System Integration Module (SIM)
Exception Control
MC68HC908MR16/MC68HC908MR32 — Rev. 4.0 Advance Information
MOTOROLA System Integration Module (SIM) 99
7.6 Exception Control
Normal, sequential program execution can be changed in thr ee different
ways:
1. Interrupts:
a. Maskable hardware CPU interrupts
b. Non-maskable software interrupt instruction (SWI)
2. Reset
3. Break interrupts
7.6.1 Interrupts
At the beginning of an interrupt, the CPU saves the CPU register
contents on the stack and sets the interrupt mask (I bit) to prevent
additional interrupts. At the end of an interrupt , the retu rn-f rom -interr upt
(RTI) instruction recovers the CPU register contents from the stack so
that normal processing can resume. Figure 7-7 shows interrupt entry
timing. Figure 7-9 shows interrupt recovery timing.
Interrupts are latched, and arbitration is performed in the SIM at the start
of interrupt processing. The arbitration result is a constant that the CPU
uses to determine which vector to fetch. Once an interrupt is latched by
the SIM, no other interrupt can take precedence, regardless of priority,
until the latched interrupt is serviced (or the I bit is cleared).
See Figure 7-8.
Figure 7-7. Interrupt Entry
MODULE
IDB
R/W
INTERRUPT
DUMMY SP SP – 1 SP – 2 SP – 3 SP – 4 VECT H VECT L
IAB
DUMMY PC – 1[7:0] PC – 1[15:8] X A CCR V DATA H V DATA L OPCODE
I BIT
START
ADDR