Datasheet

MC68HC711D3 Data Sheet, Rev. 2.1
Freescale Semiconductor 85
Chapter 8
Programmable Timer
8.1 Introduction
The M68HC11 timing system is composed of five clock divider chains. The main clock divider chain
includes a 16-bit free-running counter, which is driven by a programmable prescaler. The main timer's
programmable prescaler provides one of the four clocking rates to drive the 16-bit counter. Two prescaler
control bits select the prescale rate.
The prescaler output divides the system clock by 1, 4, 8, or 16. Taps off of this main clocking chain drive
circuitry that generates the slower clocks used by the pulse accumulator, the real-time interrupt (RTI), and
the computer operating properly (COP) watchdog subsystems. Refer to Figure 8-1.
All main timer system activities are referenced to this free-running counter. The counter begins
incrementing from $0000 as the microcontroller unit (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 or gated time accumulation modes. 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.
RTI is a programmable periodic interrupt circuit that permits pacing the execution of software routines by
selecting one of four interrupt rates.
The COP watchdog clock input (E÷2
15
) is tapped off of the free-running counter chain. The COP
automatically times out unless it is serviced within a specific time by a program reset sequence. If the COP
is allowed to time out, a reset is generated, which drives the RESET
pin low to reset the MCU and the
external system. Refer to Table 8-1 for crystal related frequencies and periods.