> Would somebody be so kind as to explain to me why you would need a 
> pull-up, or
> pull-down resistor?
>
> I'm assuming the pull-up or pull-down would connect +v or 0v to the pin in
> question to ensure that the pin 'defaults' to +v or 0v?  But would this
> junction then be connected to the collector of a transistor?  How would 
> the
> transistor pull a pin low if it has a pull-up resistor?
>
> Actually, I read something in one of the microchip documents last night 
> about
> some of the IO ports having a selectable "weak pull-up resistor".  I guess
> this is somehow related, but I suspect not enough to pull up my 1.5v to 
> 5v.


Outputs of PICs, opamps, comparators etc are used to "drive" output loads. 
some loads are ground referenced and some supply referenced - ie the other 
end is connected to ground or supply. (Some loads are referenced to an 
intermediate point but lets not get too fancy here).

Many devices can drive their outputs just as easily high (Vcc / Vdd) or low 
(ground / Vss). A typical PIC output pin would be an example of this.

BUT some outputs, by design, can eg drive or pull low but not high or vice 
versa. There are a number of good reasons (and some bad ones) for designing 
outputs to work this way. Such outputs are referred to as "open collector" 
because they are effectively the same as a transistor with no load resistor. 
If an NPN transistor with emitter to ground and collector connected to 
nothing is turned on, the collector will be clamped to ground. A load 
referenced to eg 5 volts would sink current into the transistor collector.
BUT if the transistor is turned off its collector will 'float". There is 
nothing to make it go high when the transistor is off. If a resistor is now 
placed between collector and eg +5v then when the transistor is off the 
resistor will "pull the collector up" to 5 volts, This is a "pullup 
resistor". Similarly a PNP transistor with its emitter at +5v would have a 
pull down resistor from collector to ground to pull the collector low when 
the transistor is off.

A disadvantage of an open collector output is that the pullup resistor is 
not as flexible or capable as having an active device in its place. eg when 
the resistor is a high value it does not provide much pullup current so can 
only drive a small load high. When it is a low value it will dissipate 
significant power when the transistor is turned on. The time to pull high 
depends on the time constant of the resistor and any capacitance at the 
collector and this may be slow and the pull up waveform will be an 
exponential curve as the capacitance charges. Despite this open collector 
circuits have some significant uses. These include combining multiple 
outputs together and driving loads which only require to be driven in one 
direction. ie the load acts as its own pullup resistor. (An example is a 
ULN2803 driver IC which drives high referenced loads)

Another use of pullups or pulldowns is to pull a high impedance input to a 
defined voltage level when it would otherwise "float" at an undefined 
voltage. *ALL* PIC digital input pins must ALWAYS have defined voltage 
applied - either by being driven by an output of some sort OR by having  a 
pullup to high or low. Digital inputs that are allowed to "float" can cause 
the IC to draw large currents if they float at about half supply (or near 
the transition point for ICs that are not symmetrical about high and low). 
Floating inputs can and do cause many strange and unpredictable problems.

Sometimes an IC may be somewhat weaker than desired at driving high or low. 
In such cases a pullup or pulldown resistor can be added to help provide 
more current in the desired direction. This has the effect of making the IC 
work harder in the opposite direction but this is often a worthwhile 
compromise. eg getting an LM324 quad opamp (or LM358 dual version) to drive 
really close to ground is helped by an output pulldown resistor.

Some ICs are open collector and must have a pullup or equivalent on the 
output. Look at the datasheet for the LM339 quad comparator (or LM393 dual 
version) as an example. This is probably about the cheapest IC in the world, 
and one of the oldest, but still useful and used in quantity.

OK? :-)


        Russell McMahon



_______________________________________________
http://www.piclist.com
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist