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PIC16C5X

DS30453D-page 40

Preliminary

2002 Microchip Technology Inc.

8.2

Prescaler

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

Timer0 module, or as a postscaler for the Watchdog

Timer (WDT), respectively (Section 9.2.1). For simplic-

ity, this counter is being referred to as “prescaler”

throughout this data sheet. Note that 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

the

TMR0

(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 nei-

ther readable nor writable. On a RESET, the prescaler

contains all '0's.

8.2.1

SWITCHING PRESCALER

ASSIGNMENT

The prescaler assignment is fully under software con-

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

execution). To avoid an unintended device RESET, the

following instruction sequence (Example 8-1) must be

executed when changing the prescaler assignment

from Timer0 to the WDT.

EXAMPLE 8-1:

CHANGING PRESCALER

(TIMER0

→WDT)

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

To change prescaler from the WDT to the Timer0 mod-

ule, use the sequence shown in Example 8-2. This

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

CLRWDT

instruction should be executed before switch-

ing the prescaler.

EXAMPLE 8-2:

CHANGING PRESCALER

(WDT

→TIMER0)

CLRWDT

;Clear WDT and

;prescaler

MOVLW

B'xxxx0xxx'

;Select TMR0, new

;prescale value and

;clock source

OPTION

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