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'What exactly are pullups?'
1999\03\28@060320 by

Dear Geniuses,

I've been fighting this little debate in my head for some time as to WHAT
pullup resistors do (aren't there pulldowns too?). If you set all the port
bits to 1, doesn't that logically mean they're high? I have a basic idea of
what they are, but I don't know it well enough to utilize it in my projects. I
would greatly appreciate any light that can be shed (this sentence doesn't
sound right..) on this subject.

I thank you all,

Tim Hamel

On Sun, Mar 28, 1999 at 06:02:28AM -0500, Tim Hamel wrote:
> I've been fighting this little debate in my head for some time as to WHAT
> pullup resistors do (aren't there pulldowns too?). If you set all the port
> bits to 1, doesn't that logically mean they're high? I have a basic idea of

Pullups are used in two situations; one is on pins configured as inputs,
in which case it matters not what you set the output latch to, since it
has no effect on the pin. In this case, a pullup is usually a resistor connected
from the input pin to +ve voltage, which keeps it high unless something
external pulls it low (which means the external device has to sink current from
the pullup). This is typically used where the external device is e.g. a switch
to ground, or an open collector output from something else. It's required
with typical microcontroller inputs because they are high-impedance MOSFET
inputs that will just float at a random voltage unless something pulls them one
way or another.

Pulldowns are the opposite, a resistor to ground. Pullups are more common that
pulldowns mainly because in The Old Days when TTL logic ruled the input to
a TTL device naturally floated at a high level, and required only a small pullup
for noise tolerance, but a pulldown had to sink quite a large current to get
the input low.

On some PIC ports, there are built-in pullup resistors that can be turned on,
but they function just like an external pullup resistor.

The other situation where a pullup is used is on an open drain output, of which
there is one on many PIC chips - it has only half an output driver, i.e. it can
sink current, but not source it. This can be useful for driving some kinds of th
ings,
but in other situations it needs a pullup resistor (e.g. driving the base
of a switching transistor).

--
Clyde Smith-Stubbs               |            HI-TECH Software
Email: clydehtsoft.com          |          Phone            Fax
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---------------------------------------------------------------------------
HI-TECH C: compiling the real world.

----------------
Pullup resistor is a way to make the circuit has a positive reference
voltage when not driven low by any other circuit.

----------------
All electronic circuits will swing a voltage from one point to another.
Without a Load or a PullUp resistor or component feeding the +Vcc, there
is nothing to swing.

----------------
Pullup resistor can also be known as "terminating" resistor when
installed in the farest point of the bus, from the processor point of
view.  This helps in create a unique current flow in the bust towards
the resistor, avoiding reflected waves and their nasty interference,
also helping to state some overall circuit impedance.

Resuming:
---------
A pull-up resistor can be compared to everything you know about law,
moral, respect and citizenship, that makes you behave in the right track
(and speed), with a good "polarization", mostly because the sheriff car
is parked at the next corner... :)

You can also think that the sheriff's car impose a good "pull-up
resistor" when he is traveling in the middle of the traffic, everybody
looks sooo nice, isn't? nobody cares to make a long line of cars behind
him, when he is at the maximum allowable speed. No jamming, no reflected
waves, no echoes... :)

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