Re: pull-up/pull-down/short-protection resistor design

From: Johnson Liuis (gpsabove_at_yahoo.com)
Date: 03/02/05 

Date: Tue, 1 Mar 2005 17:55:26 -0700

Thank you very much, Joerg,

You mentioned "source termination", andthe energy could possibly be
reflected in case that proper transmission line termination isn't going to
happen at the far end for whatever reason. I remembered when we talked about
RF circuits and cable, the signal reflection had to be considered. Should we
consider the reflection in a digital circuit as well?
BTW, is the energy reflection caused by "resistor mismatch" at another end?

Johnson

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:1d6Vd.10486$Pz7.1796@newssvr13.news.prodigy.com...
> Hello Johnson,
>
> >Please do not laugh at these newbie questions.
> >
> >
>
> No, I won't. Every one of us has started from scratch at some point.
>
> >1) pull-up/pull-down resistor for unused pins. It seems that 10K was used
> >for some cases while 4.7K or 150K was used for other cases. Any big
> >difference when use different resistors for this purpose? Which one can
> >better sustain the noise?
> >
> >
>
> Unless you have long lines it doesn't make a lot of difference.
> Sometimes a higher value is desirable, for example when a push button
> may be held a long time and power consumption must be minimal. If you
> have a long line connected and expect a lot of static a smaller value
> might be advised.
>
> >2) short-protection/current-limiter for pins that support both Input and
> >Output. For the data bus, the pins were grounded with 150K resistor (for
> >example, D0 to D0 first, then to ground with a resistor); for some
control
> >pins, 22R resistors were used to connect them (for example, RXRDY to
RXRDY
> >with a resistor). So what is the benefit to connect the data bus to
ground
> >with a big resistor, to speed the transaction time or sustain noise? ...
> >
>
> Usually this is done when a data bus or part of it could go into
> "tri-state" (where all drivers are turned off from the bus) or where a
> line could be floating for other reasons. Floating lines must be avoided

> in digital design. If a line would, say, float to half the supply
> voltage all kinds of grief can happen. Such as erratic logic behavior
> and overheating of chips.
>
> Also, long data busses need to be terminated. Really long ones at both
> ends. The classic kind is "Thevenin" where 220/330 resistor combos hang
> between VCC and ground. I don't use that a lot because it wastes power.
> There are many other methods such as AC termination but this gets to be
> a bit tricky.
>
> >What is the benefit to connect the pins with a resistor without grounding
them?
> >
> >
>
> If you mean a resistor in series with the line: This can have two
> reasons but often it is not to protect anything. One reason is to
> provide a little bit of a low pass into the line, maybe to reduce
> electro-magnetic interference (EMI). But usually that also requires a
> capacitor on the other side.
>
> The second reason might be "source termination", where it is assumed
> that proper transmission line termination isn't going to happen at the
> far end for whatever reason. This way there is at least a resistor at
> the source that can dissipate reflected energy. But you would see
> resistors more in the range of 50-150 ohms, not 22 ohms. They would have
> to match the characteristic impedance of the connected line.
>
> For short circuit protection you'd either need some active circuitry or
> a resistor that is large enough so the device can drive into VCC or
> ground through that resistor without overheating. The "absolute maximum
> ratings" should tell how many mA that can be and whether that is for
> just one pin or for all of them. 22 ohms is kind of small for logic
> devices, unless it is a heavy duty bus driver.
>
> Regards, Joerg
>
> http://www.analogconsultants.com

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