> I have 2 projects one using a 16F628 and another using a 12F675 both 
> running
> on 5V. Is there an exact value to use as a pull up resistor, or is 
> it even
> nessasary?

Briefly:    Input pins must always be within a voltage range specified 
in the data sheet. Depending on your circuit you may need pullup or 
pulldown resistors to accomplish this.

_______

Despite the weird interpretations of your question ... :-)

ALL input pins MUST have their levels defined at all times and be 
maintained (approximately) inside the supply rails (How approximately 
is in the data sheet, but it's usually safe to assume that an input 
should never go outside supply rails).

An input level is "defined" when it is connected to a voltage source 
by a resistance such that in the worst case the pin voltage can be 
guaranteed to be within specs.

Put less formally and more usefully, if a pin is connected to a 
voltage Vss <= Voltage <= Vdd by a resistance of not more than 1 
megohm then odds are that it will behave OK. You can sometimes use 
more than 1 meg and usually would use somewhat less.

SO - if an input pin is driven by a signal source that is always 
within the above mentioned voltage range and the signal source 
resistance is always <= 1M then you don't need pullups. However, if 
the signal source sometimes floats then a pullup (or pull down) will 
be needed to define the pin voltage under these conditions.



        RM 

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist