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I have been working on design for which I have a reference schematic. I came across the following crystal setup:

enter image description here

I've always seen crystals being connected to two pins on an MCU, like XIN and XOUT with its corresponding capacitors to make up for the required load capacitance.

In the reference design, the crystal is connected only to the XIN pin—in fact, there is no XOUT pin. At the beginning I thought it was an active oscillator but it is indeed a simple crystal.

How does this work? Does it have to do with series/parallel resonance of the crystal?

Thanks

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There is no reason why the resonant element can't be connected to a rail.

See, for example, DOI 10.1109/JSSC.1984.1052122 A One-Pin Crystal Oscillator for VLSI Circuits, Santos et al..

enter image description here

Ignoring bias components in both cases, the circuit is really similar to the Pierce oscillator conventionally used (using complimentary transistor and grounded at a different point):

schematic

simulate this circuit – Schematic created using CircuitLab

Both capacitors must be integrated into the chip for this to work so the load capacitance on the crystal is fixed. The authors claim lower crystal dissipation due to smaller feedback capacitors, which would be an advantage in some cases.

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    \$\begingroup\$ I think you could improve the answer by explaining how does one-pin oscillator works, what makes it different from the usual two-pin configuration if there is any difference. \$\endgroup\$ – Unknown123 Apr 2 '19 at 23:23
  • \$\begingroup\$ @Unknown123 added info \$\endgroup\$ – Spehro Pefhany Apr 3 '19 at 3:30
  • \$\begingroup\$ Why this circuit works without using both the XIN and XOUT pin? Is it depends on the uC fuses configuration? \$\endgroup\$ – Unknown123 Apr 3 '19 at 4:00
  • \$\begingroup\$ @Unknown123 Seems like it has only one configuration with the crystal grounded on one side. Nothing to do with fuses. There are still two connections to the crystal, but one is ground, and the load capacitors are built-in so you should use the recommended crystal type or specification. \$\endgroup\$ – Spehro Pefhany Apr 3 '19 at 4:02
  • \$\begingroup\$ For example, the ATmega8535 uC on page 25 stated that there are six options available to configure the clock source. If I'm going to use lets say 16MHz one-pin crystal configuration, do you think I need to use the External Clock fuse? \$\endgroup\$ – Unknown123 Apr 3 '19 at 4:08

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