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question about microcontroler oscillator

Discussion in 'Electronic Components' started by Don McKenzie, Oct 29, 2007.

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  1. Don McKenzie

    Don McKenzie Guest

    doesn't sound like a good waveform for micro operation.

    http://www.intel.com/design/mcs51/applnots/23065901.pdf
    have a look at page 13, this is what the original data sheets specify
    for 8051's.

    Don...




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  2. robb

    robb Guest

    saga continues,
    my repair project device uses a phillips MAB 8051 AH
    microcontroller.

    while trying to isolate trouble with a badly shaped clock signal
    i happened across microcontroller oscillator.

    the oscillator CSA 12.00 MT has a slightly warped sine wave out
    of one side and a very bad dent in the crest of the sine wave on
    the other side like...
    _
    / \__
    / \
    -/------------\

    is the output of the oscillator suppose to be clean sine wave ?
    and same on both sides ?

    oscillator is connected exactly as 8051 datasheet recommends,
    with the two pins from the oscillator each to small caps which
    connect to 0v ref


    thanks for any help,
    robb
     
  3. Eeyore

    Eeyore Guest

    What value are the caps ? I think you mean you have a ceramic resonator btw, not
    an oscillator.

    Graham
     
  4. Pieter

    Pieter Guest

    Hi,

    Make sure your scope probe is set to 10x, and has a low capacitance.
    The probe may have more capacitance tha the small capacitors near the
    crystal (10 pF?) and may load the signal. So you may be measuring what
    it does when you load it with a capacitor.

    Regards,
    Pieter
     
  5. robb

    robb Guest

     
  6. Eeyore

    Eeyore Guest

    I'd expect them to be the same value too (and about 30pF typically - check the
    resonator data sheet). See any 8051 application note too. Is the 8051 version
    you're using CMOS btw ?

    Graham
     
  7. robb

    robb Guest

    thanks for help Graham,

    the values for caps (grey w/black top) are 15p and 33p

    after reading the intel oscillator spec i expected them to be the
    same value but then maybe i missed something (well, alot)

    thanks for help/reply,
    robb
     
  8. robb

    robb Guest

    the (phillips 8051ah)
    datasheet says +5v depletion-load,N-chanel,silicon-gate,N500 MOS

    so my slight comprehension leads me to "yes ?"

    i am having difficulty finding the resonator datasheet which is
    why i am troubling "s.e.b"
    my reliable finds come from alldatasheet.com but not for the
    resonator

    thanks for help Graham,
    robb
     
  9. I was thinking of that before I saw it. Look at the thread above this one
    called "Search OpAmp chip as a voltage follower" for a description of using
    a FET to buffer a high impedance source. That same idea will let you get a
    look at that waveform without disturbing it much. The FET and a single
    resistor to a stable DC rail should be all you need, then scope the FET
    output signal.

    FET input op-amp as non-inverting unity gain buffer might also be enough,
    and more likely to be close at hand.
     
  10. TT_Man

    TT_Man Guest

    If you are seeing 3-4 volts swing on one of the pins, then I would say the
    resonator is working ok. Don't worry too much about the wiggle in the
    waveform.
     
  11. Eeyore

    Eeyore Guest

    No, that's not a CMOS part and I'm puzzled why you're using it. Is this some old
    piece of equipment ?

    Where do you buy it from ? The "CSA 12.00 MT" marking is the same as ones I've
    sourced from Farnell (Murata ?) and works fine with 2 x 33pF.

    Graham
     
  12. Eeyore

    Eeyore Guest

    A FET input probe is NOT required to look at the 8051oscillator for routine
    purposes. A 10x probe is fine.

    Graham
     
  13. Eeyore

    Eeyore Guest

    Yes, it'll be around 3-4 V pk-pk on one pin (Osc 2 ?) and about a couple of
    hundred mV on the other one.

    Graham
     
  14. Eeyore

    Eeyore Guest

    From memory, yes they are a bit 'warped' as you put it. They're certainly not
    clean square waves. I wouldn't worry about it.

    Graham
     
  15. Eeyore

    Eeyore Guest

    I doubt it. It expect it's simply different to the usual 80C51 CMOS version that
    I'm familiar with. That NMOS version must be VERY old.

    It IS being used. That's what the XTAL1 and XTAL2 pins are for. See any 8051
    datasheet.

    Graham
     
  16. robb

    robb Guest

    Yes, you are correct Graham.
    It is the main control board for a vintage "computer controlled"
    sewing machine (~25 years old)

    i am trying to restore it to functioning ... as a "fun /
    learning" electronic hobby project
    it has just enough complexity not to completely overwhelm
    (hobby-ist) me ;
    micro-controller, stepper motors and controls, memory, VFD user
    interface and programming buttons etc.....
    it was already on the board.

    i was trying to backtrace the CLK signal going to the VFD driver
    IC which had some big spikey ring in the leading edges. i was led
    back to the 8051 and was checking various pins and noticed warped
    ? sine waves at the oscillator pins XTAL1 / XTAL2

    thanks for your help,
    robb
     
  17. robb

    robb Guest

    Thanks for reply,
    that is what i see about 4 volts height on wave forms on both
    pins . One is slightly bent sine wave the other has a big dent in
    the wave

    thanks for help
    robb
     
  18. robb

    robb Guest

    well i am seeing about 4 volts on one side (the 33 pF side)
    and about 3.8 volts on the (15pF) side ?

    so does that mean i have a problem ?

    this 8051 chip supposedly has and internal oscillator ? why would
    that not be used ?
    i mean is there a reason one would not use internal oscillator if
    there were one.

    thanks for all the help everyone, it helps me learn,
    robb
     
  19. Didn't say it was necessary. So long as the wave crosses the zero point
    regularly enough to make the timer work who cares what shape it is? Nothing
    much routine about this anyway, the guy's new to it and wants to be sure of
    what he sees.

    I was just pointing out a useful similarity with another post. If someone
    wants to learn, there's no doubt that adding a FET and a resistor and a DC
    cource is a neat way of making sure a signal involving small capacitances
    might be less disturbed than by loading it directly with a cable
    connection. It's not a question of whether it's necessary, but whether or
    not it's useful. Robb wants to learn, so that's a good cheap way to do it.
    It would confirm that the waveform really is as he sees it, and that it's
    not being changed by his connection to it.
     
  20. Ryan Weihl

    Ryan Weihl Guest

    Don, are you using the ground clip at the probe?
    long ground leads distort the picture.
    rw

    --
     
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