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Can I use one shared crystal?

Discussion in 'Electronic Design' started by SBS, Apr 9, 2007.

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  1. SBS

    SBS Guest

    Hi,
    I have 2 IC (ATmega8 and an ST device) on the same
    board where each them needs a crystal of 16MHz.

    Can I use one parallel resonant shared crystal for both
    devices?

    What about the parallel load capacitance of the crystal?

    Thanks and regards.
     
  2. There are two problems to solve to use a single crystal to
    drive two chips. The two inputs may not settle to the same
    DC bias voltage. And the extra load of the second chip may
    slightly affect the first. You solve the first problem by
    coupling the output of the first oscillator to the input of
    the second. You will have to find out by trial whether the
    chip actually connected as an oscillator is loaded too much
    by having its output coupled to the input of the second chip.

    A problem free way to drive both chips from a single crystal
    would be to build an independent oscillator with the crystal
    and a cmos inverter, or use a canned oscillator. Use the
    output of either of these to drive both clock inputs. If
    the single inverter oscillator, I would capacitively couple
    to the inputs to let each chip find its own bias point. If
    the canned oscillator with a logic level output, direct
    connection will work fine.
     
  3. SBS

    SBS Guest

    Donald () ha scritto:

    ::: Can I use one parallel resonant shared crystal for both
    ::: devices?

    :: Yes, but you can not connect the two crystal pins together
    :: on the two processor.

    Then I can't connect both input (and so output) pins of the
    devices together to one lead of the crystal?

    :: Connect the crystal to one chip, and with a TinyLogic device
    :: like NC7WZ17 from Fairchild or TI, connect the output of the
    :: device with the crystal to the input of the other. ( leave the
    :: output side open )
    ::
    :: This will isolate the two oscillator circuits.

    Do I really need to isolate the two oscillator circuits or can I
    connect them as I have written above?

    If I would have to use another logic chip, then I may use 2
    crystal for an even better solution, don't I?

    Thank you.
     
  4. Donald

    Donald Guest

    Yes, but you can not connect the two crystal pins together on the two
    processor.

    Check the data sheets of each part. One pin will be an input, the other
    an output.

    Connect the crystal to one chip, and with a TinyLogic device like
    NC7WZ17 from Fairchild or TI, connect the output of the device with the
    crystal to the input of the other. ( leave the output side open )

    This will isolate the two oscillator circuits.

    good luck

    donald
     
  5. Because the signal swing is large enough to saturate each
    input, regardless of the exact zero input bias voltage of
    either. The bias voltage is important only if the signal
    swing is small and must be centered on that bias point to
    achieve a full swing output.
     
  6. Why can you connect the output of the canned oscillator directly to each
    oscillator input without coupling caps?
     
  7. Donald

    Donald Guest

    What he ( John ) said.

    If you don't like that answer given, please try it and let us know what
    you've found.

    donald
     
  8. Donald

    Donald Guest

    Too quick on the SEND button.

    This is not a unknown problem.

    I am sure many here have tried this before as well.

    I tried it with 6502 processors years ago. The osc circuit would work
    with each one seperatly but together, nothing, dead, natha.

    I had an experienced friend look at my circuit, and he laughed his butt off.

    He ask me if I have a clue what I was doing, I said "well, it should
    work" and he laughed again.

    So, the answer is "don't do it".

    But please try it for your self.

    donald
     
  9. SBS

    SBS Guest

    John Popelish () ha scritto:

    :: There are two problems to solve to use a single crystal to
    :: drive two chips. The two inputs may not settle to the same
    :: DC bias voltage. And the extra load of the second chip may
    :: slightly affect the first.


    Exaustive explanation and good advices, thank you.
     
  10. SBS

    SBS Guest

    Donald () ha scritto:

    :: I tried it with 6502 processors years ago. The osc circuit would work
    :: with each one seperatly but together, nothing, dead, natha.
    ::
    :: I had an experienced friend look at my circuit, and he laughed his butt
    off.
    ::
    :: He ask me if I have a clue what I was doing, I said "well, it should
    :: work" and he laughed again.

