Pull-up resistor

[Steve]

Hi,

I'm learning about microcontrollers and I want to know what a pull-up
resistor is and why do we need to "enable it" when switching a pin for
input? Thanks!

Steve

 

[John Popelish]

Steve wrote:

Hi,

I'm learning about microcontrollers and I want to know what a pull-up
resistor is and why do we need to "enable it" when switching a pin for
input? Thanks!

Steve

If the input (which looks like a pair of small capacitor between the
input and the supply rails, to the outside world) is driven by a
signal source than can pull the voltage up to the positive supply or
down to the negative supply voltage (as a normal logic output does),
the internal pull up resistor is not needed. But if the input signal
comes from a grounding switch that can only open or close, and thus,
only drain the input voltage down to the negative rail voltage, there
is nothing to make the input voltage leave the grounded state when the
switch opens, unless you add some other source of charging current for
the input. This is the function of a pull up resistor, whether it is
internal or external to the micro.

A pull up resistor has been provided, internally, just as a
convenience, to reduce the external parts count. But this internal
resistor that connects between the positive supply and the input has
to be connected to the input by an internal switch which you turn on
by configuring an internal storage register (thus enabling this
function if you need it).

--
John Popelish

 

[JeffM]

...I want to know what a pull-up resistor is.
Steve

Your're in luck.
On Google Groups your vague Subject line brings up similar posts.
Read the previous threads.
http://groups.google.com/groups?dq=&hl=en&lr=lang_en&ie=UTF-8&safe=off&threadm=3ffebaee%241%40clarion.carno.net.au&prev=/groups%3Fhl%3Den%26lr%3Dlang_en%26ie%3DUTF-8%26safe%3Doff%26group%3Dsci.electronics.basics