PIC16F684-ISL Microchip Technology Inc., PIC16F684-ISL Datasheet - Page 49

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PIC16F684-ISL

Manufacturer Part Number
PIC16F684-ISL
Description
Manufacturer
Microchip Technology Inc.
Datasheets

Specifications of PIC16F684-ISL

Case
N/A
Notes
NEW
Date_code
11+
5.4
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 PS<2:0> 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,
BSF 1, x....etc.) will clear the prescaler. When
assigned to WDT, a CLRWDT instruction will clear the
prescaler along with the Watchdog Timer.
5.4.1
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
(Example 5-1 and Example 5-2) must be executed
when changing the prescaler assignment from Timer0
to WDT.
TABLE 5-1:
REGISTERS ASSOCIATED WITH TIMER0
Address
Name
Bit 7
Bit 6
01h
TMR0
Timer0 Module register
0Bh/8Bh INTCON
GIE
PEIE
81h
OPTION_REG
RAPU
INTEDG
85h
TRISA
Legend:
— = Unimplemented locations, read as ‘0’, u = unchanged, x = unknown. Shaded cells are not used by the Timer0
module.
 2004 Microchip Technology Inc.
EXAMPLE 5-1:
BCF
STATUS,RP0
CLRWDT
CLRF
TMR0
BSF
STATUS,RP0
MOVLW
b’00101111’
MOVWF
OPTION_REG
CLRWDT
MOVLW
b’00101xxx’
MOVWF 1,
MOVWF
OPTION_REG
BCF
STATUS,RP0
To change prescaler from the WDT to the TMR0
module, use the sequence shown in Example 5-2. This
precaution must be taken even if the WDT is disabled.
EXAMPLE 5-2:
CLRWDT
sequence
BSF
STATUS,RP0
MOVLW
b’xxxx0xxx’
MOVWF
OPTION_REG
BCF
STATUS,RP0
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
T0IE
INTE
RAIE
T0IF
INTF
T0CS
T0SE
PSA
PS2
PS1
TRISA5 TRISA4 TRISA3 TRISA2 TRISA1 TRISA0 --11 1111 --11 1111
Preliminary
PIC16F684
CHANGING PRESCALER
(TIMER0 WDT)
;Bank 0
;Clear WDT
;Clear TMR0 and
; prescaler
;Bank 1
;Required if desired
; PS2:PS0 is
; 000 or 001
;
;Set postscaler to
; desired WDT rate
;Bank 0
CHANGING PRESCALER
(WDT TIMER0)
;Clear WDT and
; prescaler
;Bank 1
;Select TMR0,
; prescale, and
; clock source
;
;Bank 0
Value on
Value on
Bit 0
all other
POR, BOD
Resets
xxxx xxxx uuuu uuuu
RAIF
0000 0000 0000 0000
PS0
1111 1111 1111 1111
DS41202C-page 47

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