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As i am currently designing a project involving a stm32f446ve that requires semi good timing, I am looking into using a crystal oscillator.

As this is still an early prototype, I would like to be able to fit as many different crystals as possible, and after some research determined the 3.2 x 2.5mm footprint is perfect for my need as it is also fairly small.

However, there are slight differences in the pin out. Namely, some wire the crystal itself to the bottom left and top right pin, while others wire it to the bottom right and top left pin. As an example:

http://www.mouser.com/ds/2/40/cx3225sb_e-514166.pdf

vs

http://www.mouser.com/ds/2/122/ecx-32-6206.pdf

To be able to use both, I though it might be a good idea to connect both pins on either side, to connect to the crystal no matter what pins on that side it is connected to. However I have never worked with crystals before and am not sure if shorting the two pins on either side of a crystal is a good idea. As far as I can see however, the pins to which the crystal is not connected are also not connected to anything internally.

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    \$\begingroup\$ Some packages connect both otherwise-unused pins to the metal case around the crystal, with the intent that it can be grounded for shielding. Connecting the pads as you suggest would short the crystal on these packages. Both of the datasheets you linked have the same pinout, though: I think you misread the second one, since the schematic is shown from the bottom. \$\endgroup\$
    – Abe Karplus
    Jul 21, 2016 at 9:08
  • \$\begingroup\$ @dim Posted. I'd started it as a comment because I hadn't checked the datasheets, and so thought there was an actual problem that I didn't have an answer for. \$\endgroup\$
    – Abe Karplus
    Jul 21, 2016 at 9:22
  • \$\begingroup\$ Why not stick to 2 pin packages? \$\endgroup\$
    – Scott Seidman
    Jul 21, 2016 at 9:23
  • \$\begingroup\$ @ScottSeidman the standard two pin HC49/US is huge, and this package comes in a lot of frequencies. Do you have any other specific packages you recommend? \$\endgroup\$
    – user3718254
    Jul 21, 2016 at 9:34
  • \$\begingroup\$ There are dozens, and most are nust fine. Go to a vendor site, limit your search to two pin packages, and sort by the number in stock. \$\endgroup\$
    – Scott Seidman
    Jul 21, 2016 at 9:36

2 Answers 2

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Connecting separate pads of the crystal is a bad idea, but you don't need to.

It's a bad idea because (on some packages) the metal case around the crystal is connected to the spare pins, and so connecting them would short the crystal through the case.

You don't need to because the pinout is standardized for this package—the crystal is on pins 1 and 3. The datasheet you linked for the ECX-32 is confusing, because it shows the schematic from underneath, but it still follows the standard.

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The two packages are the same, the first datasheet shown the pins from top view, the second one shown the pins from bottom view.

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    \$\begingroup\$ I believe my answer already covers this point. Did you have anything to add to the discussion? \$\endgroup\$
    – Abe Karplus
    Mar 2, 2017 at 8:34

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