Pull up resistor

[Chris W]

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

--
Chris W

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[Andrew Holme]

Chris W wrote:

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

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.

 

[www.interfacebus.com]

This page provides the calculation for pull-up resistors:
http://www.interfacebus.com/IC_Output_Input_Pullup_Resistor_Values.html

 

[john jardine]

"www.interfacebus.com" <ldavis8@cfl.rr.com> wrote in message
news:1110551072.728434.42070@l41g2000cwc.googlegroups.com...

This page provides the calculation for pull-up resistors:
http://www.interfacebus.com/IC_Output_Input_Pullup_Resistor_Values.html

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

 

[PeteS]

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