Multifunction Timer (MFT)
Notes:
Simple 68HC05 devices have a simple timer, the multifunction timer (MFT). Consisting of a 15-bit ripple counter, the lower eight bits of which can be read from the timer counter register (TCR), the MFT is clocked at one fourth the internal MCU clock frequency.
The MFT is essentially a circuit for generating various periodic interrupts. Bits in the timer status/control register (TSCR) can enable interrupts for overflow of the lower eight bits of the ripple counter (flagged by TOF, enabled by TOIE, and cleared by TOFR) and overflow of the real-time interrupt (flagged by RTIF, enabled by RTIE, and cleared by RTIFR).
One of four real-time interrupts rates can be selected by the RT[1:0] bits in TSCR. Overflows from TCR clock the upper seven bits of the ripple counter which serve as the time base for real-time interrupts.
Interrupts can be generated every 1024 internal clock cycles by the timer overflow interrupt and every 16384, 32768, 65536, or 131,072 internal clock cycles by the real-time interrupt.