Programmable Oscillator

Discussion in 'General Electronics Chat' started by CVMichael, May 19, 2010.

  1. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
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    OMG !!! if I only knew they existed earlier !

    I spend hours to make this: http://forum.allaboutcircuits.com/showpost.php?p=244923&postcount=9 (see attachment), when there is a chip that does all that and much better !

    Most of the "Programmable Oscillators" I found it seems that the factory will program the chip to a fixed frequency, then they give it you... what's the point of that ?

    Anyways, now... this one: LTC6903/LTC6904, is another story, it seems that it can be programmed on the fly with a microcontroller, and it goes from 1kHz to 68MHz, awesome !! exactly what I need !!

    One thing that is confusing is that if you look at the sample diagram in the "TYPICAL APPLICATION" section, the clock of the Oscillator goes back into the microcontroller, pin "OSC1/CLKIN". Now the thing is, the microcontroller is PIC16F73, and it goes up to 20MHz, so how does that work ?
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    Where do you see the PIC16F73 in the datasheet?

    Am I missing something?
     
  3. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
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    Right on the first page of the LTC6903/LTC6904 datasheet.

    But I think I know what is going on... The microcontroller runs on internal clock (probably 8MHz), and that pin is used for "CLKIN".
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
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    Oh, my...jeez..

    I completely missed that.. ON THE FIRST PAGE. I read through to the Typical Apps, and missed the first page completely. My apologies.

    The PIC would be using the startup clock from the LTC6903 until the PIC sends a request via digital output for a faster clock speed. (Up to the fastest allowed for the particular uC)
     
  5. n1ist

    Active Member

    Mar 8, 2009
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    When first powered up, the LTC6903 outputs a clock of 1.039kHz. The PIC can then reprogram the clock to any value, but it should not program a value > 20MHz since the PIC isn't guaranteed to work above that. Just because the oscillator can go up to 68MHz doesn't mean that you need to do that in every application.

    BTW, this trick won't work on some other processors (like most of the Atmel AVR line) since they don't like having the clock frequency changed while they are running.
    /mike
     
  6. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
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    I can't find in the PIC16F73 datasheet anything about the internal clock... hmm

    But anyways, I am planning to buy the LTC6903 because it's too awesome not to get it, and I will run it with another microcontroller (probably PIC16F88), which has a 8MHz internal clock.
     
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