Why an input resistor? How are resistance and resistor values ​​selected?

Question

Electronics

I need to include a 16 MHz quartz resonator to the MK. Already in the ready version of the task I found such an inclusion. Containers for filtration, as I understand it. Why an input resistor? And actually, based on what and how are the values ​​\u200b\u200bof these capacitances and resistances chosen?

Answer 1

No, the containers are not for filtering. they are phase shifters.
quartz, in this case, can be replaced with an equivalent circuit of parallel-connected capacitance and inductance. then a quartz with external C13 C14 inverts the signal, and an inverting amplifier is connected inside the microcircuit from OSC_IN to OSC_OUT. in sum, an amplifier with a large positive feedback is obtained, which is self-excited at the frequency of the series resonance of quartz. here is a capacitive three-point circuit for understanding.

R6 matches the input impedance of OSC_IN to the capabilities of the crystal. without it, the small input resistance of OSC_IN will shunt the quartz to ground before stalling.
I will assume that this scheme will not work - the values ​​\u200b\u200bof the capacitances C13 C14 are too large, they are usually chosen at the level of units - tens of picofarads. look at the datasheet for mikruha and quartz.
https://meanders.ru/kvarcevyj-generator.shtml

Answer 2

Search for information on topics:
1) Load capacity of a quartz resonator
2) Terminator resistor
There is nothing super complicated, don't be afraid, I'm just too lazy to print more than a couple of sentences now x)

Answer 3

Strange scheme. Where is this from? The OSC input on most MKs is high-resistance, so your resistor will be at the error level i.e. The presence or absence of the scheme will not affect the operation in any way. Another thing is capacitors, their capacitance exceeds the maximum allowable by several thousand times, at this frequency they are all stupidly closed

Answer 4

All denominations are incorrect and even the diagram is slightly wrong. I don’t remember what to advise in Russian. Here in English:
Design Guidelines for Quartz Crystal Oscillators. Statek TN-30.
HCMOS Crystal Oscillators. Fairchild AN-340.