short crystal question

[qtommer]

I have read the app note of Microchip crystal selection and would like to verify some stuff cuz what I have been told is otherwise from what I have read.

1) most 4 MHz crystals that we use for PICs are based on the Pierce architecture?

2) People tell me caps used with crystals are for filtering noise.
However, I read that the caps are for providing the required phase shift so that a 360 phase shift is obtained. Apart from that it is also for coupling the signals to the amplifier network.

Can the pros please confirm that what I've read is true

 

[MikeMl]

The crystal-capacitor network used with the internal CMOS inverter inside a PIC is indeed a Pierce (special subset of Collpitts) oscillator. The capacitors are required to create a 2 port feedback network which exhibits a 180 degree phase shift at the parallel-resonance of the crystal. They are NOT there for filtering noise. That was written by some NOOB that doesn't have a clue!

 

[ericgibbs]

hi,
A short pdf, supplementary to the info that Mike has posted.