Calibration

With the best intentions in the world, a 4MHz Clock is NOT a 4MHz clock. It is a few Hz fast, or a few Hz slow, but is is NOT exactly 4000000.0000Hz. so the Calibration feature is used to speed-up or slow-down the clock slightly to compensate, and to compensate any lag in software instructions used to service the interrupt.

Interrupt ENABLE

I only ENABLE interrupts where they are needed. In other places they are either not needed, or not wanted. This also makes for smaller and more efficient code.