[SI-LIST] Re: How to select the pullup/pulldwon resistor
- From: "Ross Wellington" <welling01@xxxxxxxxxxxxxxxxxxxx>
- To: <bmgman@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 29 Nov 2002 21:03:32 -0700
One other location where you might see a pull-up is in the output of a
comparator (like an LM339). This is usually an open collector or open drain
situation.
The pull-up selection will be optimized for capacitance drive and required
edge rate for operation. In this case, it may be 200 to 510 Ohms for faster
edge rates or 4.7K to 100K for slower requirements or timing.
Ross
----- Original Message -----
From: "Mike Brown" <bmgman@xxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Friday, November 29, 2002 6:40 PM
Subject: [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. Th off 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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