Datasheet

 1996-2013 Microchip Technology Inc. Preliminary DS40143E-page 51
PIC16C55X
7.3.1 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 7-1) 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 7-1: CHANGING PRESCALER
(TIMER0WDT)
To change prescaler from the WDT to the TMR0
module use the sequence shown in Example 7-2. This
precaution must be taken even if the WDT is disabled.
EXAMPLE 7-2: CHANGING PRESCALER
(WDTTIMER0)
TABLE 7-1: REGISTERS ASSOCIATED WITH TIMER0
BCF STATUS, RP0 ;Skip if already in
;Bank 0 CLRWDT Clear WDT
CLRF TMR0 ;Clear TMR0 & Prescaler
BSF STATUS, RP0 ;Bank 1
MOVLW '00101111’b ;These 3 lines (5, 6, 7)
MOVWF OPTION ;Are required only if
;Desired PS<2:0> are
;CLRWDT 000 or 001
MOVLW '00101xxx’b ;Set Postscaler to
MOVWF OPTION ;Desired WDT rate
BCF STATUS, RP0 ;Return to Bank 0
CLRWDT ;Clear WDT and 
;prescaler
BSF STATUS, RP0
MOVLW b'xxxx0xxx' ;Select TMR0, new 
;prescale value and
;clock source
MOVWF OPTION
BCF STATUS, RP0
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 xxxx xxxx uuuu uuuu
0Bh/8Bh INTCON GIE
Reserved T0IE INTE RBIE T0IF INTF RBIF 0000 000x 0000 000x
81h OPTION
RBPU INTEDG T0CS T0SE PSA PS2 PS1 PS0 1111 1111 1111 1111
85h TRISA
— — — TRISA4 TRISA3 TRISA2 TRISA1 TRISA0 ---1 1111 ---1 1111
Legend: — = Unimplemented locations, read as ‘0’,
Note 1: Shaded bits are not used by TMR0 module.