Datasheet

PIC16F818/819
DS39598F-page 98  2001-2013 Microchip Technology Inc.
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.
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.
The WDT can be permanently disabled by clearing con-
figuration bit, WDTEN (see Section 12.1 “Configuration
Bits”).
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.
FIGURE 12-8: WATCHDOG TIMER BLOCK DIAGRAM
TABLE 12-5: SUMMARY OF WATCHDOG TIMER REGISTERS
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.
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.
From TMR0 Clock Source
(Figure 6-1)
To TMR0 (Figure 6-1)
Postscaler
INTRC
WDT
Enable Bit
0
1
M
U
X
PSA
8-to-1 MUX
PS2:PS0
0
1
MUX
PSA
WDT
Time-out
Note: PSA and PS2:PS0 are bits in the OPTION_REG register.
8
31.25 kHz
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
81h,181h OPTION_REG RBPU INTEDG T0CS T0SE PSA PS2 PS1 PS0
2007h Configuration bits
(1)
LVP 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.