HC705J2 datasheet(49/78 Pages) MOTOROLA

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MC68HC705J2

RESETS AND INTERRUPTS

MOTOROLA

5-3

5.2 Interrupts

An interrupt temporarily stops normal processing to process a particular event.

Unlike a reset, an interrupt does not stop the operation of the instruction being

executed. An interrupt takes effect when the current instruction completes its

execution. An interrupt saves the CPU registers on the stack and loads the

program counter with a user-defined interrupt vector address.

The following

conditions produce an interrupt:

• Timer overflow or real-time interrupt request (timer interrupts)

• A logical zero applied to the IRQ pin (external interrupt)

• SWI instruction (software interrupt)

The CPU does the following things to begin servicing an interrupt:

• Stores the contents of the CPU registers on the stack as shown in Figure

5-2

ACCUMULATOR

CONDITION CODE REGIST

INDEX REGISTER

PROGRAM COUNTER LOW

PROGRAM COUNTER HIGH

TOWARD HIGHER ADDRESSES

(HIGHEST STACK ADDRESS IS $00

TOWARD LOWER ADDRESSES

(LOWEST STACK ADDRESS IS $0

UNSTACK

STACK

Figure 5-2. Interrupt Stacking Order

• Sets the interrupt mask to prevent further interrupts

• Loads the program counter with the contents of the appropriate interrupt

vector locations:

— $0FF8 and $0FF9 (timer interrupt vector)

— $0FFA and $0FFB (external interrupt vector)

— $0FFC and $0FFD (software interrupt vector)

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