Quartz crystal on PIC18, without series resistor

Discussion in 'Embedded Systems and Microcontrollers' started by aeroguy, Jan 30, 2012.

  1. aeroguy

    Thread Starter Member

    Sep 16, 2009
    40
    2
    Hello,

    I have a low drive level quartz crystal (tuning fork type) that I'm using to run timer1 on my PIC18F45K22.

    The datasheet for the crystal ( http://www.ecsxtal.com/store/pdf/ECS-3x8.pdf ) recommends a 330k series resistor to limit drive power to 1 uW, but suggests that one sizes it "as necessary."

    I tried to configure my crystal this way, but I'm developing on a breadboard, and ended up with too much distance between the crystal and the PIC. I eliminated the resistor, and was able to configure it physically close to the appropriate pins on the PIC, and had success (the oscillator is driving the timer). Note, that if the resistor is truly needed I could bend the I/O pin of the PIC, do some solder work, etc. to make this happen.

    Though the crystal/timer is working well, I'm concerned that I'm overdriving the thing and will break it soon.

    I turned to the PIC datasheet. Page 440 of the datasheet tabulates DC characteristics of the SOSC pins to which this crystal is connected.

    The datasheet suggests a typical current flow through the SOSC pins as a function of the Vdd level. However, it also mentions that the numbers are rough and will vary depending on the characteristics of my resonator (as expected).

    It seems that my research is circular, and am having trouble deciding if I need to get this series resistor in place, or if I can leave my circuit as is...

    Does anyone have experience with this matter?

    kind regards,
    Eric
     
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,675
    2,722
    I have never had an issue with overdriving crystal oscillators on a PIC, and I have never used a series resistor. Just make sure you are using the proper oscillator configuration (i.e. low power) appropriate for the clock speed.

    Of course, this is IMHO and your mileage may vary... :D
     
  3. aeroguy

    Thread Starter Member

    Sep 16, 2009
    40
    2
    Thanks for sharing your experience, and for the chuckle!
     
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