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ST90158 - INTERRUPTS

4 INTERRUPTS

4.1 INTRODUCTION

The ST9 responds to peripheral and external

events through its interrupt channels. Current pro-

gram execution can be suspended to allow the

ST9 to execute a specific response routine when

such an event occurs, providing that interrupts

have been enabled, and according to a priority

mechanism. If an event generates a valid interrupt

request, the current program status is saved and

control passes to the appropriate Interrupt Service

Routine.

The ST9 CPU can receive requests from the fol-

lowing sources:

– On-chip peripherals

– External pins

– Top-Level Pseudo-non-maskable interrupt

According to the on-chip peripheral features, an

event occurrence can generate an Interrupt re-

quest which depends on the selected mode.

Up to eight external interrupt channels, with pro-

grammable input trigger edge, are available. In ad-

dition, a dedicated interrupt channel, set to the

Top-level priority, can be devoted either to the ex-

ternal NMI pin (where available) to provide a Non-

Maskable Interrupt, or to the Timer/Watchdog. In-

terrupt service routines are addressed through a

vector table mapped in Memory.

Figure 19. Interrupt Response

4.2 INTERRUPT VECTORING

The ST9 implements an interrupt vectoring struc-

ture which allows the on-chip peripheral to identify

the location of the first instruction of the Interrupt

Service Routine automatically.

When an interrupt request is acknowledged, the

peripheral interrupt module provides, through its

Interrupt Vector Register (IVR), a vector to point

into the vector table of locations containing the

start addresses of the Interrupt Service Routines

(defined by the programmer).

Each peripheral has a specific IVR mapped within

its Register File pages.

The Interrupt Vector table, containing the address-

es of the Interrupt Service Routines, is located in

the first 256 locations of Memory pointed to by the

ISR register, thus allowing 8-bit vector addressing.

For a description of the ISR register refer to the

chapter describing the MMU.

The user Power on Reset vector is stored in the

first two physical bytes in memory, 000000h and

000001h.

The Top Level Interrupt vector is located at ad-

dresses 0004h and 0005h in the segment pointed

to by the Interrupt Segment Register (ISR).

With one Interrupt Vector register, it is possible to

address several interrupt service routines; in fact,

peripherals can share the same interrupt vector

most significant bits of the vector are user pro-

grammable to define the base vector address with-

in the vector table, the least significant bits are

controlled by the interrupt module, in hardware, to

select the appropriate vector.

Note: The first 256 locations of the memory seg-

ment pointed to by ISR can contain program code.

4.2.1 Divide by Zero trap

The Divide by Zero trap vector is located at ad-

dresses 0002h and 0003h of each code segment;

it should be noted that for each code segment a

Divide by Zero service routine is required.

Warning. Although the Divide by Zero Trap oper-

ates as an interrupt, the FLAG Register is not

pushed onto the system Stack automatically. As a

result it must be regarded as a subroutine, and the

service routine must end with the RET instruction

(not IRET ).

NORMAL

PROGRAM

FLOW

INTERRUPT

SERVICE

ROUTINE

IRET

INSTRUCTION

INTERRUPT

VR001833

CLEAR

PENDING BIT

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