MC68HC908GR8 Datasheet, PDF (48/286 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Resets and Interrupts
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.
CLI
LDA #$FF
BACKGROUND
ROUTINE
INT1
PSHH
PULH
RTI
INT1 INTERRUPT SERVICE ROUTINE
INT2
PSHH
PULH
RTI
INT2 INTERRUPT SERVICE ROUTINE
Figure 4-4. Interrupt Recognition Example
The LDA opcode is prefetched by both the INT1 and INT2 RTI instructions. However, in the case of the
INT1 RTI prefetch, this is a redundant operation.
NOTE
To maintain compatibility with the M6805 Family, the H register is not
pushed on the stack during interrupt entry. If the interrupt service routine
modifies the H register or uses the indexed addressing mode, save the H
register and then restore it prior to exiting the routine.
MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7
48
Freescale Semiconductor