MC68HC11KW1 Datasheet, PDF (153/238 Pages) Motorola, Inc – High-density complementary metal oxide semiconductor HCMOS) microcontroller unit

9
TIMING SYSTEM
The MC68HC11KW1 contains three 16-bit timers. Figure 9-1 provides a diagram of the entire
timing system. The main timer, Timer 1, is described in the following paragraphs; refer to
Section 9.2 and Section 9.3 for descriptions of Timer 2 and Timer 3.
9.1
Timer 1
Timer 1 is the standard M68HC11 timing system, composed of several clock divider chains. The
main clock divider chain includes a 16-bit free-running counter, driven by a programmable
prescaler.
The prescaler output divides the system clock by 1, 4, 8, or 16. Taps from this main clocking chain
drive-circuitry generate the slower clocks used by the pulse accumulator, the real-time interrupt
(RTI), and the computer operating properly (COP) watchdog subsystems, which are also
described in this section. Refer to Figure 9-1.
9
All main timer system activities are referenced to this free-running counter. The counter begins
incrementing from $0000 as the MCU comes out of reset, and continues to the maximum count,
$FFFF. At the maximum count, the counter rolls over to $0000, sets an overflow flag and continues
to increment. As long as the MCU is running in a normal operating mode, there is no way to reset,
change or interrupt the counting. The capture/compare subsystem features three input capture
channels, four output compare channels and one channel that can be selected to perform either
input capture or output compare. Each of the three input capture functions has its own 16-bit input
capture register (time capture latch) and each of the output compare functions has its own 16-bit
compare register. All timer functions, including the timer overflow and RTI, have their own interrupt
controls and separate interrupt vectors.
The pulse accumulator contains an 8-bit counter and edge select logic. The pulse accumulator
can operate in either event counting mode or gated time accumulation mode. During event
counting mode, the pulse accumulator’s 8-bit counter increments when a specified edge is
detected on an input signal. During gated time accumulation mode, an internal clock source
increments the 8-bit counter while an input signal has a predetermined logic level.
The real-time interrupt (RTI) is a programmable periodic interrupt circuit that permits pacing the
execution of software routines by selecting one of four interrupt rates.
MC68HC11KW1
TIMING SYSTEM
9-1