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crystal vs crystal oscillator — Parallax Forums

crystal vs crystal oscillator

TCTC Posts: 1,019
edited 2014-03-19 10:29 in Propeller 1
Hello all,

I have been wondering for some time, Would there any be benefits using a 80Mhz Crystal Oscillator instead of a 5Mhz Crystal (pll16x) for the prop? And if so, What would those benefits be?

Thanks
TC

The links are just for reference.

Comments

  • TubularTubular Posts: 4,703
    edited 2014-03-18 13:23
    It depends on your application, TC. Because crystal frequency is affected by temperature, tcxo's have less frequency variation by an order of magnitude or so, This allows them to keep time (or sync, etc) more accurately in the long term. Normal crystals are fine for video use.

    There is a 4 pin footprint that works with both crystals and tcxo's such as the fox924 jmg suggested for P2. So if designing a product you have the choice of what to load.

    On thing to keep in mind - using an 80 MHz input you are stuck at 80 MHz. But using a 5MHz clock (tcxo or crystal) you can select the PLL multiplier 1x, 2x, 4x, 8x, 16x which gives you more flexibility
  • TCTC Posts: 1,019
    edited 2014-03-18 18:33
    Tubular wrote: »
    It depends on your application, TC. Because crystal frequency is affected by temperature, tcxo's have less frequency variation by an order of magnitude or so, This allows them to keep time (or sync, etc) more accurately in the long term. Normal crystals are fine for video use.

    Ahh, that makes sense.
    There is a 4 pin footprint that works with both crystals and tcxo's such as the fox924 jmg suggested for P2. So if designing a product you have the choice of what to load.

    Ok, that thing is small, and impressive. thanks.
    On thing to keep in mind - using an 80 MHz input you are stuck at 80 MHz. But using a 5MHz clock (tcxo or crystal) you can select the PLL multiplier 1x, 2x, 4x, 8x, 16x which gives you more flexibility

    Most of my projects I am running at 80Mhz. Do I need to? Don't know, and don't care(right now). Every project I have worked on to date have been powered from the mains(with a wall wort), not batteries. But When the time comes to run a project on battery power, you can bet I am going to slow down the prop as much as I can.
  • jmgjmg Posts: 15,173
    edited 2014-03-18 21:14
    Tubular wrote: »
    There is a 4 pin footprint that works with both crystals and tcxo's such as the fox924 jmg suggested for P2. So if designing a product you have the choice of what to load.

    Another new device, available in even smaller packages, is the SiLabs Si504 series.

    This is not as stable as FOX924, but Si504 (20ppm best) has the nice feature it can move to any frequency, (32KHz ~ 100MHz) with a 4 byte REAL (and an optional 16b integer fine trim)

    You order the Si504 with a preset frequency, and that is what it powers up at.
  • TCTC Posts: 1,019
    edited 2014-03-19 03:57
    jmg wrote: »
    Another new device, available in even smaller packages, is the SiLabs Si504 series.

    This is not as stable as FOX924, but Si504 (20ppm best) has the nice feature it can move to any frequency, (32KHz ~ 100MHz) with a 4 byte REAL (and an optional 16b integer fine trim)

    You order the Si504 with a preset frequency, and that is what it powers up at.

    Ok, that one is cool. Having the option to on the fly, change the clock frequency. I can see that one being used for some cool stuff.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2014-03-19 10:29
    TC wrote: »
    Most of my projects I am running at 80Mhz. Do I need to? Don't know, and don't care(right now). Every project I have worked on to date have been powered from the mains(with a wall wort), not batteries. But When the time comes to run a project on battery power, you can bet I am going to slow down the prop as much as I can.

    Running slower may only be necessary for a micro-power project. The Propeller itself doesn't use enough power in relation to motors and other add-ons to really need to slow down the clock rate to get more battery life.

    But if you plan to hand a microcontroller in a tree and take environmental samples everyday over a year, a slower clock rate can certainly help.

    If you don't really know why you would use another rate besides 80Mhz, you probably have enough to learn without changing the timing. Going fast might be useful with some video, but video can be a pretty advanced topic.

    IOW, learn as much as you can with the conventional Propeller clock rate and crystal. When you do have a real need to change, it will become obvious.
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