AVR Internal Clock

Discussion in 'Embedded Systems and Microcontrollers' started by Mazaag, Aug 24, 2007.

  1. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
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    Hi guys,

    I was wondering if there is an efficient way to access the internal oscillator of the AVR to be able to share it with other external IC's ?

    thanks
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    If you need a distributed clock then it is probably best to use an external clock and distributed it using a clock distribution IC.

    hgmjr
     
  3. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
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    what is the purpose of a distribution IC ? why do i need one ? why can't i just share the output from the external clock ? and how do i implement an external Crystal/Resonator clock ?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    214
    The high frequency associated with the clock is very suceptable to the capacitance that can build up when it is connected to multiple devices. The risetime of the clock can be degraded significantly when subjected to the high capacitiance. The clock risetime specification is often very specific about the quality of the clock signal.

    By using a clock distribution IC (a.k.a.clock buffer, clock driver) this capacitance degradation is avoided.
    This should be as simple as connecting the crystal or resonator across the OSC input terminals of the AVR along with the recommended amount of capacitance for the crystal being used. Be sure to observe the maximum clock frequency for the AVR device in question when selecting the crystal that you plan to use.

    hgmjr
     
  5. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
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    0
    Do i then take a wires from across the OSC input to use in my clock distribution IC ?
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    214
    Actually what is often done is the clock oscillator is built up as a standalone circuit which feeds the clock distribution IC. The output of the clock distribution IC then provides a fixed number of outputs. All of the microcontrollers get their own dedicated clock feed.

    hgmjr
     
  7. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    The AVR can output a square wave on an IO pin.
     
  8. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    Some AVRs can output the internal clock on PB0 -- fuse setting
     
  9. joemcder

    New Member

    Aug 27, 2007
    2
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    The internal AVR clock is not very accurate. Internal microcontroller clocks exist for the cases where we must minimize chip count and don't care much about clock quality.
    You should be aware that you can by xtal oscillators "in a can" just like the simple xtal. These are 4-pin rather than 2-pin devices. You apply power and out comes the 10 Mhz square wave (or whatever frequency). This clock can be sent to a few nearby chips OK. The microcontroller will have a fuse setting for the external clock. This is NOT the same thing as an external crystal. See the data sheet.
    As someone else pointed out is you need to send the clock all over a circuit board, you will need a clock amplifier to maintain quality.
     
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