16F54 datasheet(38/90 Pages) MICROCHIP

38 / 90 page

PIC16F5X

DS41213C-page 36

 2004 Microchip Technology Inc.

7.2.1

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 7-1) must be

executed when changing the prescaler assignment

from Timer0 to the WDT.

EXAMPLE 7-1:

CHANGING PRESCALER

(TIMER0

→WDT)

To change prescaler from the WDT to the Timer0

module, use the sequence shown in Example 7-2. This

sequence must be used even if the WDT is disabled. A

CLRWDT

instruction

should

executed

before

switching the prescaler.

EXAMPLE 7-2:

CHANGING PRESCALER

(WDT

→TIMER0)

FIGURE 7-5:

BLOCK DIAGRAM OF THE TIMER0/WDT PRESCALER

CLRWDT

;Clear WDT

CLRF

TMR0

;Clear TMR0 &

;Prescaler

MOVLW

B'00xx1111’

;Last 3 instructions

;in this example

OPTION

;are required only if

;desired

CLRWDT

;PS<2:0> are 000 or 001

MOVLW

B'00xx1xxx’

;Set Prescaler to

OPTION

;desired WDT rate

CLRWDT

;Clear WDT and

;prescaler

MOVLW

B'xxxx0xxx'

;Select TMR0, new

;prescale value and

;clock source

OPTION

T0CKI

T0SE(1)

TCY ( = FOSC/4)

Sync

Cycles

TMR0 reg

8-bit Prescaler

8 - to - 1MUX

MUX

Watchdog

Timer

PSA(1)

WDT

Time-Out

PS<2:0>(1)

PSA(1)

WDT Enable bit

Data Bus

PSA(1)

T0CS(1)

Note

T0CS, T0SE, PSA PS<2:0> are bits in the Option register.

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