Here is the block diagram for the Interval Timer. It is a very simple timer with a 12-bit counter and a 12-bit compare register. In front of the timer is a selector to choose the sub-clock or the internal low speed oscillator clock to run the timer. Note that the control register which controls the input clock for the Interval Timer is the same as the one which controls the input clock for the real-time clock timer. That means that both timers are always using the same time base; either the sub oscillator clock, the external 32kHz crystal or the internal 15kHz low speed oscillator.