MC68HC05X4 Datasheet, PDF (105/156 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
Core Timer
Real time interrupts (RTI)
When CTOE (core timer overflow enable) is set, a CPU interrupt request
is generated when the CTOF bit is set. Reset clears CTOE.
The core timer counter register (CTCR) is a read-only register that
contains the current value of the 8-bit ripple counter at the beginning of
the timer chain. This counter is clocked at f_OP/4 and can be used for
various functions including a software input capture. Extended time
periods can be attained using the CTIMER overflow function to
increment a temporary RAM storage location thereby simulating a 16-bit
(or more) counter.
The power-on cycle clears the entire counter chain and begins clocking
the counter. After t_PORL cycles, the power-on reset circuit is released,
which again clears the counter chain and allows the device to come out
of reset. At this point, if RESET is not asserted, the timer will start
counting up from zero and normal device operation will begin. When
RESET is asserted at any time during operation (other than POR), the
counter chain is cleared.
Real time interrupts (RTI)
The real time interrupt circuit consists of a three stage divider and a
1-of-4 selector. The clock frequency that drives the RTI circuit is f_OP/2¹⁴
(or f_OP/16384), with three additional divider stages, giving a maximum
interrupt period of 4 seconds at a bus frequency (f_OP) of 32.768kHz.
Register details are given in Core timer registers.
Computer operating properly (COP) watchdog timer
The COP watchdog timer function is implemented by taking the output
of the RTI circuit and further dividing it by eight, as shown in Figure 1.
Note that the minimum COP timeout period is seven times the RTI
period. This is because the COP will be cleared asynchronously with
respect to the value in the core timer counter register/RTI divider, hence
the actual COP timeout period will vary between 7x and 8x the RTI
period.
3-ctimer
MOTOROLA
Core Timer
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MC68HC05X4 Rev 1.0
105