Hello,

Pullup resistors have two common uses.

 1.  They are used to pull unused input pins to a Logic High, usually
 five volts. While you can tie inputs directly to five volts, a resistor
is  often used so that at a later time an output from another device can
be  connected to this unused input without harm.  If the input is
connected
 directly to five volts, then this direct connection would have to be
 removed before an output from another device can be hooked to the  input
pin.  With a pullup resistor, the connection can be made without
 removing the resistor.  This is because the resistor limits the sink
 current into the output of the driving device to a safe level.

 2.  The other use is when the output device where your signal is
 exiting from is an open collector device.  Open collector systems have
 an internal transistor that can drive the pin to LOGIC Low, but there is
 no provision to pull the line to 5 volts for a LOGIC One.   The pullup
 resistor provides this path to 5 volts.  On an open collector output if
 you omit the pullup resistor, you willl never output a LOGIC One.  The
 reason for open collector outputs is to allow several output to be
 connected in parallel.  I2C and SDI buses that allow numerous chips  to
be connected together in a bus use open collector outputs.  Any  output
in an open collector system can pull the line Low without  any  damage to
itself or another device.

Quick rule of thumb:

 If you need to connect any pin of a microcontroller to Vcc (5 volts) or
whatever the power supply is,  use a resistor of 4700 ohms.  You usually
cannot go wrong adding this resistor, it may not be necessary, but you
will be safe.  If the manufacturer or someone tells you expicitly to make
a direct connection, then do it.  However, when in doubt use a resistor
first.
(Of course the VCC power input to the chip goes directly to the power
supply with no resistor.)
 ----------
From:  pic microcontroller discussion [SMTP:PICLIST@MITVMA.MIT.EDU]
Sent:  Monday, September 30, 1996 1:16 PM
To:  Multiple recipients of list PIC
Subject:  Beginner needs help

To:           Multiple recipients of list PICLIST
<PICLIST@MITVMA.MIT.EDU>

I am new to PICs and would appreciate some help.  My question concerns
the
use of resistors.  What is a "pull-up" resistor for, why and when do you
need it?  I see them used sometimes and not other times.  Can you connect
an
I/O pin directly to 5 volts and read a logic high or is a resistor of
some
kind needed. Can you connect an I/O pin directly to another I/O pin or to
a
pin from other TTL chips, transistor etc.?  Is there some rule of thumb
that
I don't know about?  I know my ignorance is bare naked here, but bear
with me.

Thanks in advance.