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rfPIC12F675K Datasheet(PDF) 27 Page - Microchip Technology |
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rfPIC12F675K Datasheet(HTML) 27 Page - Microchip Technology |
27 / 136 page 2003-2013 Microchip Technology Inc. Preliminary DS70091B-page 27 rfPIC12F675 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. |
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