PIC16LC554 datasheet(33/96 Pages) MICROCHIP

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Preliminary

DS40143B-page 33

PIC16C55X(A)

6.3.1

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 WDT. Lines 5-7

are required only if the desired postscaler rate is 1:1

(PS<2:0> = 000) or 1:2 (PS<2:0> = 001).

EXAMPLE 6-1:

CHANGING PRESCALER

(TIMER0

→WDT)

1.BCF

STATUS, RP0

;Skip if already in

; Bank 0

2.CLRWDT

;Clear WDT

3.CLRF

TMR0

;Clear TMR0 & Prescaler

4.BSF

STATUS, RP0

;Bank 1

5.MOVLW

'00101111’b; ;These 3 lines (5, 6, 7)

6.MOVWF

OPTION

; are required only if

; desired PS<2:0> are

7.CLRWDT

; 000 or 001

8.MOVLW

'00101xxx’b

;Set Postscaler to

9.MOVWF

OPTION

; desired WDT rate

10.BCF

STATUS, RP0

;Return to Bank 0

To change prescaler from the WDT to the TMR0

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

precaution must be taken even if the WDT is disabled.

EXAMPLE 6-2:

CHANGING PRESCALER

(WDT

→TIMER0)

CLRWDT

;Clear WDT and

;prescaler

BSF

STATUS, RP0

MOVLW

b'xxxx0xxx'

;Select TMR0, new

;prescale value and

;clock source

MOVWF

OPTION

BCF

STATUS, RP0

TABLE 6-1:

REGISTERS ASSOCIATED WITH TIMER0

Address Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Value on

POR

Value on

All Other Resets

01h

TMR0

Timer0 module’s register

uuuu uuuu

xxxx xxxx

0Bh/8Bh

INTCON

GIE

T0IE

INTE

RBIE

T0IF

INTF

RBIF

0000 000x

81h

OPTION

RBPU

INTEDG

T0CS

T0SE

PSA

PS2

PS1

PS0

1111 1111

85h

TRISA

—

TRISA4 TRISA3 TRISA2 TRISA1 TRISA0

---1 1111

Legend: — = Unimplemented locations, read as ‘0’.

+ = Reserved for future use.

Note:

Shaded bits are not used by TMR0 module.

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