retroleds wrote:....The trimmer comes into the picture too - most trimmers for watches are 16-18 pf, the crystal adds 12.5pf(be sure to buy the correct ones, for a total of 30 pf....
Actually, it's a bit different: the crystal does not ADD to the capacitance (in fact, its own [shunt] capacitance is less than 2pF), but it REQUIRES an external LOAD capacitance of typically 12.5pF to be added.
This load capacitance is made of two capacitors, the trimmer being one part of it.
The actual load capacitance is the SERIES combination of both load capacitors - so you need 2 capacitors of 25pF each to get 12.5pF load capacitance (CL = 1/(1/C1+1/C2)). Accounting for stray capacitance (3-10pF on each side) leads to 15-22pF load capacitors.
This holds for a "Pierce" oscillator with "parallel" crystal.
A good description is
here (PDF) or
here (Wikipedia).