AVR-Chat

On Tue, 2005-08-09 at 10:07 -0400, wbounce wrote:
> Read the data sheet for your part 
> But this is my understanding.
> -----Original Message-----

>          Can any one give a sample code for 
> reading the value of  a i/o PIN. Also tell 
> me role of pull up resistor in doiing so.     \

As wbounce wrote, read the data sheet.


  As for the role of the pullup resistors:

Pullup resistors lock the value of the input pin to a known value (Vcc,
or logic 0 in this case).  This is to insure that a pin connected to a
tri-stated output from another chip doesn't read a false logic 1 because
the tri-state voltage is above .5 Vcc.  It is essentially a
safety/reliability issue.

   When you pull the pulled-up pin low, the Vcc drops across the pullup
resisotr, but allows the voltage at the pin itself to drop to ground
(very nearly 0 V).  

   Because the pullup resisots are integrated into the AVR chip, you
don't need to worry about values.  In general, pullup resistors want to
be between 4.7K amd 15K ohms.  This is to minimize the current drawn
through the pullup resisotor, which current otherwise represents wasted
energy and added heat (a very serious issue for portable, low-power
electronics).  Typically, larger values of pullup resistors have been
found to lead to problems with stray RF pickup causing erroneous signals
at the input pins, so they are to be avoided.  Smaller values result in
excessive heat geenration and power dissipation.  When using discrete,
external pullup resistors, the resistors should be mounted as close to
the chiup package as possible, with the shortest traces possible between
the pullup resistor and the package pin.  Again, this is to avoid RF and
other EMI interfereing with the correct operation of the circuit.


tom