Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

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Wrong Crystal

Views: 1239

#83783

Wrong Crystal | 20 November, 2019

Trying to create my own RFID reader based off the MFRC522 demo board by copying the layout and components, then later i am planning to change the footprint.

I am having trouble replicating their crystal. The mfrc522 datasheet calls for a 27.12 mhz quartz crystal, and the crystal on the existing board says 27.120 on it, seemed simple enough. But when i install the below part from digikey, the board does not work. The board begins to work again when i reinstall the crystal from the example board.

Digikey Part https://www.digikey.com/product-detail/en/LFXTAL014948BULK/1923-1441-ND/8633729

Picture, board with two crystals installed that both work, and two crystals in the background from digikey that do not work https://ibb.co/GT2Hbpv

MFRC522 Datasheet https://www.nxp.com/docs/en/data-sheet/MFRC522.pdf

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#83784

Wrong Crystal | 20 November, 2019

You bought quartz (at DigiKey), which should have 10 pF capacitors, and mounted on a demo board is from JFR - "(..) Load capacitance (CL): Series, 16pF, 20pF, 30pF, or specify (..)" - try changing the quartz capacitors, or check with an oscilloscope to see if it works.

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#83788

Wrong Crystal | 20 November, 2019

Thank you, did not consider that changing the crystal would make it necessary to change those caps. i will order some the correct size and try.

Much appreciated!

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#83814

Wrong Crystal | 26 November, 2019

I ordered a capacitor book. I reinstalled the crystal that i purchased from digikey, and swapped the two load capacitors out from 30pf to 10PF which works out well with the datasheet that calls for 10pf Load Capacitance, and this equation (CL = (C1 * C2) / (C1 + C2) + Cstray) gives me a similar result of 7PF when assuming that my stray capacitance is 2pf

but even with 10pf caps on, or 5pf caps on, the crystal from Digikey does not work, and the original crystal STILL works with the different load capacitors on, I'm missing something.

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#83815

Wrong Crystal | 26 November, 2019

Starting to get things to work. I tested the circuit without any load capacitors and had my first bit of success, thinking now that i possibly just have a lot more stray capacitance that i originally thought. probably going to use 2pf load caps.

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