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Processor Clock Oscillator Crystals

When trying to get a 37Khz crystal to start oscillating, it finally turned out that the boards were too clean electronically, and there was not enough noise in the system to kick them off. Adding a 100K series resistor solved the problem--provided the resistor was a NOISY carbon Composition type.

RealTime@ Time@, Oscillators

David VanHorn says:

Microcontroller Crystals Basic

One of the most frequently asked questions about microcontrollers has to be
"What kind of crystal capacitors do I use on this chip."
Unfortunately, that's even the wrong question!
This also applies to any other common chip that uses an external crystal timebase.

If all you wanted is to get your project going, then this is what you needed.
However, if you'd also like to minimize your EMI output, then carry on to the next section.

Microcontroller Crystals, Advanced.

So we've got the right cap values, what more could there be?

Sean Breheny says:

The purpose in using capacitors with a crystal is two fold:

#1) The oscillator consists of the inverter inside the PIC, the crystal, and external capacitance (both parasitic and actual capacitors). The total phase shift around the loop (from one inverter terminal, through the inverter, across the crystal/capacitor network, back into the inverter) has to be either 0 or 360 (the same thing) degrees for oscillation. The capacitance adjusts the phase shift of the network to allow oscillation.

#2) Crystals are designed to "see" a certain type of load. Most are designed to see a certain, specified capacitance, referred to as the Load Capacitance. In order for your crystal to operate at the correct frequency, it must see this value of capacitance at its terminals.

The total value of capacitance at the crystal's terminals is (Ca+Cp)/2, where Ca is the actual value of capacitor, per pin, that you place at the OSC1 and OSC2 pins, and Cp is the per pin parasitic capacitance. Cp is usually about 8pF or so. So, if your crystal wants to see a 20pF load, you will need to put 32pF capacitors at both OSC1 and OSC2: (32+8)/2 = 20pF This is one of the very useful and neat things that I learned from the PICLIST :-)

In [many cases], it probably "just works" because the parasitic capacitance is enough to satisfy #1 and [the] oscillator is probably running with perhaps 0.1% frequency error, a few kHz with an xtal in the several MHz range.

[This] question is almost a FAQ, and every time it comes up, there is always a debate about the last point that you make: what is the difference between "series" and "parallel" crystals. Here is the usual consensus: there is no difference. Every crystal has a parallel resonant frequency and a series resonant frequency. They are separated by a few kHz and which one you get depends on what value of external capacitance you place on the crystal (I think it ultimately has to do with #2 and exactly what frequency gives the 0 deg phase shift through the whole network). Crystals sold as "parallel" crystals achieve their rated frequency when loaded with the recommended load capacitance. Those sold as "series" crystals achieve their rated frequency when operated in series resonant mode (determined by the external capacitance, but I'm not sure how to figure this one out numerically,since it isn't specified for xtals sold as "series").

If you need a rough frequency standard, just use the (Ca+Cp)/2 formula. If you need strict accuracy, you will have to use a trimmer cap for one of the caps and use it to adjust the frequency.

See also:




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