Crystal
Load Capacitors !!! for ICs
What is a Load Capacitor? for a
crystal
Load capacitance is the amount of external circuit capacitance in
parallel with the crystal itself.
Above “load capacitors” are actually
in series (with GND.) in the circuit loop so their “Equivalent” Load
Capacitance,(Ceq=1/2C) is one-half (1/2) of the C value.
Then the IC input and trace
capacitance to the ground are summed to get the total or “Load Capacitance” the
rating you will find on 32.768 kHz (or any ) Crystals.
If chip datasheet suggesting you
choose a crystal rated for 12.5 pF with 1.5pF allotted for Cinput + stray. This
implies you put the parts close to the pins and do not use a ground plane under
the traces which adds a few pF but depends on details. (0.5pF/mm type)
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