Freescale Semiconductor MC68HC908MR32 - Manual (Page 189)

Exception Control
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
Freescale Semiconductor 189
Figure 14-9. Interrupt Recovery
14.5.1.1 Hardware Interrupts
A hardware interrupt does not stop the current instruction. Processing of a hardware interrupt begins after
completion of the current instruction. When the current instruction is complete, the SIM checks all pending
hardware interrupts. If interrupts are not masked (I bit clear in the condition code register), and if the
corresponding interrupt enable bit is set, the SIM proceeds with interrupt processing; otherwise, the next
instruction is fetched and executed.
If more than one interrupt is pending at the end of an instruction execution, the highest priority interrupt is
serviced first.
Figure 14-10 demonstrates what happens when two interrupts are pending. If an interrupt
is pending upon exit from the original interrupt service routine, the pending interrupt is serviced before the
load-accumulator-from- memory (LDA) instruction is executed.
Figure 14-10 . 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, software
should save the H register and then restore it prior to exiting the routine.
MODULE
IDB
R/W
INTERRUPT
SP – 4 SP – 3 SP – 2 SP – 1 SP PC PC + 1 IAB
CCR A X PC – 1[7:0] PC – 1[15:8] OPCODE OPERAND
I BIT
CLI
LDA
INT1
PULH
RTI
INT2
BACKGROUND ROUTINE #$FF
PSHH
INT1 INTERRUPT SERVICE ROUTINE
PULH
RTI
PSHH
INT2 INTERRUPT SERVICE ROUTINE