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Topic: What If?. (Regarding Crystals and Caps) (Read 1 time) previous topic - next topic

cjdelphi

Take any Atmel chip which relies on a 2 pin crystal oscillator.

You attach the 27pf caps, wire it all up correctly, upload and have it blink 1second of / 1 second off.

We now remove the 2 ceramic caps. (i've tried this)

In my test.

2 Attiny85s

1 Attiny, used the Caps and a 2 pin 20mhz crystal
2nd Attiny, used just the 20mhz crystal

The only visible difference was the LED's were starting to shift out of phase, maybe 1 - 2ms gain/loss every second..


But what i found interesting is the chip kept on working without the Caps.... I wish i tried to upload without the caps and just relying on the crystal now, so why
are the 2 ceramic low value cap's so vital in the role of time keeping?

retrolefty


Take any Atmel chip which relies on a 2 pin crystal oscillator.

That is really a 2 pin crystal resonator. A crystal oscillator is a different animal. A crystal resonator plus the internal clock electronics inside a AVR chip creates a crystal controlled oscillator.

You attach the 27pf caps, wire it all up correctly, upload and have it blink 1second of / 1 second off.

We now remove the 2 ceramic caps. (i've tried this)

In my test.

2 Attiny85s

1 Attiny, used the Caps and a 2 pin 20mhz crystal
2nd Attiny, used just the 20mhz crystal

The only visible difference was the LED's were starting to shift out of phase, maybe 1 - 2ms gain/loss every second..


But what i found interesting is the chip kept on working without the Caps.... I wish i tried to upload without the caps and just relying on the crystal now, so why
are the 2 ceramic low value cap's so vital in the role of time keeping?

Because the marked frequency of the crystal resonator was manufactures based on it working with a specified fixed amount of external capacitance loading. Some of the capacitance loading comes from the internal capacitance of the clock pins of the AVR and the external 'padding capacitors' supply the rest. The crystal resonator will still oscillate at different cap loading values, but just not at the exact frequency it is marked at. So it still works but at a slightly different frequency, typically higher as you lesson the cap loading.
Lefty


cjdelphi

ahhh, well out of curiosity, i went and just re-tried what i tried a few weeks back

I was wrong..


You CAN upload via ISP of an Arduino and flash the bootload and upload binaries, with nothing more than just a 20mhz crystal, no caps.
You CAN'T however run a sketch and have it run with just the 20mhz crystal, eg let's say pin 0 on/1000ms/0 off , what will happen is pin 0 will trigger high or low and not turn off, no idea what's going on here.

Interesting, to simply flash a new bootloader or upload a sketch, the Cap's don't appear to be needed!

be80be

If he doing this on a solder-less bread board the board has capacitance
That let's the chip run but the same setup on a PCB will not run. Seen people do this.
Ive even seen the solder-less bread board load a crystal using caps remove them and it  runs fine.

fungus


I've tried this and it works!!!


The 'Mythbusters' approach to science isn't always the best...

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

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