PIC18F442T-E/ML【PDF 79/134 页】IC MCU FLASH 8KX16 EE A/D 44QFN

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Sheet目录3882

75页 76页 77页 78页 [79页]80页 81页 82页 83页

1997 Microchip Technology Inc.

DS30444E - page 49

PIC16C9XX

7.3.1

SWITCHING PRESCALER ASSIGNMENT

The prescaler assignment is fully under software con-

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

execution.

Note:

To avoid an unintended device RESET, the

following instruction sequence (shown in

Example 7-1) must be executed when

changing the prescaler assignment from

Timer0 to the WDT. This precaution must

be followed even if the WDT is disabled.

EXAMPLE 7-1:

CHANGING PRESCALER (TIMER0

→WDT)

To change prescaler from the WDT to the Timer0 mod-

ule use the precaution shown in Example 7-2.

EXAMPLE 7-2:

CHANGING PRESCALER (WDT

→TIMER0)

CLRWDT

;Clear WDT and prescaler

BSF

STATUS, RP0 ;Select Bank1

MOVLW

b'xxxx0xxx' ;Select TMR0, new prescale value and

MOVWF

OPTION_REG

;clock source

BCF

STATUS, RP0 ;Select Bank0

TABLE 7-1: REGISTERS ASSOCIATED WITH TIMER0

BSF

STATUS, RP0

;Select Bank1

Lines 2 and 3 do NOT have to

be included if the nal desired

prescale value is other than 1:1.

If 1:1 is nal desired value, then

a temporary prescale value is

set in lines 2 and 3 and the nal

prescale value will be set in lines

10 and 11.

MOVLW

b'xx0x0xxx'

;Select clock source and prescale value of

MOVWF

OPTION_REG

;other than 1:1

BCF

STATUS, RP0

;Select Bank0

CLRF

TMR0

;Clear TMR0 and prescaler

BSF

STATUS, RP1

;Select Bank1

MOVLW

b'xxxx1xxx'

;Select WDT, do not change prescale value

MOVWF

OPTION_REG

;

CLRWDT

;Clears WDT and prescaler

10) MOVLW

b'xxxx1xxx'

;Select new prescale value and WDT

11) MOVWF

OPTION_REG

;

12) BCF

STATUS, RP0

;Select Bank0

Address

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Value on

Power-on

Reset

Value on all

other resets

01h, 101h

TMR0

Timer0 module’s register

xxxx xxxx

uuuu uuuu

0Bh, 8Bh,

10Bh, 18Bh

INTCON

GIE

PEIE

T0IE

INTE

RBIE

T0IF

INTF

RBIF

0000 000x

0000 000u

81h, 181h

OPTION RBPU INTEDG

T0CS

T0SE

PSA

PS2

PS1

PS0

1111 1111

85h

TRISA

—

PORTA Data Direction Control Register

--11 1111

Legend: x = unknown, u = unchanged, - = unimplemented locations read as '0'. Shaded cells are not used by Timer0.

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