For anyone who is trying to create accurate interrupt times on the pic, you must be aware
that when you load the rtcc with a value, the prescaler is cleared to zero. What this
means is that when the rtcc reaches 0, and your interrupt service routine starts, the
prescaler has continued to accumulate instruction counts. Lets say that it takes a dozen
or so counts to finally get around to clearing the interrupt flag, incrementing or
decrementing your time counter, and then loading the rtcc with your preload value.
The prescaler has been accumulating instruction ticks and will be incrementing the rtcc
when it rolls over. However when you preload the rtcc, those ticks get wiped out and
reset to zero.
Every interrupt cycle loses time!
The solution is to pick your crystal frequency and your prescaler value
such that you never write to the rtcc. It should be allowed to overflow.
This way, the prescaler is never cleared to zero and no clock ticks are
lost.
