PIC18LF4331-ESLASHML datasheet(129/392 Pages) MICROCHIP

129 / 392 page

 2010 Microchip Technology Inc.

DS39616D-page 129

PIC18F2331/2431/4331/4431

12.1

Timer0 Operation

Timer0 can operate as a timer or as a counter.

Timer mode is selected by clearing the T0CS bit. In

Timer mode, the Timer0 module will increment every

instruction cycle (without prescaler). If the TMR0 regis-

ter is written, the increment is inhibited for the following

two instruction cycles. The user can work around this

by writing an adjusted value to the TMR0 register.

Counter mode is selected by setting the T0CS bit. In

Counter mode, Timer0 will increment, either on every

rising or falling edge of pin, RC3/T0CKI/T5CKI/INT0.

The incrementing edge is determined by the Timer0

Source Edge Select bit (T0SE). Clearing the T0SE bit

selects the rising edge.

When an external clock input is used for Timer0, it must

meet certain requirements. The requirements ensure

the external clock can be synchronized with the internal

phase clock (TOSC). Also, there is a delay in the actual

incrementing of Timer0 after synchronization.

12.2

Prescaler

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

module. The prescaler is not readable or writable.

The PSA and T0PS<2:0> bits determine the prescaler

assignment and prescale ratio.

Clearing bit, PSA, will assign the prescaler to the

Timer0 module. When the prescaler is assigned to the

Timer0 module, prescale values of 1:2, 1:4, ..., 1:256

are selectable.

When assigned to the Timer0 module, all instructions

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

TMR0, BSF TMR0, x

..., etc.) will clear the prescaler

count.

12.2.1

SWITCHING PRESCALER

ASSIGNMENT

The prescaler assignment is fully under software con-

trol (i.e., it can be changed “on-the-fly” during program

execution).

12.3

Timer0 Interrupt

The TMR0 interrupt is generated when the TMR0

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

TMR0IF bit. The interrupt can be masked by clearing

the TMR0IE bit. The TMR0IF bit must be cleared in

software by the Timer0 module Interrupt Service

Routine before re-enabling this interrupt. The TMR0

interrupt cannot awaken the processor from Sleep

mode, since the timer requires clock cycles, even when

T0CS is set.

12.4

16-Bit Mode Timer Reads and

Writes

TMR0H is not the high byte of the timer/counter in

16-bit mode, but is actually a buffered version of the

high byte of Timer0 (refer to Figure 12-2). The high byte

of the Timer0 counter/timer is not directly readable nor

writable. TMR0H is updated with the contents of the

high byte of Timer0 during a read of TMR0L. This pro-

vides the ability to read all 16 bits of Timer0 without

having to verify that the read of the high and low byte

were valid due to a rollover between successive reads

of the high and low byte.

A write to the high byte of Timer0 must also take place

through the TMR0H Buffer register. Timer0 high byte is

updated with the contents of TMR0H when a write

occurs to TMR0L. This allows all 16 bits of Timer0 to be

updated at once.

TABLE 12-1:

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

T016BIT

T0CS

T0SE

PSA

T0PS2

T0PS1

T0PS0

TRISA

TRISA7(1)

TRISA6(1)

PORTA Data Direction Register

Legend:

Shaded cells are not used by Timer0.

Note 1:

RA6 and RA7 are enabled as I/O pins depending on the oscillator mode selected in Configuration Word 1H.

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