Pull up vs pulldown

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For logic inputs, we can either use pull up resistors to 5V for pull down to
ground. What are the pros and cons for boths and which is better to use and
why?
Thanks



Re: Pull up vs pulldown


Usually pull-ups are used. This is mainly because TTL devices or open
collector outputs are able to handle that kind of inputs. TTL devices
are able to sink more current (to Low) than to source current (to Hi)
As a rule of thumb: use pull up instead of using pull down.
Just my one Euro cent
Cheers
Klaus


wt skrev:
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Re: Pull up vs pulldown


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Pullups fit in better with the characteristics of the older logic
families such as TTL, because TTL inputs and outputs were very
assymetrical in their '1' and '0' input and output voltage margins,
input currents and output currents.  The result of this is that
pulldowns for TTL needed to be no more than a few hundred ohms, and a
TTL output couldn't drive such a low-valued pulldown high, and a
switch or other device which could pull the pullup high would have to
pass (waste) many tens of mA.  However, pullups could be many kohms,
and a TTL output could easily drive such a pullup low.

If the characteristics of the input and output circuitries you're
using are symmetrical in terms of input voltage margins, input
currents and output currents, then the question pullups or pulldowns
comes down to your own choice.  If they're not symmetrical, then that
assymmetry will dictate which is to be preferred (which is pullups in
ever case I've ever seen).


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