MC9S08RG60 Datasheet, PDF (58/232 Pages) Motorola, Inc – Microcontrollers

Resets, Interrupts, and System Configuration
5.3 MCU Reset
Resetting the MCU provides a way to start processing from a known set of initial conditions. During reset,
most control and status registers are forced to initial values and the program counter is loaded from the
reset vector ($FFFE:$FFFF). On-chip peripheral modules are disabled and I/O pins are initially configured
as general-purpose high-impedance inputs with pullup devices disabled. The I bit in the condition code
register (CCR) is set to block maskable interrupts until the user program has a chance to initialize the stack
pointer (SP) and system control settings. SP is forced to $00FF at reset.
The MC9S08RC/RD/RE/RG has six sources for reset:
• Power-on reset (POR)
• Low-voltage detect (LVD)
• Computer operating properly (COP) timer
• Illegal opcode detect
• Illegal address (16K and 8K devices only)
• Background debug forced reset
• The reset pin (RESET)
Each of these sources, with the exception of the background debug forced reset, has an associated bit in
the system reset status register. Whenever the MCU enters reset, the reset pin is driven low for 34 internal
bus cycles where the internal bus frequency is one-half the OSC frequency. After the 34 cycles are
completed, the pin is released and will be pulled up by the internal pullup 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 timer periodically. If the application program gets lost and fails to reset the COP before it
times out, a system reset is generated to force the system back to a known starting point. The COP
watchdog is enabled by the COPE bit in SOPT (see 5.8.4 System Options Register (SOPT) for additional
information). The COP timer is reset by writing any value to the address of SRS. This write does not affect
the data in the read-only SRS. Instead, the act of writing to this address is decoded and sends a reset signal
to the COP timer.
After any reset, the COP timer is enabled. This provides a reliable way to detect code that is not executing
as intended. If the COP watchdog is not used in an application, it can be disabled by clearing the COPE
bit in the write-once SOPT register. Also, the COPT bit can be used to choose one of two timeout periods
(2¹⁸ or 2²⁰ cycles of the bus rate clock). Even if the application will use the reset default settings in COPE
and COPT, the user must write to write-once SOPT during reset initialization to lock in the settings. That
way, they cannot be changed accidentally if the application program gets lost.
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MC9S08RC/RD/RE/RG
Freescale Semiconductor