Internal Inputprotection in the PIC's.

Did you know it? Microchip does not use diodes to protect the inputs in PIC's. They use transistors! 
----------*--------- Input
          !
       /--!--\
      !  \!/  !
Vdd --!---!-- !
      !   !   !
       \--!--/
          !
         Vss
When the input exceed Vdd (+0.5V to 0.6V), then the current is lead into Vss. It means that there is a diode (a transistor diode) between Input and Vdd, but a lot of current is lead into ground.

This affect the lowcost programmer, and R1 was added to avoid to high current into ground.

If using an input pin as diode to Vdd is changed to an external diode, then you need no resistor! And the current driving MCLR is much higher. Look in the FILE section.

http://www.ebar.dtu.dk/~c888600/files/newpic5.gif

Best regards, Jens Dyekjær Madsen

E-Mail address: Jens.Madsen@post3.tele.dk