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Forum Index : Microcontroller and PC projects : Crystal Oscillator query
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| Bowden_P Senior Member  Joined: 20/03/2019 Location: United KingdomPosts: 166 |
Hi, I am looking for advice on how to use a small SMD quartz crystal with a Microchip micro's on-chip oscillator module, seeeking to get the drive level within the crystal's spec. I am intending to use an ABRACON ABM3 20MHz SMD crystal, with a PIC16F1779 micro. The crystal has a drive level spec. of 10uW typical, 100uW max, with an ESR of typically 30 Ohms, and a load capacitance of 18pF. The micro's spec. doesn't mention any oscillator o/p amplitude, but if I assume an o/p swing of 2V peak-to-peak, this equates to 0.707V rms. With an ESR of 30 Ohms this will make a drive level of 16.6mW to the crystal, way over its max spec. I gather a series resistor can be added to reduce this. If I assume a drive level of about 30uW would be OK, this is equivalent to 30mV rms, or about 1/24 of the oscillator o/p. So, if I add a series resistor of 24 x 30 Ohms, or about 720 Ohms, does this fix the drive level problem? Or are these calculations plain wrong! With best regards, Paul. Nothing so constant as change. |
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phil99 Guru  Joined: 11/02/2018 Location: AustraliaPosts: 2957 |
That output swing is with respect to ground, but the other end of the crystal isn't connected to ground. The clock input pin may be a fairly high impedance even at the crystal operating frequency. The datasheet for the micro might have some information regarding that and what sort of crystal is needed. Sometimes loading capacitors are needed on either side of the crystal which could complicate things further. |
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| Bowden_P Senior Member Joined: 20/03/2019 Location: United KingdomPosts: 166 |
Hi Phil99, Thanks for your reply. The output swing will be somewhere between Gnd and Vdd, but I doubt it will be that amplitude, hence my assumption of 2V pk-pk. Just a guess, however. I don't know what effect the loading caps. will have either, but are usually the same as the 18pF crystal load value. In past projects I have used crystals in small metal can-type packages with wire leads, and haven't worried about drive levels or load caps. For this project reliability is more important, hence my query! With best regards, Paul. Nothing so constant as change. |
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| phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2957 |
Page 104 of the Microchip PIC16(L)F1777/8/9 datasheet has a little info, but for "low drive level" crystals says consult the crystal datasheet. |
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| Bowden_P Senior Member Joined: 20/03/2019 Location: United KingdomPosts: 166 |
Hi Phil, Yes, I looked up the ABRACON site. They suggested using a current probe to measure crystal current in order to calculate the required series resistor, or a more suck-it-and-see approach of increasing the resistor value until it stops oscillating, then divide that value by the crystal ESR to get a Gain Margin for the oscillator amplifier. Not sure what I would do with that. The resistor and the o/p load cap. form a voltage divider, so as you say it gets complicated here. Thanks for your interest, Paul. Nothing so constant as change. |
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| mozzie Senior Member Joined: 15/06/2020 Location: AustraliaPosts: 197 |
G'day Paul, It might pay to take a look at AN588, this will hopefully steer you in the right direction. AN00588 Regards, Lyle. Edited 2026-01-27 11:19 by mozzie |
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| Bowden_P Senior Member Joined: 20/03/2019 Location: United KingdomPosts: 166 |
Hi Mozzie, Thanks for the link, I'll give it a good read. This topic gets more complicated than I thought. Before now I just connected a crystal, and it always worked, but now I'm a bit more concerned about getting it right. With best regards, Paul. Nothing so constant as change. |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 5643 |
Paul, Not all crystal oscillator circuits inside micro's are capable of working with these low power crystals. The resistor value you calculated is in the ballpark. Typical values for low power crystals can be 1k to 4k. Then, before you settle on a value, try to run the circuit warm (+50C) and cold (-20C) with a resistor dual the value (so if you goal for 1k, use a 2k resistor). Most important in these tests is to make sure the micro starts up cold and warm. A 18pf low power crystal is not easy to use. You most likely end up with 2 x 33pf (or 27pf). The 2k resistor in combination with the 33pf load capacitors give quite some phase shift. That is why most low power crystals are 12pf. Happy Engineering Volhout P.S. check 3.5.2 and 3.5.3 of attached ST document on how to measure drive level with simple means. A 1pF probe is used, but if you have a probe that is 10pF input, you can change the 33pf board capacitor (for the test only) with 22pF, parallel to the probe. cd00221665-oscillator-design-guide-for-stm8af-al-s-stm32-mcus-and-mpus-stmicroelectronics.zip Edited 2026-01-27 17:29 by Volhout PicomiteVGA PETSCII ROBOTS |
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