MSP430F47197: Crystal Capacitance Load Setting XCAPxPF

To TI team and members

MSP430F47197, SLAU056J Document: page 295 Section 5.2.3 Oscillator Quotes:

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The software-selectable XCAPxPF bits configure the internally provided load capacitance for the LFXT1 crystal. The internal pin capacitance plus the parasitic 2-pF pin capacitance combine serially to form the load capacitance. The load capacitance can be selected as 1, 6, 8, or 10 pF. Additional external capacitors can be added if necessary.

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Question 0:
   SLAS626C-March 2011 page 42 Note 1 mentions that parasitic bond and package capacitance is already included in the load capacitance value.  if pin parasitic capacitance is 2pF and we select load capacitance to be XCAPxPF 10pF, does that mean the capacitance load of the MSP430F47197 is approximately 1.7pF?  (Since capacitance are in series = 1/2 + 1/10)pF
 

Question1:
    The The SLAA225-November 2004 Application Report: MSP430 LFXT1 Oscillator Accuracy page 1 Figure 1 shows Capacitave Load outside of the MSP430x4xx IC.  There is no reference or location for the XCAPxPF (internal load capacitance).

    This is just an example, but if my crystal design requires a load capacitance (CL) of 7pF, what is the recommended method?

My calculation / thinking method is the following
    Setting MSP430F47197  XCAPxPF to "11"  (~6pF), then add external capacitor CL of 1pF,   Then load capacitance being used would be CLtotal = (6 + 1)pF = 7pF.   is this the correct way of thinking?

(for the detailed designers, yes, for now I am ignoring Cstray/CImirror/Cseries/Camplifier capacitance to simplify some calculations, I just want to know how this XCAPxPF capacitance effects in determining external load capacitor for the crystal in my design )

-or-

is it best to have no capacitance load and just select by software XCAPxPF to "01" (6 pF) and not worry about it?


Question 2:
     For XCAPxPF what is the MIN and MAX range for the capacitance? is assuming having a 10% tolerance, is this thinking correct?

Regards,
D.T.