PIC12F508 datasheet(38/98 Pages) MICROCHIP

38 / 98 page

PIC12F508/509/16F505

DS41236A-page 36

Preliminary

 2004 Microchip Technology Inc.

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.

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 pro-

gram 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)

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

executed

before

switching the prescaler.

EXAMPLE 6-2:

CHANGING PRESCALER

(WDT

→ TIMER0)

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.

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

CLRWDT

;Clear WDT and

;prescaler

MOVLW

‘xxxx0xxx’

;Select TMR0, new

;prescale value and

;clock source

OPTION

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