Which LSE Crystal to use on Discovery Board

LHuxl.1 · ‎2023-02-01

I have an STM32L100C-DISCO board (MB110B Rev. B) and I want to install the LSE crystal, which is not populated by default. I consulted the schematic in the UM1656 "user manual for the 32L100CDISCOVERY discovery kit". Crystal X3 has a part number of "MC306-G-06Q-32.768 (JFVNY)" on the schematic. I consulted the JFVNY datasheet. The footprint didn't look right, but I purchased some crystals anyways. Of course the crystals are much too small to solder to the pads on the board.

What crystal should I purchase to match the X3 footprint on the MB110B discovery board?

Peter BENSCH · ‎2023-02-01

To be absolutely sure, a look at the AN2867 mentioned by @Imen DAHMEN is very useful.

But if it's just for this one board, you can essentially use any clock crystal that fits on the pins.

For example, if it's one with two leads, solder it between pin 1 (marked with the white dot) and pin 4 (the one with R24 written next to it). You also have to solder R24 (solder bridge), R25 (solder bridge), C15 and C16: starting from the side at JP2: C16, R25, R24, C15. The capacitors of the original crystal have 6.8pF each, according to AN2867 you can determine their value more exactly, whereby for laboratory applications a thumb value of 4.7...8.2pF can be used.

Then, as described in the reference manual RM0038, section 20.3.11, you can digitally correct the accuracy of the RTC with a Smooth Digital Calibration. Some additional information and explanation on how the smooth calibration works can be found here.

Good luck!

Regards

/Peter

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Imen.D · ‎2023-02-01

Hello @LHuxl.1 and welcome to the Community :)

Please have a look at this AN2867 Application note Oscillator design guide for STM8AF/AL/S, STM32 MCUs and MPUs. This document will help you for selecting crystals and for the STM32 products.

Imen

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Thanks
Imen

Peter BENSCH · ‎2023-02-01

To be absolutely sure, a look at the AN2867 mentioned by @Imen DAHMEN is very useful.

But if it's just for this one board, you can essentially use any clock crystal that fits on the pins.

For example, if it's one with two leads, solder it between pin 1 (marked with the white dot) and pin 4 (the one with R24 written next to it). You also have to solder R24 (solder bridge), R25 (solder bridge), C15 and C16: starting from the side at JP2: C16, R25, R24, C15. The capacitors of the original crystal have 6.8pF each, according to AN2867 you can determine their value more exactly, whereby for laboratory applications a thumb value of 4.7...8.2pF can be used.

Then, as described in the reference manual RM0038, section 20.3.11, you can digitally correct the accuracy of the RTC with a Smooth Digital Calibration. Some additional information and explanation on how the smooth calibration works can be found here.

Good luck!

Regards

/Peter

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Bubbles · ‎2023-02-01

This crystal?

MC-306 | Crystal Unit | Product | Epson crystal device (epsondevice.com)

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LHuxl.1 · ‎2023-02-01

Thanks for the quick response Imen. I looked up almost a dozen crystals from Table 7 of that application note, and none of the crystals that I looked at is larger than 2 mm in any dimension. The pads on the MB110B board appear to be for a crystal which is approximately 5 x 7 mm.

You can see how large the footprint for this crystal must be by looking at Figure 10 on page 9/30 of document UM1656. Crystal X3 is situated near the center of the board.

I was hoping to get an actual design part number from the MB110B bill of materials. Is the BOM available?

LHuxl.1 · ‎2023-02-01

Yes, I now own 10 crystals that look just like the one in that datasheet. It is larger than most 32 kHz crystals, but it is still too small. Since the design capacitive loading on these crystals is so small, I don't think that it's a good idea to somehow bodge a crystal onto the board.

Peter BENSCH · ‎2023-02-01

Yes, even the board is obsolete you can still find the BOM.

Regards

/Peter

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LHuxl.1 · ‎2023-02-01

Thanks Peter, you broke the code. I had the part orientation wrong. If I rotate the MC306 part 90 degrees, I should be able to solder the crystal to the board. Sorry for the trouble.

Peter BENSCH · ‎2023-02-01

Things happen, everyone, absolutely everyone makes a mistake. :grinning_face:

It's great that it's working now.

If the problem is solved, please mark this thread as answered by selecting Select as best for the preferred answer, as also explained here. This will help other users find that answer faster.

Regards

/Peter

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