DM163006 datasheet(89/320 Pages) MICROCHIP

89 / 320 page

 2001 Microchip Technology Inc.

Advance Information

DS39541A-page 89

PIC18C601/801

8.0

INTERRUPTS

PIC18C601/801 devices have 15 interrupt sources and

an interrupt priority feature that allows each interrupt

source to be assigned a high priority level, or a low pri-

ority level. The high priority interrupt vector is at

000008h and the low priority interrupt vector is at

000018h. High priority interrupt events will override any

low priority interrupts that may be in progress.

There are 10 registers that are used to control interrupt

operation. These registers are:

• RCON

• INTCON

• INTCON2

• INTCON3

• PIR1, PIR2

• PIE1, PIE2

• IPR1, IPR2

It is recommended that the Microchip header files sup-

plied with MPLAB® IDE be used for the symbolic bit

names in these registers. This allows the assembler/

compiler to automatically take care of the placement of

these bits within the specified register.

Each interrupt source has three bits to control its oper-

ation. The functions of these bits are:

• Flag bit to indicate that an interrupt event

occurred

• Enable bit that allows program execution to

branch to the interrupt vector address when the

flag bit is set

• Priority bit to select high priority or low priority

The interrupt priority feature is enabled by setting the

IPEN bit (RCON register). When interrupt priority is

enabled, there are two bits that enable interrupts glo-

bally. Setting the GIEH bit (INTCON register) enables

all interrupts that have the priority bit set. Setting the

GIEL bit (INTCON register) enables all interrupts that

have the priority bit cleared. When the interrupt flag,

enable bit and appropriate global interrupt enable bit

are set, the interrupt will vector immediately to address

000008h or 000018h, depending on the priority level.

Individual interrupts can be disabled through their cor-

responding enable bits.

When the IPEN bit is cleared (default state), the inter-

rupt priority feature is disabled and interrupts are

compatible with PICmicro® mid-range devices. In Com-

patibility mode, the interrupt priority bits for each source

have no effect. The PEIE bit (INTCON register)

enables/disables all peripheral interrupt sources. The

GIE bit (INTCON register) enables/disables all interrupt

sources. All interrupts branch to address 000008h in

Compatibility mode.

When an interrupt is responded to, the Global Interrupt

Enable bit is cleared to disable further interrupts. If the

IPEN bit is cleared, this is the GIE bit. If interrupt prior-

ity levels are used, this will be either the GIEH or GIEL

bit. High priority interrupt sources can interrupt a low

priority interrupt.

The return address is pushed onto the stack and the

PC is loaded with the interrupt vector address

(000008h or 000018h). Once in the Interrupt Service

Routine, the source(s) of the interrupt can be deter-

mined by polling the interrupt flag bits. The interrupt

flag bits must be cleared in software before re-enabling

interrupts, to avoid recursive interrupts.

The "return from interrupt" instruction, RETFIE, exits

the interrupt routine and sets the GIE bit (GIEH or GIEL

if priority levels are used), which re-enables interrupts.

For external interrupt events, such as the INT pins or

the PORTB input change interrupt, the interrupt latency

will be three to four instruction cycles. The exact

latency is the same for one or two cycle instructions.

Individual interrupt flag bits are set, regardless of the

status of their corresponding enable bit or the GIE bit.

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