PIC16F818/819
WDT time-out period values may be found in
Section 15.0 “Electrical Characteristics” under
parameter #31. Values 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)
For PIC16F818/819 devices, the WDT is driven by the
INTRC oscillator. When the WDT is enabled, the
INTRC (31.25 kHz) oscillator is enabled. The nominal
WDT period is 16 ms and has the same accuracy as
the INTRC oscillator.
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 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 (Watchdog
Timer wake-up). The TO bit in the Status register 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 con-
figuration bit, WDTEN (see Section 12.1 “Configuration
Bits”).
FIGURE 12-8:
WATCHDOG TIMER BLOCK DIAGRAM
From TMR0 Clock Source
(Figure 6-1)
0
Postscaler
8
M
U
X
1
INTRC
31.25 kHz
PS2:PS0
8-to-1 MUX
PSA
WDT
Enable Bit
To TMR0 (Figure 6-1)
0
1
MUX
PSA
WDT
Time-out
Note: PSA and PS2:PS0 are bits in the OPTION_REG register.
TABLE 12-5: SUMMARY OF WATCHDOG TIMER REGISTERS
Address
81h,181h OPTION_REG
2007h
Configuration bits(1)
Name
Bit 7
RBPU INTEDG
LVP
Bit 6
Bit 5
Bit 4
Bit 3
PSA
Bit 2
Bit 1
Bit 0
T0CS
T0SE
PS2
PS1
PS0
BOREN MCLRE FOSC2 PWRTEN WDTEN FOSC1 FOSC0
Legend: Shaded cells are not used by the Watchdog Timer.
Note 1: See Register 12-1 for operation of these bits.
DS39598E-page 98
2004 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
