mc9s08qg8 Freescale Semiconductor, Inc, mc9s08qg8 Datasheet - Page 59

• Manufacturer Part Number: mc9s08qg8
• Description: Hcs08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

When the 1-kHz clock source is selected, the COP counter is re-initialized to zero upon entry to stop mode.
The COP counter begins from zero after the MCU exits stop mode.
5.5
Interrupts provide a way to save the current CPU status and registers, execute an interrupt service routine
(ISR), and then restore the CPU status so processing resumes where it was before the interrupt. Other than
the software interrupt (SWI), which is a program instruction, interrupts are caused by hardware events such
as an edge on the IRQ pin or a timer-overflow event. The debug module can also generate an SWI under
certain circumstances.
If an event occurs in an enabled interrupt source, an associated read-only status flag will become set. The
CPU will not respond until and unless the local interrupt enable is a 1 to enable the interrupt. The I bit in
the CCR is 0 to allow interrupts. The global interrupt mask (I bit) in the CCR is initially set after reset,
which masks (prevents) all maskable interrupt sources. The user program initializes the stack pointer and
performs other system setup before clearing the I bit to allow the CPU to respond to interrupts.
When the CPU receives a qualified interrupt request, it completes the current instruction before responding
to the interrupt. The interrupt sequence obeys the same cycle-by-cycle sequence as the SWI instruction and
consists of:
While the CPU is responding to the interrupt, the I bit is automatically set to avoid the possibility of another
interrupt interrupting the ISR itself (this is called nesting of interrupts). Normally, the I bit is restored to 0
when the CCR is restored from the value stacked on entry to the ISR. In rare cases, the I bit can be cleared
inside an ISR (after clearing the status flag that generated the interrupt) so that other interrupts can be
serviced without waiting for the first service routine to finish. This practice is not recommended for anyone
other than the most experienced programmers because it can lead to subtle program errors that are difficult
to debug.
The interrupt service routine ends with a return-from-interrupt (RTI) instruction which restores the CCR,
A, X, and PC registers to their pre-interrupt values by reading the previously saved information from the
stack.
When two or more interrupts are pending when the I bit is cleared, the highest priority source is serviced
first (see
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Saving the CPU registers on the stack
Setting the I bit in the CCR to mask further interrupts
Fetching the interrupt vector for the highest-priority interrupt that is currently pending
Filling the instruction queue with the first three bytes of program information starting from the
address fetched from the interrupt vector locations
Table
Interrupts
For compatibility with M68HC08 devices, the H register is not
automatically saved and restored. It is good programming practice to push
H onto the stack at the start of the interrupt service routine (ISR) and restore
it immediately before the RTI that is used to return from the ISR.
5-2).
MC9S08QG8 and MC9S08QG4 Data Sheet, Rev. 1.01
NOTE
Chapter 5 Resets, Interrupts, and General System Control
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