Experiments with unusual crystals

Experiments with unusual crystals

Postby Joseph Watson » Mon May 11, 2015 9:15 pm

I am not having a problem. I just thought I would tell about some simple experiments that I tried.

I recently started playing with a PIC18F25K20. The little development board I am using has a 3-pin socket into which a crystal can be placed which permits easily changing crystals. Oddly, the middle hole of the crystal socket is not grounded as I thought it might have been since that would make it readily able to accept a resonator in place of a crystal. The board does include a 15pf capacitor from each side of the crystal socket to ground so maybe having the middle hole grounded would result in a bit too much capacitance. The intended clock frequency in this system is based on using a 16 MHz crystal which is scaled up via a PLL internal to the PIC resulting in a 64 MHz clock.

In any case, after playing with a couple of different crystals (16 MHz and 4 MHz), I started poking around in my stock of crystals and resonators to see what else I might try. I tried a 16 MHz resonator and found that it worked well anyway. Of course, the capacitors on the board were still present so not having a ground for the capacitors built into the resonator did not seem to matter much. Next, I tried a 20 MHz crystal which worked and so the PIC must have been over-clocking at 80 MHz for the brief period that I let it run.

Then I became more adventurous and tried a 3-pin 10.7 MHz ceramic filter from the I.F. circuit of an FM radio. That worked. I tried a 2-pin 4.5 MHz ceramic filter from an old TV. That worked. I tried some 2-pin crystals from 49 MHz walkie talkies which operate at 3rd overtone so the crystals are actually around 16.5 MHz. They worked. I tried a 2-pin 455 KHz ceramic filter from an AM radio circuit which did not work, nor did a 32,767 Hz watch crystal.

Of course, I know that I was pushing the PIC in ways for which it was not intended. For one thing, I never tried changing the Config settings related to the oscillator. I was just poking crystals into the socket to see what ran. Has anyone else had similar experiences with the use of odd crystals or resonators?
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Re: Experiments with unusual crystals

Postby tunelabguy » Mon May 18, 2015 3:40 am

Joseph Watson wrote:..nor did a 32,767 Hz watch crystal..

The 32kHz watch crystal probably would have worked, except that it burned out immediately. Those crystals have are order of magnitude lower power rating. Normal crystal circuits just drive them too hard and they fracture. To use a watch crystal you need to limit the power, usually by a very large series resistor from the "driving end" of the crystal.
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Re: Experiments with unusual crystals

Postby Joseph Watson » Mon May 18, 2015 6:40 pm

That makes sense. I never thought about that. Thanks.
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Re: Experiments with unusual crystals

Postby Olin Lathrop » Wed May 27, 2015 10:02 pm

tunelabguy wrote:The 32kHz watch crystal probably would have worked, except that it burned out immediately.


It's actually quite unlikely to have worked. 33 kHz is probably way below the minimum input frequency of the 4x PLL. These things all have a range of frequencies over which they operate.

Those crystals have are order of magnitude lower power rating. Normal crystal circuits just drive them too hard and they fracture. To use a watch crystal you need to limit the power, usually by a very large series resistor from the "driving end" of the crystal.


On a PIC, you don't need to do that usually. The LP crystal driver mode is intended for such low power crystals.
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Re: Experiments with unusual crystals

Postby tunelabguy » Wed May 27, 2015 11:44 pm

Olin Lathrop wrote:
tunelabguy wrote:The 32kHz watch crystal probably would have worked, except that it burned out immediately.


It's actually quite unlikely to have worked. 33 kHz is probably way below the minimum input frequency of the 4x PLL. These things all have a range of frequencies over which they operate.

Those crystals have are order of magnitude lower power rating. Normal crystal circuits just drive them too hard and they fracture. To use a watch crystal you need to limit the power, usually by a very large series resistor from the "driving end" of the crystal.


On a PIC, you don't need to do that usually. The LP crystal driver mode is intended for such low power crystals.

As Mr. Watson said in his first posting, "For one thing, I never tried changing the Config settings related to the oscillator." So it is unlikely he would have been in LP crystal driver mode. And even if the 4x PLL doesn't work, the basic oscillator front end could easily burn out the watch crystal. But you are right. He had several factors against that watch crystal working.
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