Chapter 5 Resets, Interrupts, and General System Control
MC9S08DZ128 Series Data Sheet, Rev. 1
86
Freescale Semiconductor
bus cycles. After the 34 cycles are completed, the pin is released and will be pulled up by the internal
pull-up resistor, unless it is held low externally. After the pin is released, it is sampled after another 38
cycles to determine whether the reset pin is the cause of the MCU reset.
5.4
Computer Operating Properly (COP) Watchdog
The COP watchdog is intended to force a system reset when the application software fails to execute as
expected. To prevent a system reset from the COP timer (when it is enabled), application software must
reset the COP counter periodically. If the application program gets lost and fails to reset the COP counter
before it times out, a system reset is generated to force the system back to a known starting point.
for additional information). If the COP watchdog is not used in an application, it can be disabled by
clearing COPT bits in SOPT1.
The COP counter is reset by writing 0x0055 and 0x00AA (in this order) to the address of SRS during the
selected timeout period. Writes do not affect the data in the read-only SRS. As soon as the write sequence
is done, the COP timeout period is restarted. If the program fails to do this during the time-out period, the
MCU will reset. Also, if any value other than 0x0055 or 0x00AA is written to SRS, the MCU is
immediately reset.
information) selects the clock source used for the COP timer. The clock source options are either the bus
clock or an internal 1-kHz clock source. With each clock source, there are three associated time-outs
controlled by the COPT bits in SOPT1.
Table 5-6 summarizes the control functions of the COPCLKS and
COPT bits. The COP watchdog defaults to operation from the 1-kHz clock source and the longest time-out
(210 cycles).
When the bus clock source is selected, windowed COP operation is available by setting COPW in the
SOPT2 register. In this mode, writes to the SRS register to clear the COP timer must occur in the last 25%
of the selected timeout period. A premature write immediately resets the MCU. When the 1-kHz clock
source is selected, windowed COP operation is not available.
The COP counter is initialized by the rst writes to the SOPT1 and SOPT2 registers and after any system
reset. Subsequent writes to SOPT1 and SOPT2 have no effect on COP operation. Even if the application
will use the reset default settings of COPT, COPCLKS, and COPW bits, the user should write to the
write-once SOPT1 and SOPT2 registers during reset initialization to lock in the settings. This will prevent
accidental changes if the application program gets lost.
The write to SRS that services (clears) the COP counter should not be placed in an interrupt service routine
(ISR) because the ISR could continue to be executed periodically even if the main application program
fails.
If the bus clock source is selected, the COP counter does not increment while the MCU is in background
debug mode or while the system is in stop mode. The COP counter resumes when the MCU exits
background debug mode or stop mode.