12C508 datasheet(28/113 Pages) MICROCHIP

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PIC12C5XX

DS40139E-page 28

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”

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

PS2:PS0

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 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)

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

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)

CLRWDT

;Clear WDT and

;prescaler

MOVLW

'xxxx0xxx'

;Select TMR0, new

;prescale value and

;clock source

OPTION

FIGURE 6-5:

BLOCK DIAGRAM OF THE TIMER0/WDT PRESCALER

TCY ( = Fosc/4)

Sync

Cycles

TMR0 reg

8-bit Prescaler

8 - to - 1MUX

MUX

Watchdog

Timer

PSA

WDT

Time-Out

PS2:PS0

Note: T0CS, T0SE, PSA, PS2:PS0 are bits in the OPTION register.

PSA

WDT Enable bit

Data Bus

PSA

T0CS

T0SE

GP2/T0CKI

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