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2009 Microchip Technology Inc.

DS39636D-page 117

PIC18F2X1X/4X1X

10.3

Prescaler

An 8-bit counter is available as a prescaler for the Timer0

module. The prescaler is not directly readable or writable;

its value is set by the PSA and T0PS2:T0PS0 bits

(T0CON<3:0>),

which

determine

the

prescaler

assignment and prescale ratio.

Clearing the PSA bit assigns the prescaler to the

Timer0 module. When it is assigned, prescale values

from 1:2 through 1:256 in power-of-2 increments are

selectable.

When assigned to the Timer0 module, all instructions

writing to the TMR0 register (e.g., CLRF TMR0, MOVWF

TMR0, BSF TMR0, etc.) clear the prescaler count.

10.3.1

SWITCHING PRESCALER

ASSIGNMENT

The prescaler assignment is fully under software

control and can be changed “on-the-fly” during program

execution.

10.4

Timer0 Interrupt

The TMR0 interrupt is generated when the TMR0

FFFFh to 0000h in 16-Bit mode. This overflow sets the

TMR0IF flag bit. The interrupt can be masked by clear-

ing the TMR0IE bit (INTCON<5>). Before re-enabling

the interrupt, the TMR0IF bit must be cleared in software

by the Interrupt Service Routine.

Since Timer0 is shut down in Sleep mode, the TMR0

interrupt cannot awaken the processor from Sleep.

TABLE 10-1:

REGISTERS ASSOCIATED WITH TIMER0

Note:

Writing to TMR0 when the prescaler is

assigned to Timer0 will clear the prescaler

count but will not change the prescaler

assignment.

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Reset

Values

on page

TMR0L

Timer0 Register, Low Byte

TMR0H

Timer0 Register, High Byte

INTCON

GIE/GIEH PEIE/GIEL TMR0IE

INT0IE

RBIE

TMR0IF

INT0IF

RBIF

T0CON

TMR0ON

T08BIT

T0CS

T0SE

PSA

T0PS2

T0PS1

T0PS0

TRISA

RA7(1)

RA6(1)

RA5

RA4

RA3

RA2

RA1

RA0

Legend: Shaded cells are not used by Timer0.

Note 1: PORTA<7:6> and their direction bits are individually configured as port pins based on various primary

oscillator modes. When disabled, these bits read as ‘0’.

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