number preloaded into timer (PIC18)

Discussion in 'Homework Help' started by TheRekz, Dec 13, 2009.

  1. TheRekz

    Thread Starter New Member

    Oct 25, 2009
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    If I want a timer with a period of 1 second
    The external oscillator is 8 MHZ
    The prescaler is set to 128


    What number should be preloaded into the timer? I remember there is a formula to do this, but I can't find it... can anyone help me
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    The oscillator outputs 8,000,000 per second. What is left after dividing by 128?
     
  3. TheRekz

    Thread Starter New Member

    Oct 25, 2009
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    It's 62,500, I know... but is that the number that should be preloaded to the timer?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    How many counts of (8,000,000/128) = one second?
     
  5. TheRekz

    Thread Starter New Member

    Oct 25, 2009
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    what do you mean?? how do I know that?
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    It's implicit in the unit - Hertz, which is a measure of cycles per second.
     
  7. TheRekz

    Thread Starter New Member

    Oct 25, 2009
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    so you mean I need to divide 1/(8,000,000/128)
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    For a digital clock that runs with an line operated supply, you can arrange to use part of the AC waveform to produce a 60 Hz waveform. If you apply that signal to a counter, one second will have elapsed every time the count reaches 60.

    A 60 Hz waveform goes through 60 repetitions in one second. An 8 MHz waveform goes through 8,000,000 repetitions in one second. Frequency is always cycles (or repetitions) per second.
     
  9. TheRekz

    Thread Starter New Member

    Oct 25, 2009
    13
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    so you're saying that 62,500 cycles is equal to 1 second... therefore I should set the count to 62500?
     
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