PIC12F635ESLASHST datasheet(57/196 Pages) MICROCHIP

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Preliminary

DS41232B-page 55

PIC12F635/PIC16F636/639

5.4

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 PS<2:0> 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.

5.4.1

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 5-1

and

Example 5-2) must be executed when changing the

prescaler assignment from Timer0 to WDT.

EXAMPLE 5-1:

CHANGING PRESCALER

(TIMER0

→ WDT)

To change prescaler from the WDT to the TMR0

module, use the sequence shown in Example 5-2. This

precaution must be taken even if the WDT is disabled.

EXAMPLE 5-2:

CHANGING PRESCALER

(WDT

→ TIMER0)

TABLE 5-1:

REGISTERS ASSOCIATED WITH TIMER0

BCF

STATUS,RP0

;Bank 0

BCF

STATUS,RP1

;

CLRWDT

;Clear WDT

CLRF

TMR0

;Clear TMR0 and

; prescaler

BSF

STATUS,RP0

;Bank 1

BCF

STATUS,RP1

;

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

BCF

STATUS,RP1

;

CLRWDT

;Clear WDT and

;prescaler

BSF

STATUS,RP0

;Bank 1

BCF

STATUS,RP1

;

MOVLW

b’xxxx0xxx’

;Select TMR0,

;prescale, and

;clock source

MOVWF

OPTION_REG

;

BCF

STATUS,RP0

;Bank 0

BCF

STATUS,RP1

;

Address

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Value on

POR, BOD,

WUR

Value on

all other

Resets

01h

TMR0

Timer0 Module Register

xxxx xxxx uuuu uuuu

0Bh/8Bh INTCON

GIE

PEIE

T0IE

INTE

RAIE

T0IF

INTF

RAIF

0000 0000 0000 0000

81h

OPTION_REG

RAPU

INTEDG

T0CS

T0SE

PSA

PS2

PS1

PS0

1111 1111 1111 1111

85h

TRISA

—

TRISA5 TRISA4 TRISA3 TRISA2 TRISA1 TRISA0 --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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