mc68hc916y3 Freescale Semiconductor, Inc, mc68hc916y3 Datasheet - Page 165

• Manufacturer Part Number: mc68hc916y3
• Description: Mc68hc16y3 16 Bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

5.8.3 Interrupt Acknowledge and Arbitration
MC68HC16Y3/916Y3
USER’S MANUAL
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
interrupts is complete.
The CPU16 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 CPU16 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 CPU16 recognizes
the higher-level request.
When the CPU16 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
decoded by modules or external devices that have requested interrupt service, to de-
termine whether the current interrupt acknowledge cycle pertains to them, and it is
latched into the interrupt priority mask field in the CPU16 condition code register to
preclude further interrupts of lower priority during interrupt service.
Modules or external devices that have requested interrupt service must decode the IP
mask value placed on the address bus during the interrupt acknowledge cycle and re-
spond 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 SCIM2, has an IARB field in its configuration
register. IARB fields can be assigned values from %0000 to %1111. In order to imple-
ment an arbitration scheme, each module that can request interrupt service must be
assigned a unique, non-zero IARB field value during system initialization. Arbitration
priorities range from %0001 (lowest) to %1111 (highest) — if the CPU16 recognizes
an interrupt service request from a source that has an IARB field value of %0000, a
spurious interrupt exception is processed.
Do not assign the same arbitration priority to more than one module.
When two or more IARB fields have the same nonzero value, the
CPU16 interprets multiple vector numbers at the same time, with
unpredictable consequences.
Freescale Semiconductor, Inc.
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WARNING
MOTOROLA
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