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Section 2.2.2 of pic18f4550 datasheet says:

In HS, HSPLL, XT and XTPLL Oscillator modes, a crystal or ceramic resonator is connected to the OSC1 and OSC2 pins to establish oscillation. Figure 2-2 shows the pin connections. The oscillator design requires the use of a parallel cut crystal.

Note: Use of a series cut crystal may give a frequency out of the crystal manufacturer’s specifications.

How do I know the type of a crystal? Which type is more widespread?

I'm planning to buy a crystal from here http://belchip.by/section/?selected_section=02156&filter_var=&page=1&limit=120&query=&sorting=price

How do I select the right one, there is no any data about them. Maybe it's possible to tell the type by their look?

Also why, when and how does the type matter?

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  • \$\begingroup\$ If a load capacitance is stated you can assumed that a crystal is intended for parallel operation. I checked one of those crystals and it had a 35 pF load capacitance. \$\endgroup\$ – Leon Heller Jul 15 '16 at 18:27
  • \$\begingroup\$ Parallel cut crystals are most common, and are used by most MCUs. If you use a series crystal instead, it will still need feedback capacitors and the frequency will be out by a small amount. Series crystals are most commonly used in overtone oscillators (3x and 5x). \$\endgroup\$ – Leon Heller Jul 15 '16 at 19:16
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Every crystal that I know of resonates at two frequencies, one due to internal series components and one due to parallel series components. Additionally the two frequencies are very close.

So if one has markings on it that indicate it is series cut then running it in a circuit that naturally only works at a parallel resonance means that the xtal accuracy is called into question; not much but enough in a critical design to affect performance.

In other words, the PIC will force any xtal to run at parallel resonance (because it just won't oscillate at series resonance).

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