M68HC11K_13 Datasheet, PDF (120/290 Pages) Freescale Semiconductor, Inc – M68HC11K Family Technical Data

Freescale Semiconductor, Inc.
Resets and Interrupts
5.5.1 Non-Maskable Interrupts
Non-maskable interrupts can interrupt CPU operations at any time. The
most common use for such an interrupt is for serious system problems,
such as program runaway or power failure. The three sources of
non-maskable interrupt are:
• XIRQ pin
• Illegal opcode trap
• Software interrupt instruction (SWI)
5.5.1.1 Non-Maskable Interrupt Request (XIRQ)
The XIRQ input is an updated version of the non-maskable NMI input of
earlier MCUs. Upon reset, both the X bit and I bit of the CCR are set to
inhibit all maskable interrupts and XIRQ. After minimum system
initialization, software can clear the X bit by a transfer from accumulator
A to condition code register (TAP) instruction, enabling XIRQ interrupts.
Thereafter, software cannot set the X bit and the XIRQ interrupt
becomes non-maskable.
I bit-related interrupts do not affect the X bit, which has a higher priority
than they do in the interrupt priority logic. When an I bit-related interrupt
occurs, the CPU sets the I bit after stacking the CCR byte, but the X bit
remains unaffected. When an X bit-related interrupt occurs, the CPU
sets both the X and I bits after stacking the CCR. The RTI instruction
restores the X and I bits to their pre-interrupt request state when it pulls
the CCR from the stack.
5.5.1.2 Illegal Opcode Trap
The MCU includes an illegal opcode detection circuit to avoid attempting
to process undefined opcodes or opcode sequences. This mechanism
works for all unimplemented opcodes on all four opcode map pages.
When the circuit detects an illegal opcode, it generates an interrupt. The
CPU responds by pushing the current value of the program counter,
which is actually the address of the first byte of the illegal opcode, on the
stack. The illegal opcode service routine can use this stacked address
as a pointer to the illegal opcode to correct it. To avoid repeated
Technical Data
120
Resets and Interrupts
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M68HC11K Family
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