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

Update:
It turns out that `SOURCE` and `LOCKED` are each 32-bit registers, which I am trying to set unconditionally, so a `write()` actually is appropriate in this case.  But, I had mixed things up and had you asked me, I _thought_ I was doing a RMW, so I'm grateful you pointed that out.

Anyone have any thoughts?