After web researches and a first discussion about general comprehension, I am a bit lost with the load frequency calculation:
I see a design which uses two 33pF external capacitors (C1 and C2),with a uC which has two 4pf input capacitors (Cin and Cout) and with an guesstimated stray capacitance of 2pf, the above formula gives a c_load = (4+33)*(4+33)/(4+33+33+4)+2 = 20.5pF
This design uses this configuration with the following quartz ECS-.327-CDX-1128 which has the following characteristics:
- FL = 32 768Hz (load frequency)
- C0 capacitance = 2 pF (shunt capacitance)
- C1 capacitance = 3,7 pF (mean of 3,0 and 4,4pF) (motional capacitance)
- CL = 6 pF (load capacitance expected to get the load frequency above)
I use the following formula to get the series frequency Fs (thanks to all the quartz characteristics above), which I use then in the same formula but changing the load capacitance value with previous calculated one.
I got:
- Fs = 26 632 Hz
- Fl (with Cload = 20.5pF) = 28 813 Hz
Even if load capacitance specified is not respected here, I am surprised that the load frequency is such far from the frequency expected. Indeed, this not the same order of magnitude as it can be pointed out at TI application report , as shown below, where the frequency drift is more about some Hz or ten of Hz (note: this is an example, quartz characteristics are not the same)
So I guess I am misunderstanding something which makes me make wrong interpretation leading to a wrong calculation. Can you help please ?
I have found another formula here, as shown below, but the result (Fs = 26 070Hz and FL=28 292Hz) are still really far.
