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0.0 ohm resistor??

Discussion in 'Electronic Basics' started by Ant_Magma, Feb 25, 2006.

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  1. Ant_Magma

    Ant_Magma Guest

    Guys, i'm currently working on National Semiconductor's DP83847
    Ethernet Transceiver.

    In their reference designs, there's this 0.0 ohm resister at the output
    pin of the 25Mhz oscillator

    http://geocities.com/Antonio_Magma/0resistor.gif

    Is there such thing as a 0 ohm resistor? How do i even build it? Just
    ignore it and place a wire?
     
  2. Pooh Bear

    Pooh Bear Guest

    It's a 'dummy' component. Maybe it was originally inteneded to have a value
    but it was found unnecessary.

    Graham
     
  3. PeteS

    PeteS Guest

    As Graham noted, it could well be a dummy component. In some designs,
    the chip mfrs are not completely clear in their datasheets and there
    are times I will drop in a zero ohm resistor as a placeholder for some
    other value if I need to use it (drive limit to a crystal is a pretty
    common use).

    Another place I have used them is where I need a shorted link, but
    which way around is in doubt. I have (in one of my designs) a resistive
    touch panel and I use optional zero ohm resistors to make sure I get
    the directional sense correct (even if the manufacturer changes the
    panel).

    There are myriad other uses, of course.

    In your case, replace it with a wire.

    Cheers

    PeteS
     
  4. Bob Masta

    Bob Masta Guest

    Another use for 0 ohm resistors is to act like a jumper.
    They are better than the little wires you might bend
    up yourself for a one-off, since automatic insertion
    equipment can handle them just like regular resistors.

    Best regards,


    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Home of DaqGen, the FREEWARE signal generator
     
  5. Pooh Bear

    Pooh Bear Guest

    Agreed.

    In a cost-sensitive applications, a 'zero ohm resistor' or link may be used
    to 'cross tracks' in order to make possible a 'single-sided' printed circuit
    board in place of a double-sided one.

    Much low cost consumer equipment uses single sided pcbs.

    It's rare to see such links identified as components though.

    Graham
     
  6. Ant_Magma

    Ant_Magma Guest

    Thx 4 ur replies guys. I'll replace them with wires.
     
  7. Chris

    Chris Guest

    Hi, Ant. It would have been good if you'd indicated a link to the
    National reference design -- your external oscillator driver isn't in
    the data sheet or the first couple of app notes on the National
    website.

    http://cache.national.com/ds/DP/DP83847.pdf
    http://www.national.com/appinfo/networks/files/dp83847_IBM_ppc440gp.pdf

    The IC either uses a crystal oscillator or an external oscillator. The
    data sheet says that, for the external oscillator, drive X1 directly,
    and leave X2 open. So, ignore the 0.0 ohm resistor, and just connect
    the output of the external oscillator to X1.

    I'd guess IBM just needed a jumper on their board.

    It's easier to get help if you show us where to look.

    Good luck
    Chris
     
  8. Ant_Magma

    Ant_Magma Guest

    Ah, thx Chris. I'll keep that in mind.

    Since u mentioned about leaving X2 open this leads to my next question.

    1) In the description on the datasheet, the X2 is described as the
    primary clock reference output and this pin isn't used. Does it mean
    that this pin outputs a 25mhz reference frequency?

    2) From the reference designs in the link by Chris (IBM), some of the
    pins at the MII interface have a 49.9ohm resistor some dont. However,
    in the reference design by National, they are replaced with 0.0ohm
    resistors. Which is which? What purpose does this resistors serve? Does
    it mean that it's optional (i want to save PCB space without the
    resistors)?

    3) If i have other components that requires 25mhz oscillator, is it ok
    to share it with this chip? Will it pose any inteference problems? Or
    must every difference component have their own oscillators and not
    share?

    Thx guys...
     

  9. Bull. It IS a component, because it has to be on the BOM, and if its
    surface mount, it has to be loaded on the P-N-P machine, or placed by
    hand. Jumpers, on the other hand are classes as wires, not resistors.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  10. Chris

    Chris Guest

    Hi, Ant. If you look at the data sheet, you'll see (view in fixed font
    or M$ Notepad):

    |
    | | |
    | | |
    | | |
    | | |
    | | |
    | | |
    | | X1 X2 |
    | '--o-------o---'
    | | |0.0 ohms
    | | .-.
    | | | |
    | | | |
    | | _ '-'
    | | | | |
    | o-|| ||-o
    |22pF| |_| |22pF
    | --- ---
    | --- ---
    | | |
    | === ===
    | GND GND
    |
    (created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

    On p.26, it shows that the X2 output is necessary to feed energy back
    into the crystal for oscillation. Don't connect X2, no oscillation. R
    can be 0.0 ohms (for the type of crystal specified in the data sheet).
    Long story.

