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PIC16C717/770/771

DS41120B-page 128

2002 Microchip Technology Inc.

12.10.1

INT INTERRUPT

External interrupt on RB0/INT pin is edge triggered:

either rising if bit INTEDG (OPTION_REG<6>) is set,

or falling, if the INTEDG bit is clear. When a valid edge

appears

the

RB0/INT

pin,

flag

bit

INTF

(INTCON<1>) is set. This interrupt can be disabled by

clearing enable bit INTE (INTCON<4>). Flag bit INTF

must be cleared in software in the interrupt service rou-

tine before re-enabling this interrupt. The INT interrupt

can wake-up the processor from SLEEP, if bit INTE was

set prior to going into SLEEP. The status of global inter-

rupt enable bit GIE decides whether or not the proces-

sor branches to the interrupt vector following wake-up.

See Section 12.13 for details on SLEEP mode.

12.10.2

TMR0 INTERRUPT

An overflow (FFh

→ 00h) in the TMR0 register will set

flag bit T0IF (INTCON<2>). The interrupt can be

enabled/disabled by setting/clearing enable bit T0IE

(INTCON<5>). (Section 2.2.2.3)

12.10.3

PORTB INTCON CHANGE

An input change on PORTB<7:0> sets flag bit RBIF

(INTCON<0>). The PORTB pin(s) which can individu-

ally generate interrupt is selectable in the IOCB regis-

ter. The interrupt can be enabled/disabled by setting/

clearing

enable

bit

RBIE

(INTCON<4>).

(Section 2.2.2.3)

12.11 Context Saving During Interrupts

During an interrupt, only the PC is saved on the stack.

At the very least, W and STATUS should be saved to

preserve the context for the interrupted program. All

registers that may be corrupted by the ISR, such as

PCLATH or FSR, should be saved.

Example 12-1 stores and restores the STATUS, W and

PCLATH registers. The register, W_TEMP, is defined in

Common RAM, the last 16 bytes of each bank that may

be accessed from any bank. The STATUS_TEMP and

PCLATH_TEMP are defined in bank 0.

The example:

Stores the W register.

Stores the STATUS register in bank 0.

Stores the PCLATH register in bank 0.

Executes the ISR code.

Restores the PCLATH register.

Restores the STATUS register

Restores W.

Note

that

W_TEMP,

STATUS_TEMP

and

PCLATH_TEMP are defined in the common RAM area

(70h - 7Fh) to avoid register bank switching during con-

text save and restore.

EXAMPLE 12-1:

Saving STATUS, W, and PCLATH Registers in RAM

#define

W_TEMP

0x70

#define

STATUS_TEMP

0x71

#define

PCLATH_TEMP

0x72

org

0x04

; start at Interrupt Vector

MOVWF

W_TEMP

; Save W register

MOVF

STATUS,w

MOVWF

STATUS_TEMP

; save STATUS

MOVF

PCLATH,w

MOVWF

PCLATH_TEMP

; save PCLATH

(Interrupt Service Routine)

MOVF

PCLATH_TEMP,w

MOVWF

PCLATH

MOVF

STATUS_TEMP,w

MOVWF

STATUS

SWAPF

W_TEMP,f

;

SWAPF

W_TEMP,w

; swapf loads W without affecting STATUS flags

RETFIE

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