IRQ7 is transition-sensitive as well as level-sensitive: a level-7 interrupt is not detected
unless a falling edge transition is detected on the IRQ7 line. This prevents redundant
servicing and stack overflow. A nonmaskable interrupt is generated each time IRQ7 is
asserted as well as each time the priority mask changes from %111 to a lower number
while IRQ7 is asserted.
Interrupt requests are sampled on consecutive falling edges of the system clock. In-
terrupt request input circuitry has hysteresis: to be valid, a request signal must be as-
serted for at least two consecutive clock periods. Valid requests do not cause
immediate exception processing, but are left pending. Pending requests are pro-
cessed at instruction boundaries or when exception processing of higher-priority ex-
ceptions is complete.
The CPU32 does not latch the priority of a pending interrupt request. If an interrupt
source of higher priority makes a service request while a lower priority request is pend-
ing, the higher priority request is serviced. If an interrupt request with a priority equal
to or lower than the current IP mask value is made, the CPU32 does not recognize the
occurrence of the request. If simultaneous interrupt requests of different priorities are
made, and both have a priority greater than the mask value, the CPU32 recognizes
the higher-level request.
4
4.7.3 Interrupt Acknowledge and Arbitration
When the CPU32 detects one or more interrupt requests of a priority higher than the
interrupt priority mask value, it places the interrupt request level on the address bus
and initiates a CPU space read cycle. The request level serves two purposes: it is de-
coded by modules or external devices that have requested interrupt service, to deter-
mine whether the current interrupt acknowledge cycle pertains to them, and it is
latched into the interrupt priority mask field in the CPU32 status register, to preclude
further interrupts of lower priority during interrupt service.
Modules or external devices that have requested interrupt service must decode the in-
terrupt priority mask value placed on the address bus during the interrupt acknowledge
cycle and respond if the priority of the service request corresponds to the mask value.
However, before modules or external devices respond, interrupt arbitration takes
place.
Arbitration is performed by means of serial contention between values stored in indi-
vidual module interrupt arbitration (IARB) fields. Each module that can make an inter-
rupt service request, including the SIM, has an IARB field in its configuration register.
IARB fields can be assigned values from %0000 to %1111. In order to implement an
arbitration scheme, each module that can initiate an interrupt service request must be
assigned a unique, non-zero IARB field value during system initialization. Arbitration
priorities range from %0001 (lowest) to %1111 (highest) — if the CPU recognizes an
interrupt service request from a source that has an IARB field value of %0000, a spu-
rious interrupt exception is processed.
WARNING
Do not assign the same arbitration priority to more than one module.
When two or more IARB fields have the same nonzero value, the
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4-46
SYSTEM INTEGRATION MODULE
MC68331
USER’S MANUAL
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