COP8CCR9IMT7SLASHNOPB datasheet(78/111 Pages) TI1

78 / 111 page

COP8CBR9, COP8CCR9, COP8CDR9

SNOS535I – OCTOBER 2000 – REVISED MARCH 2013

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Figure 5-24. VIS Operation

5.15.4 NON-MASKABLE INTERRUPT

5.15.4.1 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 associated 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.

5.15.4.2 Software Trap

The Software Trap is a special kind of non-maskable interrupt which occurs when the INTR instruction

(used to acknowledge 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 location beyond the programmed Flash memory

space, the unused 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 Register resides at this location, and to maintain the integrity 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.

Functional Description

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