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Pull up resistor

Discussion in 'Electronic Basics' started by Chris W, Mar 11, 2005.

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  1. Chris W

    Chris W Guest

    Is there a rule of thumb on what the resistance should be for a pull up
    resistor?

    --
    Chris W

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  2. Andrew Holme

    Andrew Holme Guest

    Making it too small could waste power.

    Making it too large would lead to slow rise times, and, in extreme
    cases, even noise pickup.

    1k was popular for early TTL but is unnecessarily low for many modern,
    low-power CMOS applications. 100k strikes me as being on the high side
    for most (not all) applications. 4k7 or 10k is often about right for
    CMOS.
     
  3. john jardine

    john jardine Guest

  4. PeteS

    PeteS Guest

    One thing to watch out for is when the pullup is on a bus (such as I2C
    / SMBus). The devices used for this have a section devoted to
    calculating the pullup resistance required (it's dependent on the
    number of devices attached, but that's always true).

    For pulling up a single pin, I agree on the 4k7 / 10k rule. If you have
    to pullup / down a tri-stateable bus, then bus speed becomes a major
    issue. Slow busses can be pulled with 100k or so, but fast ones ( <
    25MHz or so) need more of the order of 4k7.

    If they are faster than that, you may need to look at proper
    terminations.

    Cheers
    PeteS
     
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