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I'm having trouble with this part of my circuit. I have this crystal connected, but I see only a 3.3V DC signal on Pin 1 and 3 of the Crystal, and a 0V signal on Pin 2 and 4 of the crystal. I'm wondering two things:

  1. Does a crystal have polarity? (I don't believe it does, but want to confirm)

  2. Do the XTIN and XTOUT pins need to do something to get the crystal to oscillate? FYI, this part on the right that the crystal is connected to is FT2232H.

    Crystal

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  • \$\begingroup\$ How are you measuring it? Oscilloscope or multimeter? \$\endgroup\$
    – Tom Carpenter
    Jan 19, 2016 at 2:24
  • \$\begingroup\$ And no, crystals don't have a polarity. The two pins are essentially connected to either side of an inverter (not gate) - hence in and out. \$\endgroup\$
    – Tom Carpenter
    Jan 19, 2016 at 2:25
  • \$\begingroup\$ @TomCarpenter measured on a scope. \$\endgroup\$
    – Russell
    Jan 19, 2016 at 2:30
  • \$\begingroup\$ You should see it oscillating if you measure from the XT pins to ground, though the DC level of both pins may well be different. However you may find that the scope probe capacitance may be too much and cause it to stop oscillating especially if it is set to 1x. \$\endgroup\$
    – Tom Carpenter
    Jan 19, 2016 at 2:34
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    \$\begingroup\$ Just noticed you said you were measuring across the crystal (1 and 3) - if there is any common ground between the oscilloscope and the device (e.g. USB port) - this would just short one of the crystal pins directly to ground through the oscilloscope which would definitely stop it oscillating. \$\endgroup\$
    – Tom Carpenter
    Jan 19, 2016 at 2:38

1 Answer 1

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Your schematic symbol is wrong- it looks like the GND pins are shorting the oscillator input and output leads to ground internally, and I doubt your crystal is made that way.

Here is a typical 4 lead SMD crystal:

enter image description here

Check your crystal's load capacitance parameter- the values you show might be okay for an 8pF load spec, but not for an 18pF. (Should be about double the load capacitance number, minus a few pF).

Check also that you have Rref 12K 1% to ground from the REF pin of the FT2232H, and that the 1.8V regulator output VREGOUT is actually close to 1.8V and connected properly to VREGIN assuming you're using the usual circuit.

Usually an oscilloscope probe (even a x10, but definitely a x1) will cause the oscillator to stop when connected to OSCI, though you may be able to see a waveform with a x10 probe on OSCO. Crystals do not have polarity.

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  • \$\begingroup\$ If you look at the pin numbers on the symbol, that likely isn't an issue, 1 and 3 are the crystal. The symbol is a bit weird, but as long as the pin numbers are correct that's all that matters. \$\endgroup\$
    – Tom Carpenter
    Jan 19, 2016 at 2:44
  • \$\begingroup\$ @TomCarpenter As I said above, I doubt it's the problem at hand and the pin numbers agree (though that alone is no guarantee- pin numbering can get screwed up), but it sure looks terrible on a schematic. \$\endgroup\$
    – Spehro Pefhany
    Jan 19, 2016 at 2:48
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    \$\begingroup\$ I misread what you wrote about the symbol, I agree that it's not a good symbol! Also, typically 6MHz crystals have quite a high capacitance - around 18pF, so a load capacitance of 18pF is probably ok though maybe a bit low - I'd use 27pF as per the FTDI recommendations. \$\endgroup\$
    – Tom Carpenter
    Jan 19, 2016 at 2:49

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