[SI-LIST] Re: How to select the pullup/pulldwon resistor

History dies hard!  The 4.7k pullup and the low value pulldown are 
historic relics of the TTL era, where the high-state leakage current of 
50 uA or so tolerated a much larger value resistor than the Iil spec of 
(1.6 or 2 ma, depending on family).  In 5-volt TTL, the 4.7k value 
permitted 10 inputs to be tied together while keeping the Vih above 2.4 
volts. A single input is pulled down by a 200 ohm resistor.  2ma through 
200 ohms keeps the Vil at 400 mV, which is the Vol spec of the family.

Test requirements often dictate being able to control all the inputs of 
a device, even if some are functionally static.  Hence the addition of a 
resistor instead of tying a pin to VDD or VSS directly.

Mike is right about there being no good reason for a different value of 
R for pull up/down, if the input leakage curent specs are symmetrical. 
 I have seen board designs with multiple values of pullup, and 
asymmetric pullup/pulldown values, for no good reason.  All this 
accomplishes is to increase the manufacturing cost of the board.

Don't confuse "pullup" with "terminator" - although both pull a net up. 
 The pullup maintains a static HIGH on a net, and its value is 
determined by the leakage spec of the connected inputs and the allowable 
(Vcc -Vih).  A terminator provides a a dynamic HIGH, in the case of 
open-drain or open collector circuits, and/or an impedance matching 
function. Its minimum value is determined by the allowable Iol of the 
driver, and the maximum value by the allowable time constant of the 
low-to-high transition.  Its optimum value is somewhere near the 
impedance of the net it is connected to.

Most discrete logic families do not provide enough drive to permit the 
use of a 50 (or 70) ohm termination to VDD.  A termination to VDD is 
limited by the Iol of the driver.  Special "driver" circuits are used to 
address the Iol limitation.  Unless drivers are used, the Iol-limited 
resistance will seldom be a good match to achievable board impedances. 
Consequently, signal integrity is compromised.  

An alternative to using a driver is to terminate to a voltage less than 
Vdd.  This permits a lower value of termination R, thus better signal 
integrity.  The tradeoff is that the voltage swing on the net is 
reduced, and the power dissipation is increased.  The optimum resistance 
and Thevenin voltage may be found by plotting the Voh and Vol I-V curves 
and the termination load line.  Pick the High and Low state operating 
points and draw a line between them.  The slope of the line is the 
resistance, and the intercept is the Thevenin voltage.

Even when driver circuits are used, their specs may be marginal for 
driving heavily-loaded nets, whose loaded impedance can be in the 20-30 
ohm range.  The above technique is applicable in these cases for the 
purpose of reducing the value of the terminator resistor to achieve a 
lower reflection coefficient.

Regards

Mike

Michael Nudelman wrote:

>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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