[SI-LIST] Re: How to select the pullup/pulldwon resistor
- From: Michael Nudelman <mnudelman@xxxxxxxxxxx>
- To: 'peter zhu ' <peter.zhu@xxxxxxxxxx>,"'si-list@xxxxxxxxxxxxx '" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 29 Nov 2002 18:04:47 -0500
There are different criterias. I am pretty sure you will have many answers
and most will be correct.
Typically when you see a schematic you will see 4.7K pullups and 200-300 Ohm
pulldowns.
There are reasons for that.
On one hand there is some subconscious thinking that pulldown should be low
value (often it is substitute of direct grounding due to testability
requirements). The gut instinct tells some people that if you want to ground
something a pulldown shoudl be low so not to pickup any noise.
The truth is: pulldown will pick up as much noise with the same resistance
value as a pullup would and thus will affect the input exactly in the same
way. There is no difference netween pullup and pulldown in that respect: if
you think you can reliably pull up an input by 4.7K, you can then as
reliably pull it down with the same resistor.
However: Often times a pullup is a load of some sort, like an open
drain/collector or a tri-state bus quieting pullup.
And tehn you care about the driver being able to sink ebough current,
provided by the pullup, to provide the reliable "0" and not to overload
itself.
Whereas a pulldown, sometimes being also a bus quieter, not often seen as an
"reverse open drain" load. And more often it is just a substitute for a
GND-ing of an unused input of a logic gate, which is not being driven. Like
I pointed before, it could well be a live GND, but test engineers want to
see a resistor there: it allows for atomated testing.
And design engineers subconsiously (to be closer to a live GND) choose a low
value of 200-300 Ohms.
Although, telling the truth they migh as well choose same 4.7K pulldown, and
even save some money on a value unification (ordering more of the same part
makes it cheaper), still allowing for the testability.
The there are the PECL outputs, that need pulldowns of 200 Ohms or so...but
this is another subject.
PS> Just recently I was working with a vendor who specified a pullup for the
open drain on their chips to be 3.01 K +- 1%. Of course when asked tehy
could not explain such a pickiness in values and eventually agreed to allow
4.7K 5% resistors :-)))))
Mike.
-----Original Message-----
From: peter zhu
To: si-list@xxxxxxxxxxxxx
Sent: 11/28/02 6:46 AM
Subject: [SI-LIST] How to select the pullup/pulldwon resistor
Hi all:
In hardware design, we often encounter the problem of pullup or pulldown
resistor, i very puzzled about it:
1.how to determine pullup or pulldown.
2.how to determine the value of pullup or pulldown resistor.
These are two common hardware design problem. I hope can see some
detailed materials about it. Help me!
Regards!
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