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What is the purpose of the resistor in parallel with the resonator ? — Parallax Forums

What is the purpose of the resistor in parallel with the resonator ?

BeanBean Posts: 8,129
edited 2005-06-07 22:57 in General Discussion
I have never put a resistor in parallel with the resonator on the SX ?
Is it required ? What bad things can happen without it ?
Bean.


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Comments

  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2005-06-05 03:47
    I don't know about a resonator, but with a crystal depending on the crystal cut, a parallel resistor helps guarantee startup.

    What circuit are you referencing that uses a resistor with a resonator?

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • BeanBean Posts: 8,129
    edited 2005-06-05 04:07
    In the SX-Key manual Version 2.0 page 177 it has a table with settings for different Murata Ceramic Resonators.
    It shows either a 1Meg or 22K parallel resistor for all of them.

    Bean.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    "SX-Video·Module" Now available from Parallax for only $28.95 http://www.parallax.com/detail.asp?product_id=30012


    Product web site: www.sxvm.com

    "It's not getting what you want, it's wanting what you've got."
    ·
  • Guenther DaubachGuenther Daubach Posts: 1,321
    edited 2005-06-06 18:54
    Bean,

    the table on page 177 of the SX-Key manual 2.0 has been taken from the original SX28 data sheet published by UBICOM (you can find it there on page 22).

    On my designs, I usually follow the suggestions in the data sheet, i.e. I add a 10 kOhm resestor in parallel to the 50 MHz Murata resonator sold by Parallax. On the other hand, so far I never had problems when this resistor was missing when I did tests on an SX Tech board which does not have this resistor installed.

    Maybe, the parallel resistor allows for less oscillator circuit gain, i.e. lower DEVICE OSC?? settings still making sure that the oscillator properly starts up at power on but I never tested this so far.

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    Greetings from Germany,

    G
  • mmmm Posts: 56
    edited 2005-06-07 22:57
    It is a feedback resistor for the Oscillators active device. Most active devices can be thought of a TTL or CMOS inverter circuit made to oscillate with positive feedback(by the resonator) but the device still needs to be biased properly(by the resistor) to start oscillating and to maintain oscillations. Too large of a resistor and the feedback is lowered and the oscillator will be slow to start over environmental conditions or even Smile out, to small a resistor and the oscillators gain will be lowered and therefore the output level will be down. It's best to stick to the MFg recommended value, 100k to a couple of megs for CMOS active devices and 1 to 10 K for TTL active devices.


    Mike
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