PIC12F508-I/SL 查看數據表（PDF） - Microchip Technology

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PIC12F508-I/SL

8/14-Pin, 8-Bit Flash Microcontrollers

Microchip Technology 

PIC12F508/509/16F505

6.2 Prescaler

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

Timer0 module or as a postscaler for the Watchdog

Timer (WDT), respectively (see Section 7.6 “Watch-

dog Timer (WDT)”). For simplicity, this counter is

being referred to as “prescaler” throughout this data

sheet.

Note:

The prescaler may be used by either the

Timer0 module or the WDT, but not both.

Thus, a prescaler assignment for the

Timer0 module means that there is no

prescaler for the WDT and vice versa.

The PSA and PS<2:0> bits (OPTION<3:0>) determine

prescaler assignment and prescale ratio.

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 WDT. The prescaler is neither readable

nor writable. On a Reset, the prescaler contains all ‘0’s.

6.2.1

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 6-1) must be

executed when changing the prescaler assignment

from Timer0 to the WDT.

EXAMPLE 6-1:

CHANGING PRESCALER

(TIMER0 → WDT)

CLRWDT

;Clear WDT

CLRF TMR0

;Clear TMR0 & Prescaler

MOVLW ‘00xx1111’b ;These 3 lines (5, 6, 7)

OPTION

;are required only if

;desired

CLRWDT

;PS<2:0> are 000 or 001

MOVLW ‘00xx1xxx’b ;Set Postscaler to

OPTION

;desired WDT rate

To change the prescaler from the WDT to the Timer0

module, use the sequence shown in Example 6-2. This

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

CLRWDT instruction should be executed before

switching the prescaler.

EXAMPLE 6-2:

CHANGING PRESCALER

(WDT → TIMER0)

CLRWDT

MOVLW ‘xxxx0xxx’

OPTION

;Clear WDT and

;prescaler

;Select TMR0, new

;prescale value and

;clock source

DS41236E-page 38

© 2009 Microchip Technology Inc.

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