COP8-REF-AM datasheet(60/83 Pages) NSC

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18.0 Interrupts (Continued)

18.4 NON-MASKABLE INTERRUPT

18.4.1 Pending Flag

There is a pending flag bit associated with the non-maskable

Software Trap interrupt, called STPND. This pending flag is

not memory-mapped and cannot be accessed directly by the

software.

The pending flag is reset to zero when a device Reset

occurs. When the non-maskable interrupt occurs, the asso-

ciated pending bit is set to 1. The interrupt service routine

should contain an RPND instruction to reset the pending flag

to zero. The RPND instruction always resets the STPND

flag.

18.4.2 Software Trap

The Software Trap is a special kind of non-maskable inter-

rupt which occurs when the INTR instruction (used to ac-

knowledge interrupts) is fetched from program memory and

placed in the instruction register. This can happen in a

variety of ways, usually because of an error condition. Some

examples of causes are listed below.

If the program counter incorrectly points to a memory loca-

tion beyond the programmed Flash memory space, the un-

used memory location returns zeros which is interpreted as

the INTR instruction.

If the stack is popped beyond the allowed limit (address 06F

Hex), a 7FFF will be loaded into the PC. Since the Option

of the stack overpop protection, the Flash memory will return

a zero on an instruction fetch and a software trap will be

triggered.

A Software Trap can be triggered by a temporary hardware

condition such as a brownout or power supply glitch.

The Software Trap has the highest priority of all interrupts.

When a Software Trap occurs, the STPND bit is set. The GIE

bit is not affected and the pending bit (not accessible by the

user) is used to inhibit other interrupts and to direct the

program to the ST service routine with the VIS instruction.

Nothing can interrupt a Software Trap service routine except

for another Software Trap. The STPND can be reset only by

the RPND instruction or a chip Reset.

The Software Trap indicates an unusual or unknown error

condition. Generally, returning to normal execution at the

point where the Software Trap occurred cannot be done

reliably. Therefore, the Software Trap service routine should

re-initialize the stack pointer and perform a recovery proce-

dure that re-starts the software at some known point, similar

to a device Reset, but not necessarily performing all the

same functions as a device Reset. The routine must also

execute the RPND instruction to reset the STPND flag.

Otherwise, all other interrupts will be locked out. To the

extent possible, the interrupt routine should record or indi-

cate the context of the device so that the cause of the

Software Trap can be determined.

If the user wishes to return to normal execution from the

point at which the Software Trap was triggered, the user

must first execute RPND, followed by RETSK rather than

RETI or RET. This is because the return address stored on

the stack is the address of the INTR instruction that triggered

the interrupt. The program must skip that instruction in order

to proceed with the next one. Otherwise, an infinite loop of

Software Traps and returns will occur.

Programming a return to normal execution requires careful

consideration. If the Software Trap routine is interrupted by

another Software Trap, the RPND instruction in the service

routine for the second Software Trap will reset the STPND

flag; upon return to the first Software Trap routine, the

20006333

FIGURE 32. VIS Operation

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