Rainbow Electronics T48C862-R8 - Manual (Page 19)

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T48C862-R8
4590B–4BMCU–02/03
Interrupt Processing For processing the eight interrupt levels, the microcontroller includes an interrupt con-
troller with two 8-bit wide interrupt pending and interrupt active registers. The interrupt
controller samples all interrupt requests during every non-I/O instruction cycle and
latches these in the interrupt pending register. If no higher priority interrupt is present in
the interrupt active register, it signals the CPU to interrupt the current program execu-
tion. If the interrupt enable bit is set, the processor enters an interrupt acknowledge
cycle. During this cycle a short call (SCALL) instruction to the service routine is exe-
cuted and the current PC is saved on the return stack. An interrupt service routine is
completed with the RTI instruction. This instruction, resets the corresponding bits in the
interrupt pending/active register and fetches the return address from the return stack to
the program counter. When the interrupt enable flag is reset (triggering of interrupt rou-
tines is disabled), the execution of new interrupt service routines is inhibited but not the
logging of the interrupt requests in the interrupt pending register. The execution of the
interrupt is delayed until the interrupt enable flag is set again. Note that interrupts are
only lost if an interrupt request occurs while the corresponding bit in the pending register
is still set (i.e., the interrupt service routine is not yet finished).
It should be noted that automatic stacking of the RBR is not carried out by the hardware
and so if ROM banking is used, the RBR must be stacked on the expression stack by
the application program and restored before the RTI. After a master reset (power-on,
brown-out or watchdog reset), the interrupt enable flag and the interrupt pending and
interrupt active register are all reset.
Interrupt Latency The interrupt latency is the time from the occurrence of the interrupt to the interrupt
service routine being activated. This is extremely short (taking between 3 to 5 machine
cycles depending on the state of the core).
Figure 14. Interrupt Handling
7
6
5
4
3
2
1
0
Pr
ior
ity level
INT5 active
INT7 active
INT2 pending
SWI0
INT2 active
INT0 pending INT0 active
INT2
RTI
RTI
INT5
INT3 active
INT3
RTI
RTI
RTI
INT7
Time
Main /
Autosleep
Main /
Autosleep