    :) Thank you.
     
  11. colin

    colin Guest

    I dont see why one cant drive the other,
    a usual arangement for one IC wich seem to have a cmos inverter between the
    two pins
    is to have ~ 20pf on both input and output pins
    with the xtal between the two,
    If the output of the IC doing the oscilating is conected to the other IC
    input via a 100pf decoupling capacitor,
    what can it do but load it by 10pf or so,
    this can be compensated for by reducing the 20pf capacitor,
    ofc the output of the 2nd IC is just left open.

    you might find one ic or the other is better at driving the other.
    and you might have to play around a bit with capacitances etc.
    ofc you would need to make sure the track length to conect them is Very
    short.
    If its for even low volume production you would have to look into it a bit
    deeper.

    Colin =^.^=
     
  12. SBS

    SBS Guest

    colin () ha scritto:

    :: I dont see why one cant drive the other,

    Thank you Colin, can you draw a picture which explains
    this situation, please?

    I can't understand how to connect both devices to the single
    crystal... :-(
     
  13. Nico Coesel

    Nico Coesel Guest

    As told before: A crystal oscillator has an input and an (inverting)
    output. A crystal connected in be input and output will cause the
    whole thing to oscillate at the crystal's resonant frequency.

    If you want to use one crystal for 2 devices, you should place the
    devices close together. Connect the crystal between the crystal
    oscillator pins of device A, connect the crystal oscillator output
    from device A to the oscillator input from device B with a small
    capacitor (say 1nf).

    The datasheets should tell you more about such a setup and which pins
    are input and output.
     
  14. The biggest risk may be other signals that capacitively
    couple into the trace going to the second chip and
    contaminate the wave being fed to the crystal. If this
    trace is more than an inch long, I would probably shield it
    with a pair of ground traces (grounded at the oscillator
    end, not the receiver end).
     
  15. Donald

    Donald Guest

    Like this ??


    +------+
    | | |
    | | |
    __|__ --- 0
    \ / [ ] / \
    \ / --- / \
    0 | -----
    | | |
    | | |
    +------+----()---+
    1nf
    CPU A CPU B

    I don't think this will work.

    I think the drive of CPU A output will be loaded too much to drive the
    crystal and the input of CPU B.

    Let me know how it works.

    donald
     
  16. Nico Coesel

    Nico Coesel Guest

    The OP should give it a try. Most modern CMOS chips have a very small
    input capacitance.
     
  17. whit3rd

    whit3rd Guest

    I think there's still a miscommunication here; the chip can accept
    an external oscillator, OR oscillate a connected crystal, and
    what is recommended is that ONE CPU oscillates a crystal
    while the OTHER uses an external oscillator.

    So, one poster thinks both CPUs are in oscillate-a-crystal
    connection (and wants to connect two wires on each of the
    two), while the respondent only connects two wires to
    the crystal-oscilator chip, and one wire to the uses-external-
    oscillator
    chip.

    The TinyLogic device is just used as a buffer, so the (possibly
    sensitive) crystal wiring isn't connected to an antenna of
    interconnect
    wire. Any available gate would serve.
     
  18. SBS

    SBS Guest

    whit3rd () ha scritto:

    :: The TinyLogic device is just used as a buffer, so the
    :: (possibly sensitive) crystal wiring isn't connected to
    :: an antenna of interconnect wire. Any available gate
    :: would serve.

    BTW, I need to use a crystal with nominal frequency
    of 16 MHz. So the best soultion should be to use a crystal
    for each device, shouldn't it?
     
  19. Possibly the easiest and least expensive anyway. Unless you have some
    special need for them to run _exactly_ at the same frequency and phase.
     
  20. Rich Grise

    Rich Grise Guest

    My "little voice" is also telling me that I'd use much less than
    1 nf for the cap - maybe more like 33 ~ 120 pf.

    Thanks,
    Rich
     
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