ample, if the Software Trap routine is located at 0310 Hex,

then the vector location 0yFE and -0yFF should contain the

data 03 and 10 Hex, respectively. When a Software Trap in-

terrupt occurs and the VIS instruction is executed, the pro-

gram jumps to the address specified in the vector table.

The interrupt sources in the vector table are listed in order of

rank, from highest to lowest priority. If two or more enabled

and pending interrupts are detected at the same time, the

one with the highest priority is serviced first. Upon return

from the interrupt service routine, the next highest-level

pending interrupt is serviced.

If the VIS instruction is executed, but no interrupts are en-

abled and pending, the lowest-priority interrupt vector is

used, and a jump is made to the corresponding address in

the vector table. This is an unusual occurrence, and may be

the result of an error. It can legitimately result from a change

in the enable bits or pending flags prior to the execution of

the VIS instruction, such as executing a single cycle instruc-

tion which clears an enable flag at the same time that the

pending flag is set. It can also result, however, from inadvert-

ent execution of the VIS command outside of the context of

an interrupt.

The default VIS interrupt vector can be useful for applica-

tions in which time critical interrupts can occur during the

servicing of another interrupt. Rather than restoring the pro-

gram context (A, B, X, etc.) and executing the RETI instruc-

tion, an interrupt service routine can be terminated by return-

ing to the VIS instruction. In this case, interrupts will be

serviced in turn until no further interrupts are pending and

the default VIS routine is started. After testing the GIE bit to

ensure that execution is not erroneous, the routine should

restore the program context and execute the RETI to return

to the interrupted program.

This technique can save up to fifty instruction cycles (t

more, (50µs at 10 MHz oscillator) of latency for pending in-

terrupts with a penalty of fewer than ten instruction cycles if

no further interrupts are pending.

To ensure reliable operation, the user should always use the

VIS instruction to determine the source of an interrupt. Al-

though it is possible to poll the pending bits to detect the

source of an interrupt, this practice is not recommended. The

use of polling allows the standard arbitration ranking to be al-

tered, but the reliability of the interrupt system is compro-

mised. The polling routine must individually test the enable

and pending bits of each maskable interrupt. If a Software

Trap interrupt should occur, it will be serviced last, even

though it should have the highest priority. Under certain con-

ditions, a Software Trap could be triggered but not serviced,

resulting in an inadvertent “locking out” of all maskable inter-

rupts by the Software Trap pending flag. Problems such as

this can be avoided by using VIS instruction.

TABLE 4. Interrupt Vector Table

Arbitration

Ranking

Source

Description

Vector Address (Note 26)

(Hi-Low Byte)

(1) Highest

Software

INTR Instruction

0yFE–0yFF

(2)

Reserved

0yFC–0yFD

(3)

External

G0

0yFA–0yFB

(4)

Timer T0

Underflow

0yF8–0yF9

(5)

Timer T1

T1A/Underflow

0yF6–0yF7

(6)

Timer T1

T1B

0yF4–0yF5

(7)

MICROWIRE/PLUS

BUSY Low

0yF2–0yF3

(8)

EERAM

EERAM Write Complete

0yF0–0yF1

(9)

Reserved

0yEE–0yEF

(10)

Reserved

0yEC–0yED

(11)

Reserved

0yEA–0yEB

(12)

Reserved

0yE8–0yE9

(13)

Reserved

0yE6–0yE7

(14)

Reserved

0yE4–0yE5

(15)

Port L/Wakeup

Port L Edge

0yE2–0yE3

(16) Lowest

Default VIS

Reserved

0yE0–0yE1

Note 26: y is a variable which represents the VIS block. VIS and the vector table must be located in the same 256-byte block except if VIS is located at the last ad-

dress of a block. In this case, the table must be in the next block.

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