MC68HC908GZ8 Datasheet, PDF (53/344 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Resets and Interrupts
Interrupts
4.3 Interrupts
4.3.1 Effects
An interrupt temporarily changes the sequence of program execution to respond to
a particular event. An interrupt does not stop the operation of the instruction being
executed, but begins when the current instruction completes its operation.
An interrupt:
• Saves the CPU registers on the stack. At the end of the interrupt, the RTI
instruction recovers the CPU registers from the stack so that normal
processing can resume.
• Sets the interrupt mask (I bit) to prevent additional interrupts. Once an
interrupt is latched, no other interrupt can take precedence, regardless of its
priority.
• Loads the program counter with a user-defined vector address
MC68HC908GZ8
MOTOROLA
5
4
STACKING ³
ORDER ₂
1
•
•
•
CONDITION CODE REGISTER
ACCUMULATOR
INDEX REGISTER (LOW BYTE)⁽¹⁾
PROGRAM COUNTER (HIGH BYTE)
PROGRAM COUNTER (LOW BYTE)
•
•
•
1
2
3 _UNSTACKING
4
ORDER
5
$00FF DEFAULT ADDRESS ON RESET
1. High byte of index register is not stacked.
Figure 4-3. Interrupt Stacking Order
After every instruction, the CPU checks all pending interrupts if the I bit is not set.
If more than one interrupt is pending when an instruction is done, the highest
priority interrupt is serviced first. In the example shown in Figure 4-4, if an interrupt
is pending upon exit from the interrupt service routine, the pending interrupt is
serviced before the LDA instruction is executed.
Resets and Interrupts
For More Information On This Product,
Go to: www.freescale.com
Data Sheet
53