MC9S08EL32 Datasheet, PDF (64/356 Pages) Freescale Semiconductor, Inc – 8-Bit HCS08 Central Processor Unit (CPU)

Chapter 5 Resets, Interrupts, and General System Control
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.
After any reset, the COP watchdog is enabled (see Section 5.7.3, “System Options Register 1 (SOPT1),”
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.
The COPCLKS bit in SOPT2 (see Section 5.7.4, “System Options Register 2 (SOPT2),” for additional
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 5-1 summaries 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).
Table 5-1. COP Configuration Options
Control Bits
COPCLKS
COPT[1:0]
Clock Source
COP Overflow Count
N/A
0:0
N/A
COP is disabled
0
0:1
1 kHz
2⁵ cycles (32 ms¹)
0
1:0
1 kHz
2⁸ cycles (256 ms¹)
0
1:1
1 kHz
2¹⁰ cycles (1.024 s¹)
1
0:1
Bus
2¹³ cycles
1
1:0
Bus
2¹⁶ cycles
1
1:1
Bus
2¹⁸ cycles
1 _{Values are shown in this column based on tRTI = 1 ms. See tRTI in the appendix Section}
A.12.1, “Control Timing,” for the tolerance of this value.
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.
MC9S08EL32 Series and MC9S08SL16 Series Data Sheet, Rev. 3
64
Freescale Semiconductor