let timer_0 = rp_pac::TIMER0;

    // Switch `SOURCE` to `CLK_SYS` (instead of 1 µs tick)
    timer_0.source().modify(|source| source.clk_sys());

    // Lock configuration--`TIMER0` now read-only (until reset)
    timer_0.locked().modify(|lock| lock.set_locked(true));
}
```
It turns out that `SOURCE` is a 32-bit register, which I am trying to unconditionally set to `0x1`, so a `write()` is appropriate in this case.  But, I had mixed things up and thought I was doing a RMW, so I'm grateful you pointed that out.

Anyone have any thoughts?