P12C508 Microchip Technology, P12C508 Datasheet - Page 24

• Manufacturer Part Number: P12C508
• Description: 8-Pin / 8-Bit CMOS Microcontroller
• Manufacturer: Microchip Technology

PIC12C5XX
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 (Section 7.6). 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.
The
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 program
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.
FIGURE 6-5:
DS40139A-page 24
PSA
Note: T0CS, T0SE, PSA, PS2:PS0 are bits in the OPTION register.
Prescaler
SWITCHING PRESCALER ASSIGNMENT
WDT Enable bit
Watchdog
GP2/T0CKI
Timer
T
and
CY
Pin
( = Fosc/4)
BLOCK DIAGRAM OF THE TIMER0/WDT PRESCALER
T0SE
PS2:PS0
0
1
bits
PSA
M
U
X
(OPTION<3:0>)
0
1
T0CS
Advance Information
M
U
X
8-bit Prescaler
0
8 - to - 1MUX
Time-Out
8
MUX
WDT
1
0
1
PSA
EXAMPLE 6-1:
CLRF
CLRWDT
MOVLW
OPTION
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
CLRWDT instruction should be executed before switching
the prescaler.
EXAMPLE 6-2:
CLRWDT
MOVLW
OPTION
M
U
X
PSA
TMR0
'xxxx1xxx'
'xxxx0xxx'
PS2:PS0
Cycles
Sync
2
CHANGING PRESCALER
(TIMER0 WDT)
CHANGING PRESCALER
(WDT TIMER0)
1996 Microchip Technology Inc.
;prescaler
;prescaler
;prescale value and
;clock source
;Clear TMR0
;Clears WDT and
;Select new prescale
;value
;Clear WDT and
;Select TMR0, new
TMR0 reg
Data Bus
8