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oscillator frequency

Discussion in 'Electronic Components' started by ron, Feb 24, 2006.

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  1. ron

    ron Guest

    hi, how does one decide the operating frequency of a crystal oscillator
    required for a PIC18F452? is there any relation between the supply
    power and the frequency?
    the datasheet seems confusing.
    can anyone also kindly brief on the XT, HS, etc resonators mentioned in
    the datasheet?
     
  2. Quitus

    Quitus Guest

    First of all, take into account that MICROCHIP recommends you to no use
    this PIC for new designs ... consider using PIC18F4520.

    You must decide the frequency of the crystal depending on your
    application. The more velocity your application need to calculate or to
    do anything, the more xtal frequency you may use.

    On PIC18F452 DS you could find the range of frequencies allowed by this
    device.
    No. Supply power only affects you on POR situations because clock setup
    has an initial delay after POR that depends on supply voltage and on
    xtal frequency ... but it's really insignificant.
    I'm not sure why you see this confusing ... is so clear how each mode
    (XT, HS, ...) works. Depending on the mode you select, you'd get some
    features or others ... for example, LP mode is the best for
    low-power/low-consuming applications but it's not recommended for fast
    calculations.

    To configure the PIC with the oscilation mode you decide to use, you
    may setup correctly the "Configuration bits". That is, if you use MPLAB
    environment, on "Configure" main menu, the "Configuration Bits..."
    option. However, if you not use MPLAB, you can setup the configuration
    bits by code: see page 197 of PIC18F452 DS.

    Regards,

    Quitus.
     
  3. Add a coda here. The above is true for the 18F452, but for the 18LF452,
    which can run at much lower supply voltages, the maximum supported
    frequency does drop with voltage. The same is true for the LF4520.
    I suggest that the original poster, tries looking at the following
    application notes from Microchip:
    AN588, though written before the 18 family (and therefore not discussing
    the PLL), and AN826, give a superb overview of the reasons to chose
    resonators or crystals, and the oscillator modes.

    Best Wishes
     
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