參數(shù)資料
型號(hào): PIC12F505
廠商: Microchip Technology Inc.
元件分類: 8位微控制器
英文描述: 8/14-Pin, 8-Bit Flash Microcontrollers
中文描述: 8/14-Pin,8位閃存微控制器
文件頁數(shù): 38/100頁
文件大小: 741K
代理商: PIC12F505
PIC12F508/509/16F505
DS41236C-page 36
Preliminary
2007 Microchip Technology Inc.
6.2
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 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
SWITCHING PRESCALER
ASSIGNMENT
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.
EXAMPLE 6-1:
CHANGING PRESCALER
(TIMER0
WDT)
;Clear WDT
;Clear TMR0 & Prescaler
‘00xx1111’b ;These 3 lines (5, 6, 7)
;are required only if
;desired
;PS<2:0> are 000 or 001
‘00xx1xxx’b ;Set Postscaler to
;desired WDT rate
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:
CHANGING PRESCALER
(WDT
TIMER0)
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.
CLRWDT
CLRF
MOVLW
OPTION
TMR0
CLRWDT
MOVLW
OPTION
CLRWDT
;Clear WDT and
;prescaler
;Select TMR0, new
;prescale value and
;clock source
MOVLW
‘xxxx0xxx’
OPTION
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