Datasheet

© 2007 Microchip Technology Inc. DS41190E-page 29
PIC12F629/675
4.4 Prescaler
An 8-bit counter is available as a prescaler for the
Timer0 module, or as a postscaler for the Watchdog
Timer. For simplicity, this counter will be referred to as
“prescaler” throughout this Data Sheet. The prescaler
assignment is controlled in software by the control bit
PSA (OPTION_REG<3>). Clearing the PSA bit will
assign the prescaler to Timer0. Prescale values are
selectable via the PS2:PS0 bits (OPTION_REG<2:0>).
The prescaler is not readable or writable. When
assigned to the Timer0 module, all instructions writing
to the TMR0 register (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 Watchdog Timer.
4.4.1 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 4-1) must be executed when changing the
prescaler assignment from Timer0 to WDT.
EXAMPLE 4-1: CHANGING PRESCALER
(TIMER0→WDT)
To change prescaler from the WDT to the TMR0
module, use the sequence shown in Example 4-2. This
precaution must be taken even if the WDT is disabled.
EXAMPLE 4-2: CHANGING PRESCALER
(WDT→TIMER0)
TABLE 4-1: REGISTERS ASSOCIATED WITH TIMER0
bcf STATUS,RP0 ;Bank 0
clrwdt ;Clear WDT
clrf TMR0 ;Clear TMR0 and
; prescaler
bsf STATUS,RP0 ;Bank 1
movlw b’00101111’ ;Required if desired
movwf OPTION_REG ; PS2:PS0 is
clrwdt ; 000 or 001
;
movlw b’00101xxx’ ;Set postscaler to
movwf OPTION_REG ; desired WDT rate
bcf STATUS,RP0 ;Bank 0
clrwdt ;Clear WDT and
; postscaler
bsf STATUS,RP0 ;Bank 1
movlw b’xxxx0xxx’ ;Select TMR0,
; prescale, and
; clock source
movwf OPTION_REG ;
bcf STATUS,RP0 ;Bank 0
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Value on
POR, BOD
Value on
all other
RESETS
01h TMR0 Timer0 Module Register xxxx xxxx uuuu uuuu
0Bh/8Bh INTCON GIE PEIE T0IE INTE GPIE T0IF INTF GPIF 0000 0000 0000 000u
81h OPTION_REG GPPU INTEDG T0CS T0SE PSA PS2 PS1 PS0 1111 1111 1111 1111
85h TRISIO — — TRISIO5 TRISIO4 TRISIO3 TRISIO2 TRISIO1 TRISIO0 --11 1111 --11 1111
Legend: — = Unimplemented locations, read as ‘0’, u = unchanged, x = unknown.
Shaded cells are not used by the Timer0 module.