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CC3200 Reference design: load capacitance of 40MHz crystal

lilyhack
Expert 1160 points
Other Parts Discussed in Thread: CC3200

In the CC3200 reference design, a 40MHz 8pf SMD crystal is used (Y3). So the load capacitance should be 8pf with this crystal. But in the schematic, I see 6.2PF is used. Can anyone please explain why?

I am planning to use a 18pf (http://www.digikey.com/product-detail/en/ABM3C-40.000MHZ-D4Y-T/535-11304-2-ND/2639905) crystal instead. What load capacitance should I use?

  • 0
    Intellectual 260 points

    PCB track and CC3200 pin capacitance add up with 6.2 pF to give you 8 pF.

    The datasheet has a max load capacitance rating of 7 pF for the oscillator. I believe the oscillator drivers won't be able to drive the load you're considering.

  • 0 in reply to Lotus 77
    Expert 1160 points

    Thanks.

    I can't seem to find the datasheet of  Q24FA20H00396 to make the footprint. Found a generic Epson FA20 though. Is this the one I should use for making the footprint?

    If not, can anyone please share the right datasheet?

  • 0 in reply to lilyhack
    TI__Guru 58035 points

    Hello,

    As Lilyback mentioned , we dont recommend using an XTAL with load cap spec > 12pF. I also see that that XTAL has a ppm error of 30ppm , which does not comply with the WiFi specs (25 ppm is the wifi spec).

    You can find the datasheet for the Epson Xtal at the following location (The footprint is same for all FA-20H products).

    http://megagoal.com/ocart/index.php?route=product/product&path=20_26&product_id=51

    Regards,

    Dham

  • 0 in reply to Vivek Dham
    Prodigy 215 points

    Hi Dham 

    The crystal recommended from you in that last post is supplied from Singapore it takes couple of weeks to get to US. Is it possible to use

    8Q-40.000MEEV-T from TXC

     It has +- 10 ppm error and 8 pF load capacitance.

    If yes it is required to be connected to 6.2 pF caps?

    Thanks 

    Mehran

  • 0 in reply to m.rizi
    TI__Guru 58035 points

    Hello Mehran,

    This XTAL should be fine if your application would not see a temperature beyond -20C to 70C temperature range.

    The 6.2pF caps are to match the load cap for the TXAL (8pF). This external cap along with the PCB and component paracitics bring the total load cap to 8pF. If you are following the exact same layout as in the reference design then 6.2pF should be close to the required value. If there is a slight change , I suggest tunning the cap little bit to get to the right center frequency. At room temperature adjust the caps so that the center frequency error is < 5 ppm.

    Regards,

    Dham

  • 0 in reply to Vivek Dham
    Prodigy 185 points

    To confirm, it seems essential to have the right crystal, the device seems very picky: I've just lost three days of my life on a new board, thought I'd purchased the right crystal from Farnell/Newark, but it was not to be: that was a 12pF device. Only when I replaced the crystal with one from a CC3200 Launchpad did it suddenly start oscillating. Similarly, I tried one of the crystals I'd purchased on the Launchpad, and they definitely don't work. I'm awaiting delivery of some new crystals now.

    For the avoidance of doubt, the failure mode is that unit simply doesn't oscillate.

    What hasn't helped is that the data sheet from Epson for the FA-20H has four digits of the part number that don't indicate what they are, other than "part of the specification" with no further detail. In addition, I've been unable to find any reference to the marking of the prescribed Epson device in the Launchpad BOM, schematic or CC3200 data sheet (Q24FA20H00396). It turns out that the crystals on the three CC3200 launchpads and three CC3100boosts I have are not Epson at all, but IQD.