PIC16F87X
WDT time-out period values may be found in the Elec-
trical Specifications section under parameter #31. Val-
ues for the WDT prescaler (actually a postscaler, but
shared with the Timer0 prescaler) may be assigned
using the OPTION_REG register.
12.12 Watchdog Timer (WDT)
The Watchdog Timer is as a free running on-chip RC
oscillator which does not require any external compo-
nents. This RC oscillator is separate from the RC oscil-
lator of the OSC1/CLKIN pin. That means that the WDT
will run, even if the clock on the OSC1/CLKIN and
OSC2/CLKOUT pins of the device has been stopped,
for example, by execution of a SLEEPinstruction.
Note: The CLRWDTand SLEEPinstructions clear
the WDT and the postscaler, if assigned to
the WDT, and prevent it from timing out and
generating a device RESET condition.
During normal operation, a WDT time-out generates a
device RESET (Watchdog Timer Reset). If the device is
in SLEEP mode, a WDT time-out causes the device to
wake-up and continue with normal operation (Watch-
dog Timer Wake-up). The TO bit in the STATUS register
will be cleared upon a Watchdog Timer time-out.
.
Note: When a CLRWDT instruction is executed
and the prescaler is assigned to the WDT,
the prescaler count will be cleared, but the
prescaler assignment is not changed.
The WDT can be permanently disabled by clearing
configuration bit WDTE (Section 12.1).
FIGURE 12-10: WATCHDOG TIMER BLOCK DIAGRAM
From TMR0 Clock Source
(Figure 5-1)
0
Postscaler
8
M
1
U
WDT Timer
X
8 - to - 1 MUX
PS2:PS0
PSA
WDT
Enable Bit
To TMR0 (Figure 5-1)
0
1
MUX
PSA
WDT
Time-out
Note: PSA and PS2:PS0 are bits in the OPTION_REG register.
FIGURE 12-11: SUMMARY OF WATCHDOG TIMER REGISTERS
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
(1)
(1)
2007h
Config. bits
(1)
BODEN
CP1
CP0
PWRTE
PSA
WDTE
PS2
FOSC1
PS1
FOSC0
PS0
81h,181h
OPTION_REG
RBPU
INTEDG
T0CS
T0SE
Legend: Shaded cells are not used by the Watchdog Timer.
Note 1: See Register 12-1 for operation of these bits.
1999 Microchip Technology Inc.
DS30292B-page 133
MICROCHIP [ MICROCHIP ]
