I'm slightly bamboozled by the LFCLKCSRC options in the NRF52 datasheet / product specification.
Bit field SRC makes sense. Set to 0 for the internal RC oscillator, 1 for the xtal, and 2 to drive it from the HFCLK.
Bit field BYPASS makes sense as well. Set to 0 when using a LF XTAL that doesn't go rail-to-rail, set to 1 if using rail-to-rail XTAL.
Bit field EXTERNAL, however, makes no sense. 0 is "Disable external source (use with Xtal)". 1 is "Enable use of external source instead of Xtal (SRC needs to be set to Xtal)". Uhhh... So do I set EXTERNAL to 1 or 0 when using an external crystal oscillator?