Tim Lunn with his diabolical pseudonym wrote:

> Firstly, if using a prescaler, when does the timer start counting? is
> it after the prescale bit is set?

  There is no "prescale bit".  The timer counts when you tell it by the
OPTION register, what to count, if (in the case of an external source)
and when there is such a source.

  If prescaled, the timer counts overflows of the prescaler, so it in a
sense only "starts counting" at the first such overflow.

> Is the prescaler reset to 1:2 when a write occurs to the TMR0 register

  No, it's set to zero.

> and does this reset only affect the 3 prescaler bits?

  The prescaler size is between one and eight bits (corresponding to a
binary division ratio of from 2 to 256).  This is set by the OPTION
register however.  In fact, the prescaler is a full 8 bits, all are
reset by the TMR0 write, and the OPTION register simply selects *which*
bit will be used to clock the TMR0.  If you change the prescale value in
the OPTION register during counting, you will get indeterminate results.

> or the whole option_reg??

  The OPTION register is only used to set the "size" of the prescaler.

  This enquiry, as many, sounds as if you may not have the specification
sheet for the PIC16F84, document DS30430C from Microchip.  Have you?
--
  Cheers,
        Paul B.