PIC16F526 Datasheet, PDF (40/122 Pages) Microchip Technology – 14-Pin, 8-Bit Flash Microcontroller

PIC16F526
7.2 Prescaler
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 8.6 “Watch-
dog Timer (WDT)”). For simplicity, this counter is
being referred to as “prescaler” throughout this data
sheet.
Note:
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.
The PSA and PS<2:0> bits of the OPTION register
determine prescaler assignment and prescale ratio.
When assigned to the Timer0 module, all instructions
writing to the TMR0 register (e.g., CLRF TMR0,
MOVWF TMR0, 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.
7.2.1
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 the WDT.
EXAMPLE 7-1:
CHANGING PRESCALER
(TIMER0 WDT)
CLRWDT
;Clear WDT
CLRF TMR0
;Clear TMR0 & Prescaler
MOVLW b'00xx1111'
CLRWDT
;PS<2:0> are 000 or 001
MOVLW b'00xx1xxx' ;Set Postscaler to
OPTION
;desired WDT rate
To change the prescaler from the WDT to the Timer0
module, use the sequence shown in Example 7-2. This
sequence must be used even if the WDT is disabled. A
CLRWDT instruction should be executed before
switching the prescaler.
EXAMPLE 7-2:
CHANGING PRESCALER
(WDT TIMER0)
CLRWDT
;Clear WDT and
;prescaler
MOVLW b'xxxx0xxx' ;Select TMR0, new
;prescale value and
;clock source
OPTION
DS41326D-page 40
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