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

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

Date: Wed, 2 Mar 2005 11:49:00 -0700

Thanks, Keith,
I am using Outlook Express Newsreader. So every time I just right-click the
title, and select "Reply to Group". Is that OK?
Johnson

"CBFalconer" <cbfalconer@yahoo.com> wrote in message
news:4224FA2D.49047705@yahoo.com...
> Johnson Liuis wrote:
> >
> > Please do not laugh at these newbie questions.
> > I am looking into some sample circuit design and wants to learn
> > how to use resistors for the following purposes:
> >
> > 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?
> >
> > 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? What
> > is the benefit to connect the pins with a resistor without
> > grounding them?
>
> First, consider unused input pins. They normally control some sort
> of CMOS gate, which has one FET connected to Vcc, and another to
> ground. At normal logic levels of the input, at least one gate is
> turned off. However, at some input level both will be turned on,
> and basically create a short from Vcc to ground. This doesn't do
> the circuit, or the power dissipation, any good. So some sort of
> pullup or pulldown resistor is indicated to peg that input level.
> 10 to 100 Kohm is usually a good choice. It won't draw any power,
> and will protect the input gate against power line spikes. If you
> decide to use the input later you can usually ignore the presence
> of that pullup.
>
> Output circuits can be of three flavors. One is totem pole, where
> the output pin is pulled hard to either Vcc or ground. These will
> have good logic levels, barring fighting (of which more later).
>
> Or they can be arranged to pull only high or low. Assume low (the
> more common case). In that case something external has to supply
> the high level, and that is usually a resistor to Vcc. This allows
> multiple outputs of the same type to be directly wired together,
> and the level is low if any output is low. Also known as the wired
> and. Again, resistors in the 1 K to 10 Kohm range are often used.
> The critical thing is the output risetime, governed by the RC time
> constant, which in turn is the product of the resistance and all
> the stray capacity on that line (which includes wiring, output
> pins, and input pins).
>
> Now consider pins that can be configured as either input or output,
> or can be tri-stated to effectively remove them from the circuit,
> and are connected to other pins of the same ilk (on the same or
> different chips). Mistakes happen. Such a pin may be configured
> for output when it should be an input, and while something else is
> driving the signal line. This is fighting. The fight is usually
> resolved when something burns up. This can be protected against
> with series resistors. If each node connects to the common bus
> through, say, 470 ohms there will always be at least about 1 Kohm
> between any pair of misconfigured pins. If the logic levels are 0
> and 5 Volts, then no more than 5 mAmps will flow, and the
> dissipation will be in the resistors. The circuit won't work, but
> it won't self-destruct.
>
> --
> "If you want to post a followup via groups.google.com, don't use
> the broken "Reply" link at the bottom of the article. Click on
> "show options" at the top of the article, then click on the
> "Reply" at the bottom of the article headers." - Keith Thompson
>
>

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