Pic 16f877a

i dont understand at all what is Prescalar assingment bit and prescalar rate select bits ? what are their function ?

The prescaler in the option reg? It can be assigned to the WDT or timer0... There are 8 possible scales... from 1:1 (WDT) and 1:2 (timer0) up to 1:128 & 1:256

Timer0 is an 8 bit timer... 1:2) 2 clock pulses increase the timer.. 1:256) 256 clock pulses increase the timer

Prescalar assingment bit set which is to be used WDT or Timer0 I figured I add that Just to make sure the OP set it for what one he wanted to use.