SPMC68CK338CPV14 datasheet(46/133 Pages) FREESCALE

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MOTOROLA

MC68CK338

MC68CK338TS/D

Arbitration is performed by means of serial contention between values stored in individual module inter-

rupt arbitration (IARB) fields. Each module that can make an interrupt service request, including the

SIML, 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 ser-

vice request must be assigned a unique, non-zero IARB field value during system initialization. Arbitra-

tion 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 spurious 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 non-zero value, the CPU32L interprets multiple

vector numbers at the same time, with unpredictable consequences.

Because the EBI manages external interrupt requests, the SIML IARB value is used for arbitration be-

tween internal and external interrupt requests. The reset value of IARB for the SIML is %1111, and the

reset IARB value for all other modules is %0000.

Although arbitration is intended to deal with simultaneous requests of the same priority, it always takes

place, even when a single source is requesting service. This is important for two reasons: the EBI does

not transfer the interrupt acknowledge read cycle to the external bus unless the SIML wins contention,

and failure to contend causes the interrupt acknowledge bus cycle to be terminated early, by a bus error.

When arbitration is complete, the module with the highest arbitration priority must terminate the bus

cycle. Internal modules place an interrupt vector number on the data bus and generate appropriate in-

ternal cycle termination signals. In the case of an external interrupt request, after the interrupt acknowl-

edge cycle is transferred to the external bus, the appropriate external device must decode the mask

value and respond with a vector number, then generate data and size acknowledge (DSACK) termina-

tion signals, or it must assert the autovector (AVEC) request signal. If the device does not respond in

time, the EBI bus monitor asserts the bus error signal (BERR), and a spurious interrupt exception is

taken.

Chip-select logic can also be used to generate internal AVEC or DSACK signals in response to interrupt

requests from external devices. Chip-select address match logic functions only after the EBI transfers

an interrupt acknowledge cycle to the external bus following IARB contention. If a module makes an

interrupt request of a certain priority, and the appropriate chip-select registers are programmed to gen-

erate AVEC or DSACK signals in response to an interrupt acknowledge cycle for that priority level, chip-

select logic does not respond to the interrupt acknowledge cycle, and the internal module supplies a

vector number and generates internal cycle termination signals.

For periodic timer interrupts, the PIRQL field in the periodic interrupt control register (PICR) determines

PIT priority level. A PIRQL value of %000 means that PIT interrupts are inactive. By hardware conven-

tion, when the CPU32L receives simultaneous interrupt requests of the same level from more than one

SIML source (including external devices), the periodic interrupt timer is given the highest priority, fol-

lowed by the IRQ pins. Refer to 3.4.6 Periodic Interrupt Timer for more information.

3.9.2 Interrupt Processing Summary

A summary of the interrupt processing sequence follows. When the sequence begins, a valid interrupt

service request has been detected and is pending.

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