12F508 Microchip Technology, 12F508 Datasheet - Page 38

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PIC12F508/509/16F505
6.2
An 8-bit counter is available as a prescaler for the
Timer0 module or as a postscaler for the Watchdog
Timer (WDT), respectively (see Section 7.6 “Watch-
dog Timer (WDT)”). For simplicity, this counter is
being referred to as “prescaler” throughout this data
sheet.
The PSA and PS<2:0> bits (OPTION<3:0>) determine
prescaler assignment and prescale ratio.
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 WDT. The prescaler is neither readable
nor writable. On a Reset, the prescaler contains all ‘0’s.
6.2.1
The prescaler assignment is fully under software
control (i.e., it can be changed “on-the-fly” during pro-
gram execution). To avoid an unintended device Reset,
the following instruction sequence (Example 6-1) must
be executed when changing the prescaler assignment
from Timer0 to the WDT.
DS41236B-page 36
Note:
Prescaler
The prescaler may be used by either the
Timer0 module or the WDT, but not both.
Thus, a prescaler assignment for the
Timer0 module means that there is no
prescaler for the WDT and vice versa.
SWITCHING PRESCALER
ASSIGNMENT
Preliminary
EXAMPLE 6-1:
To change the prescaler from the WDT to the Timer0
module, use the sequence shown in Example 6-2. This
sequence must be used even if the WDT is disabled. A
CLRWDT instruction should be executed before
switching the prescaler.
EXAMPLE 6-2:
CLRWDT
CLRF
MOVLW
OPTION
CLRWDT
MOVLW
OPTION
CLRWDT
MOVLW
OPTION
TMR0
‘00xx1111’b ;These 3 lines (5, 6, 7)
‘00xx1xxx’b ;Set Postscaler to
‘xxxx0xxx’
CHANGING PRESCALER
(TIMER0
CHANGING PRESCALER
(WDT
;Clear WDT
;Clear TMR0 & Prescaler
;are required only if
;desired
;PS<2:0> are 000 or 001
;desired WDT rate
© 2005 Microchip Technology Inc.
;Clear WDT and
;prescaler
;Select TMR0, new
;prescale value and
;clock source
TIMER0)
WDT)