12F629 datasheet(31/136 Pages) MICROCHIP

31 / 136 page

 2010 Microchip Technology Inc.

DS41190G-page 31

PIC12F629/675

4.4

Prescaler

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

Timer0 module, or as a postscaler for the Watchdog

Timer. For simplicity, this counter will be referred to as

“prescaler” throughout this Data Sheet. The prescaler

assignment is controlled in software by the control bit

PSA (OPTION_REG<3>). Clearing the PSA bit will

assign the prescaler to Timer0. Prescale values are

selectable via the PS2:PS0 bits (OPTION_REG<2:0>).

The prescaler is not readable or writable. When

assigned to the Timer0 module, all instructions writing

to the TMR0 register (e.g., CLRF 1,

MOVWF 1,

BSF 1,

x....etc.

) will clear the prescaler. When

assigned to WDT, a CLRWDT instruction will clear the

prescaler along with the Watchdog Timer.

4.4.1

SWITCHING PRESCALER

ASSIGNMENT

The prescaler assignment is fully under software

control (i.e., it can be changed “on the fly” during

program execution). To avoid an unintended device

Reset, the following instruction sequence (Example 4-

1) must be executed when changing the prescaler

assignment from Timer0 to WDT.

EXAMPLE 4-1:

CHANGING PRESCALER

(TIMER0

WDT)

To change prescaler from the WDT to the TMR0

module, use the sequence shown in Example 4-2. This

precaution must be taken even if the WDT is disabled.

EXAMPLE 4-2:

CHANGING PRESCALER

(WDT

TIMER0)

TABLE 4-1:

REGISTERS ASSOCIATED WITH TIMER0

BCF

STATUS,RP0

;Bank 0

CLRWDT

;Clear WDT

CLRF

TMR0

;Clear TMR0 and

; prescaler

BSF

STATUS,RP0

;Bank 1

MOVLW

b’00101111’ ;Required if desired

MOVWF

OPTION_REG

; PS2:PS0 is

CLRWDT

; 000 or 001

;

MOVLW

b’00101xxx’ ;Set postscaler to

MOVWF

OPTION_REG

; desired WDT rate

BCF

STATUS,RP0

;Bank 0

CLRWDT

;Clear WDT and

; postscaler

BSF

STATUS,RP0

;Bank 1

MOVLW

b’xxxx0xxx’ ;Select TMR0,

; prescale, and

; clock source

MOVWF

OPTION_REG

;

BCF

STATUS,RP0

;Bank 0

Address

Name

Bit 7Bit 6Bit 5Bit 4Bit 3Bit 2Bit 1Bit 0

Value on

POR, BOD

Value on

all other

Resets

01h

TMR0

Timer0 Module Register

xxxx xxxx

uuuu uuuu

0Bh/8Bh

INTCON

GIE

PEIE

T0IE

INTE

GPIE

T0IF

INTF

GPIF

0000 0000

0000 000u

81h

OPTION_REG

GPPU

INTEDG

T0CS

T0SE

PSA

PS2

PS1

PS0

1111 1111

85h

TRISIO

—

TRISIO5 TRISIO4 TRISIO3 TRISIO2 TRISIO1 TRISIO0 --11 1111

--11 1111

Legend:

— = Unimplemented locations, read as ‘

0’, u = unchanged, x = unknown.

Shaded cells are not used by the Timer0 module.

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