PIC16F87XA
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.
14.13 Watchdog Timer (WDT)
The Watchdog Timer is a free running on-chip RC oscil-
lator, which does not require any external components.
This RC oscillator is separate from the RC oscillator 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 1: The CLRWDT and SLEEP instructions
clear the WDT and the postscaler, if
assigned to the WDT, and prevent it from
timing out and generating
RESET condition.
a device
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 regis-
ter will be cleared upon a Watchdog Timer time-out.
2: 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 14.1).
FIGURE 14-11:
WATCHDOG TIMER BLOCK DIAGRAM
From TMR0 Clock Source
(Figure 5-1)
0
Postscaler
M
1
U
WDT Timer
X
8
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.
TABLE 14-7: SUMMARY OF WATCHDOG TIMER REGISTERS
Address
Name
Bit 7
(1)
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
2007h
Config. bits
BODEN(1)
INTEDG
CP1
CP0
PWRTE(1)
PSA
WDTE
PS2
FOSC1
PS1
FOSC0
PS0
81h,181h OPTION_REG RBPU
T0CS
T0SE
Legend: Shaded cells are not used by the Watchdog Timer.
Note 1: See Register 14-1 for operation of these bits.
2001 Microchip Technology Inc.
Advance Information
DS39582A-page 153
MICROCHIP [ MICROCHIP ]