    Anyway, this is a typical setup for a crystal-based clock on
    microcontrollers. The business behind the chip is described in
    numerous places, particularly Don Lancaster's CMOS Cookbook. He
    doesn't get into the math. For a somewhat more detailed
    non-mathematical explanation, see Chapter 5 of Art of Electronics
    (Horowitz/Hill).

    Unless it specifically says so in the processor/uC data sheet, NEVER
    use X2 for anything except oscillator feedback. You're loading down
    the oscillator, and you could easily inhibit oscillation unless you do
    it right.

    I don't have experience with your chip. I'd really recommend you
    download all of the information at National (datasheet, appnotes,
    design guide, &c) and go through it carefully. Your questions kind of
    indicate you haven't done that, yet. I believe from quick scanning
    that your answers to 2) is there somewhere, relating to changes if
    you're using the chip for high-speed operation. The answers are almost
    always in the documentation provided with the chip -- they do know what
    they're doing, and they don't want to make it any harder to use the
    chip.

    If neither this or other s.e.b. responses are sufficient, try
    sci.electronics.design. Someone there may have designed in the chip.
    Make sure you're up to speed on what you're doing, first.

    Good luck
    Chris
     
  11. Pooh Bear

    Pooh Bear Guest

    It's rare to see it identified as a component on a schematic , as per the OP's
    link.

    I've seen BOMs where such links are accounted for simply by a single length of
    tinned copper wire equal to the total use too.

    Graham
     
  12. Ant_Magma

    Ant_Magma Guest

    Thx Chris, i've already read the datasheets and the reference designs.
    I'm not lazy, i'm just new.

    I'm confused becoz the datasheet says to connect the oscillator to X1
    and leave X2 open. And yet in the circuit its connecting both X1 and X2
    like you showed in your post. So which is which?
     
  13. Guest

    Do you have an oscillator (power, ground, out) or just a crystal (two
    leads)?

    With the oscillator option you connect only one pin to the chip, with
    the crystal you connect two because the active part of the oscillator
    circuit is inside the chip.

    If you want to share the clock and are using an external oscillator
    (one pin to the chip) you could use a buffer to do so. You might even
    get away with simply using a gate from a hex inverter or quad nand gate
    to seperately clock eached driven chip.
     
  14. DJ Delorie

    DJ Delorie Guest

    Most likely you've got a board that's designed to be used multiple
    ways, with the zero ohm resistors acting like configuration jumpers.
    Populate resistor for configuration X, don't populate for
    configuration Y.
     
  15. Ant_Magma

    Ant_Magma Guest

    Ic, so i have 2 options with my crystal oscillator depending on whether
    i want to use just the crystal or a oscillator.
     

  16. You must be stuck in the thru hole world. 0 Ohm resistors are common
    in RF designs, and every surface mount board RF I've worked on has had
    them designated as a component. I saw a lot of them on the digiatal
    boards, as well.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  17. PeteS

    PeteS Guest

    This is very common.

    X1 and X2 are connected (internally) to an oscillator amplifier.

    if you are using a crystal, then the crystal is connected across X1 and
    X2. Alternatively, *if you happen to have a 25MHz clock available and
    wish to use it for this chip*, then connect the clock (from somewhere
    else in the system) to X1 and leave X2 open.

    In terms of the device, X1 is the input to the oscillator, X2 is it's
    output.

    So if you are using a crystal, connect it as shown.

    Here's what the datasheet says:
    (P25 in acrobat reader. P26 of datasheet)

    [Highlights mine]
    3.7 Crystal Oscillator Circuit
    The DsPHYTER II supports an external CMOS level oscillator source or a
    crystal resonator device. *****If an external clock source is used, X1
    should be tied to the clock source and X2 should be left
    floating******. In either case, the clock source must be a 25 MHz
    0.005% (50 PPM) CMOS oscillator or a 25 MHz (50 PPM), parallel, 20 pF
    load crystal resonator.
    Figure 10 below shows a typical connection for a crystal resonator
    circuit. The load capacitor values will vary with the crystal vendors;
    check with the vendor for the recommended loads.
    The oscillator circuit was designed to drive a parallel resonance AT
    cut crystal with a minimum drive level of 500µW and a maximum of 1mW.
    If a crystal is specified for a lower
    drive level, a current limiting resistor should be placed in series
    between X2 and the crystal.
    As a starting point for evaluating an oscillator circuit, if the
    requirements for the crystal are not known, CL1 and CL2 should be set
    at 22 pF, and R1 should be set at 0Ω.

    Cheers

    PeteS
     
  18. Rich Grise

    Rich Grise Guest

    Another use for 0 ohm resistors is to act like a jumper. They are better
    than the little wires you might bend up yourself for a one-off, since
    automatic insertion equipment can handle them just like regular resistors.[/QUOTE]


    http://www.irctt.com/pdf_files/ZEROHM.pdf

    Cheers!
    Rich
     
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