PIC18F242 datasheet(107/332 Pages) MICROCHIP

107 / 332 page

DS39564C-page 105

PIC18FXX2

10.1

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 TMR0L reg-

ister is written, the increment is inhibited for the follow-

ing two instruction cycles. The user can work around

this by writing an adjusted value to the TMR0L 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 RA4/T0CKI. The increment-

ing edge is determined by the Timer0 Source Edge

Select bit (T0SE). Clearing the T0SE bit selects the ris-

ing edge. Restrictions on the external clock input are

discussed below.

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.

10.2

Prescaler

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

module. The prescaler is not readable or writable.

The PSA and T0PS2:T0PS0 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 TMR0L register (e.g., CLRF

TMR0,

MOVWF

TMR0,

BSF

TMR0,

....etc.) will clear the

prescaler count.

10.2.1

SWITCHING PRESCALER ASSIGNMENT

The prescaler assignment is fully under software con-

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

execution).

10.3

Timer0 Interrupt

The TMR0 interrupt is generated when the TMR0 reg-

ister overflows from FFh to 00h in 8-bit mode, or FFFFh

to 0000h in 16-bit mode. This overflow sets the TMR0IF

bit. The interrupt can be masked by clearing the

TMR0IE bit. The TMR0IE bit must be cleared in soft-

ware by the Timer0 module Interrupt Service Routine

before re-enabling this interrupt. The TMR0 interrupt

cannot awaken the processor from SLEEP, since the

timer is shut-off during SLEEP.

10.4

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 10-2). 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 10-1:

REGISTERS ASSOCIATED WITH TIMER0

Note:

Writing to TMR0L 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

Value on

POR, BOR

Value on

All Other

RESETS

TMR0L

Timer0 Module Low Byte Register

xxxx xxxx

uuuu uuuu

TMR0H

Timer0 Module High Byte Register

0000 0000

INTCON

GIE/GIEH

PEIE/GIEL

TMR0IE

INT0IE

RBIE

TMR0IF

INT0IF

RBIF

0000 000x

0000 000u

T0CON

TMR0ON

T08BIT

T0CS

T0SE

PSA

T0PS2

T0PS1

T0PS0

1111 1111

TRISA

—

PORTA Data Direction Register

-111 1111

Legend: x = unknown, u = unchanged, - = unimplemented locations read as '0'. Shaded cells are not used by Timer0.

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