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I have a quartz crystal and I am connecting it to a microcontroller's EXTAL and XTAL pins.

  • Can I connect it anyway?
  • Do the crystals have polarity?
  • In EXTAL and XTAL, which is input and which is output pins?
Electric_90
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Freshman
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  • Crystals don't have polarity. If it works one way, it will work the other way too. – user57037 Feb 05 '19 at 05:04
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    Electronic_Maniac - Hi, I see you haven't accepted any answers on your (currently 21) previous questions. While not mandatory, this is one of the few ways that question askers effectively say "thanks". Therefore your choice not to accept any answers could be perceived as not wanting to say thanks. If you didn't intend to give readers that impression, then perhaps review answers to your previous questions and consider accepting the one which best helped in each case? In case you didn't read them already, see the [tour] and [help]. – SamGibson Feb 05 '19 at 05:37
  • not accepting any answers isn't cool. – uhoh Feb 05 '19 at 08:01
  • I am sorry. I usually thank people for their valuable support and answers in the comment section. I will go and do this. I apologize to all my friends for being unaware of this – Freshman Feb 05 '19 at 08:21
  • 3 hours later and none accepted. – Andy aka Feb 05 '19 at 11:55
  • I have accepted and vote the answer which has helped me. But it comes as,"Votes cast by those having less than 15 reputation are recorded but not displayed post score. – Freshman Feb 05 '19 at 13:11
  • @Electronic_Maniac - Hi, Thanks for your responses. You seem to be confused about the difference between upvoting (which as you said, will not change the displayed value until your reputation score is higher) and accepting. You said: "I have accepted [...] the answer which has helped me." No, you haven't yet. None of your questions has an accepted answer. Please read my previous comment again, especially the first link. Thanks :-) – SamGibson Feb 06 '19 at 14:12

1 Answers1

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Do the crystals have polarity?

No.

In EXTAL and XTAL, which is input and which is output pins?

This might vary between different MCUs (check the datasheet for yours). The convention I have seen on MCUs is shown in the following diagram (and many others - this diagram was just picked at random):

quartz crystal connections to MCU

(Image Source: Freescale Semiconductor Application Note AN2049: Some Characteristics and Design Notes for Crystal Feedback Oscillators, page 2)

As explained in that appnote and shown on the diagram, EXTAL is an input and XTAL is an output.



Update: As kindly commented by TimWescott, the E of the EXTAL pin is likely to be because that's where an External oscillator signal (e.g. from a crystal oscillator output or other driver) would be connected, thereby showing that EXTAL is an input.

While that fits my observations with various MCUs, it's always possible that a manufacturer chose to deviate from the diagram and common labeling above. So if you're unsure, check the relevant MCU datasheet e.g. looking for a diagram similar to the one above.

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SamGibson
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    I'm pretty sure that the 'E' in "EXTAL" means external, i.e., it's the pin to which you apply an external oscillator, and, hence, an input. – TimWescott Feb 05 '19 at 06:02
  • @Tim - Hi, Agreed completely. However I don't have a definitive source which I can link, that states this. – SamGibson Feb 05 '19 at 06:06
  • True. I suspect this is up to the individual manufacturer -- you could set up shop and call them FRED and SAM, and let people wonder. The only thing it would affect would be your sales. – TimWescott Feb 05 '19 at 06:17
  • @Tim - I've tried to capture the situation in an update. Thanks for the comments :-) – SamGibson Feb 05 '19 at 07:34
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    EXTAL is indeed the input pin for an external clock signal, see e.g. datasheet, section 5 – Huisman Feb 05 '19 at 08:13