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When TSC assertion takes effect, internal signals are forced to values that can cause inadvertent mode selection. Once the output drivers change state, the MCU must be powered down and restarted before normal operation can resume.
3.8 Interrupts
Interrupt recognition and servicing involve complex interaction between the central processing unit, the system integration module, and a device or module requesting interrupt service.
The CPU32 provides for eight levels of interrupt priority (0–7), seven automatic interrupt vectors, and
200 assignable interrupt vectors. All interrupts with priorities less than seven can be masked by the interrupt priority (IP) field in the status register. The CPU32 handles interrupts as a type of asynchronous exception.
Interrupt recognition is based on the states of interrupt request signals iIRQ[7:1] and the IP mask value.
Each of the signals corresponds to an interrupt priority. IRQ1 has the lowest priority, and IRQ7 has the highest priority.
The IP field consists of three bits. Binary values %000 to %111 provide eight priority masks. Masks prevent an interrupt request of a priority less than or equal to the mask value (except for IRQ7) from being recognized and processed. When IP contains %000, no interrupt is masked. During exception processing, the IP field is set to the priority of the interrupt being serviced.
Interrupt request signals can be asserted by external devices or by microcontroller modules. Request lines are connected internally by means of a wired NOR — simultaneous requests of differing priority can be made. Internal assertion of an interrupt request signal does not affect the logic state of the corresponding MCU pin.
External interrupt requests are routed to the CPU via the external bus interface and SIM interrupt control logic. The CPU treats external interrupt requests as though they come from the SIM.
External IRQ[6:1] are active-low level-sensitive inputs. External IRQ7 is an active-low transition-sensitive input. IRQ7 requires both an edge and a voltage level for validity.
IRQ[6:1] are maskable. IRQ7 is nonmaskable. The IRQ7 input is transition-sensitive in order to prevent redundant servicing and stack overflow. A nonmaskable interrupt is generated each time IRQ7 is asserted, and each time the priority mask changes from %111 to a lower number whileIRQ7 is asserted.
Interrupt requests are sampled on consecutive falling edges of the system clock. Interrupt request input circuitry has hysteresis. To be valid, a request signal must be asserted for at least two consecutive clock periods. Valid requests do not cause immediate exception processing, but are left pending. Pending requests are processed at instruction boundaries or when exception processing of higher-priority exceptions 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 pending, the higher priority request is serviced. If an interrupt request of equal or lower priority than the current IP mask value is made, the
CPU does not recognize the occurrence of the request in any way.
3.8.1 Interrupt Acknowledge and Arbitration
Interrupt acknowledge bus cycles are generated during exception processing. When the CPU detects one or more interrupt requests of a priority higher than the interrupt priority mask value, it performs a
CPU space read from address $FFFFF : [IP] : 1.
The CPU space read cycle performs two functions: it places a mask value corresponding to the highest priority interrupt request on the address bus, and it acquires an exception vector number from the interrupt source. The mask value also serves two purposes: it is latched into the CCR IP field in order to
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mask lower-priority interrupts during exception processing, and it is decoded by modules that have requested interrupt service to determine whether the current interrupt acknowledge cycle pertains to them.
Modules that have requested interrupt service decode the IP value placed on the address bus at the beginning of the interrupt acknowledge cycle, and if their requests are at the specified IP level, respond to the cycle. Arbitration between simultaneous requests of the same priority is performed by means of serial contention between module interrupt arbitration (IARB) field bit values.
Each module that can make an interrupt service request, including the SIM, has an IARB field in its configuration register. An IARB field can be assigned a value from %0001 (lowest priority) to %1111 (highest priority). A value of %0000 in an IARB field causes the CPU to process a spurious interrupt exception when an interrupt from that module is recognized.
Because the EBI manages external interrupt requests, the SIM IARB value is used for arbitration between internal and external interrupt requests. The reset value of IARB for the SIM is %1111, and the reset IARB value for all other modules is %0000. Initialization software must assign different IARB values in order to implement an arbitration scheme.
Each module must have a unique IARB value. When two or more IARB fields have the same nonzero value, the CPU interprets multiple vector numbers simultaneously, with unpredictable consequences.
Arbitration must always take place, even when a single source requests service. This point is important for two reasons: the CPU interrupt acknowledge cycle is not driven on the external bus unless the SIM wins contention, and failure to contend causes an interrupt acknowledge bus cycle to be terminated by a bus error, which causes a spurious interrupt exception to be taken.
When arbitration is complete, the dominant module must place an interrupt vector number on the data bus and terminate the bus cycle. In the case of an external interrupt request, because the interrupt acknowledge cycle is transferred to the external bus, an external device must decode the mask value and respond with a vector number, then generate bus cycle termination signals. If the device does not respond in time, a spurious interrupt exception is taken.
The periodic interrupt timer (PIT) in the SIM can generate internal interrupt requests of specific priority at predetermined intervals. By hardware convention, PIT interrupts are serviced before external interrupt service requests of the same priority. Refer to 3.2.7 Periodic Interrupt Timer for more information.
3.8.2 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.
A. The CPU finishes higher priority exception processing or reaches an instruction boundary.
B. Processor state is stacked. The contents of the status register and program counter are saved.
C. The interrupt acknowledge cycle begins:
1.
FC[2:0] are driven to %111 (CPU space) encoding.
2.
The address bus is driven as follows. ADDR[23:20] = %1111; ADDR[19:16] = %1111, which indicates that the cycle is an interrupt acknowledge CPU space cycle; ADDR[15:4]
= %111111111111; ADDR[3:1] = the level of the interrupt request being acknowledged; and ADDR0 = %1.
3.
Request priority level is latched into the IP field in the status register from the address bus.
D. Modules or external peripherals that have requested interrupt service decode the request level in ADDR[3:1]. If the request level of at least one interrupting module or device is the same as the value in ADDR[3:1], interrupt arbitration contention takes place. When there is no contention, the spurious interrupt monitor asserts BERR, and a spurious interrupt exception is processed.
E. After arbitration, the interrupt acknowledge cycle can be completed in one of three ways:
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