RFPIC12C509AF datasheet(32/104 Pages) MICROCHIP

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rfPIC12C509AG/509AF

DS70031A-page 30

Preliminary

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 (Section 8.6). For simplicity,

this counter is being referred to as “prescaler” through-

out 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 PS2:PS0 bits (OPTION<3:0>) determine

prescaler assignment and prescale ratio.

When assigned to the Timer0 module, all instructions

writingtothe

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.

6.2.1

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 6-1) must be

executed when changing the prescaler assignment

from Timer0 to the WDT.

EXAMPLE 6-1:

Changing Prescaler

(Timer0

→WDT)

1.CLRWDT

;Clear WDT

2.CLRF

TMR0

;Clear TMR0 & Prescaler

3.MOVLW

'00xx1111’b ;These 3 lines (5, 6, 7)

4.OPTION

; are required only if

; desired

5.CLRWDT

;PS<2:0> are 000 or 001

6.MOVLW

'00xx1xxx’b ;Set Postscaler to

7.OPTION

; desired WDT rate

To change prescaler from the WDT to the Timer0 mod-

ule, 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 switch-

ing the prescaler.

EXAMPLE 6-2:

Changing Prescaler

(WDT

→Timer0)

CLRWDT

;Clear WDT and

;prescaler

MOVLW

'xxxx0xxx'

;Select TMR0, new

;prescale value and

;clock source

OPTION

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