Electronic – 16MHz Crystal with 2p load capacity: What load capacitors are necessary

1. Yes, a 32.768 kHz crystal is often referred to as a 32 kHz crystal. I think it's rather sloppy personally, as it only costs another 4 characters to make certain of avoiding any potential confusion.

2. Yes, they will all work. Unless you need the frequency to be absolutely spot on then e.g. 22pF will do.

3. If you look at the notes in the datasheet it says they are for design guidance only, for exact values consult the crystal manufacturer. So the crystal datasheet is the one to go from. As Endolith says the microcontroller doesn't affect the correct load capacitance value (well maybe a tiny bit with the pin capacitance)

I agree with Olin that the 16F84A is an antique - if you grab one of the newer PICs you will give yourself far more options. The PIC16F1828 is a nice part, internal RC/PLL up to 32MHz and loads of nice peripherals. Probably won't be much more than the 16F84A, may even be cheaper.

My seat-of-the-pants understanding for load capacitors (corrections invited) goes like this:

When a crystal is cut for a certain load capacitance, it is measured with that capacitance across it during final factory trimming. There is nothing magical about the value. It is simply a way of saying, that if you design your circuit to present that same capacitance, then your crystal will be within the stated (.005% or whatever) tolerance.

So, you add up all the capacitance in your circuit, and then add in what's needed to bring it up to the spec. We'll use your numbers. The stray capacitance due to the traces on the board obviously will vary with the board, so let's guess 1.3 pf. A number I made up, to go with the capacitance of the microprocessor's oscillator, stated to be 1.7 pf. So, we've got 3 pf in parallel with the crystal. The crystal wants 18pf, so we have to make up the 15 pf difference with discrete parts.

Since the two load capacitors are in series (Gnd->cap->xtal->cap->Gnd), we double the cap value to 30pf. Two 30 pf caps in series give us the 15 pf we're looking for.

Note 1. I tried searching for typical PCB stray capacitance. It was all over the map. Suffice it to say, that as the hardware gets smaller, the capacitance will keep getting smaller. A lot of typical values claimed less than 1 pf.

Note 2. If there is more capacitance than spec, the crystal will oscillate at a lower frequency than specified. If there's less, then it's higher. You can see, that if you want to trim the oscillator to spec, it's easier to shoot for a lower capacitance and add some later, than to try the opposite.

Note 3. For fun, look up "gimmick capacitor".

Note 4. My "seat of the pants" explanation is sufficient as an introduction, and this technique works in many cases, but not everywhere. For a more in-depth look at the EE principles behind those capacitors, see this answer.