Trying to get the USB com. working.
Now there's a little details that I have seen stated but not previously understood:

The 18F13K50/14K50 requires the use of an external crystal of 12 or 48 Mhz ONLY! The internal clock circuitry is in fact simpler than what the 2550/4550 has, and the only option to feed the USB oscillator is either directly or using a x4 PLL. CPU frequency can then be derived from the PLL using ÷1, 2, 3 or 4.

No problem for my USB mode, but for my low power mode running at 12Mhz is out of question. That will require the use of the internal RC oscillator, and a loss of timing precision between the temperature samples (which is not overly critical).

An other solution is to use the second external oscillator and wire a 32,768kHz crystal to it (with a bit more PCB real estate used).