Pull up resistor

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

Chris W

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Chris W
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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.
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Andrew Holme

This page provides the calculation for pull-up resistors:

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Interesting to see the logic family comparison data . Ta!. regards john

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john jardine

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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