nRF52: Setting LFCLKSRC to use external crystal oscillator?

[CurtisHx]

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?

[RK]

Don't really see the confusion there. And there's also Table 25 which lists all the possibilities.

If EXTERNAL is 0, the default, then external source is disabled and you can use RC, XTAL or HF SYNTH depending on the value of SRC. If EXTERNAL is 1 then you are using an external source but that you must also set SRC to XTAL in that case plus setting BYPASS to whatever value you need.

Just look at Table 25.

[CurtisHx]

I somehow missed that table. That makes a bit more sense. I was getting confused on the "External" flag. When I saw "External", I was thinking "external crystal oscillator", but that's apparently not right. If I understand correctly, "External" is used when you have something other than a crystal generating the 32khz clock, such as another micro bit-banging the 32khz clock.

[RK]

Yes - that's exactly what external means, an entirely separate self-contained source for the square wave