PIC16CR54C
following instruction sequence (Example 6-1) must be
executed when changing the prescaler assignment from
Timer0 to the WDT.
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) (WDT postscaler not implemented on
PIC16C52), respectively (Section 6.1.2). For simplicity,
this counter is being referred to as “prescaler”
throughout this data sheet. Note that 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.
EXAMPLE 6-1: CHANGING PRESCALER
(TIMER0→WDT)
1.CLRWDT
2.CLRF
;Clear WDT
TMR0
;Clear TMR0 & Prescaler
3.MOVLW '00xx1111’b ;These 3 lines (5, 6, 7)
4.OPTION
; are required only if
; desired
5.CLRWDT
;PS<2:0> are 000 or 001
6.MOVLW '00xx1xxx’b ;Set Postscaler to
7.OPTION ; desired WDT rate
The PSA and PS2:PS0 bits (OPTION<3:0>) determine
prescaler assignment and prescale ratio.
To change 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
CLRWDTinstruction should be executed before switching
the prescaler.
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.
EXAMPLE 6-2: CHANGING PRESCALER
(WDT→TIMER0)
CLRWDT
;Clear WDT and
6.2.1
SWITCHING PRESCALER ASSIGNMENT
;prescaler
MOVLW 'xxxx0xxx'
;Select TMR0, new
;prescale value and
;clock source
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
OPTION
FIGURE 6-6: BLOCK DIAGRAM OF THE TIMER0/WDT PRESCALER
TCY ( = Fosc/4)
Data Bus
8
0
M
U
X
1
M
U
X
T0CKI
pin
1
Sync
2
Cycles
TMR0 reg
0
T0SE
T0CS
PSA
0
1
8-bit Prescaler
M
U
X
8
Watchdog
Timer
8 - to - 1MUX
PS2:PS0
PSA
1
0
WDT Enable bit
MUX
PSA
WDT
Time-Out
Note: T0CS, T0SE, PSA, PS2:PS0 are bits in the OPTION register.
DS40191A-page 24
Preliminary
1998 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
