PIC16F87X Datasheet, PDF (133/218 Pages) Microchip Technology – 28/40-pin 8-Bit CMOS FLASH Microcontrollers

PIC16F87X
12.12 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 SLEEP instruction.
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.
The WDT can be permanently disabled by clearing
configuration bit WDTE (Section 12.1).
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.
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.
FIGURE 12-10:
WATCHDOG TIMER BLOCK DIAGRAM
From TMR0 Clock Source
(Figure 5-1)
WDT Timer
0
1
M
U
X
Postscaler
8
WDT
Enable Bit
PSA
8 - to - 1 MUX
PS2:PS0
To TMR0 (Figure 5-1)
0
1
MUX
PSA
Note:
PSA and PS2:PS0 are bits in the OPTION_REG register.
WDT
Time-out
TABLE 12-7: SUMMARY OF WATCHDOG TIMER REGISTERS
Address
Name
Bit 7
Bit 6
Bit 5
2007h
Config. bits
(1)
BODEN⁽¹⁾ CP1
81h,181h OPTION_REG RBPU INTEDG T0CS
Legend: Shaded cells are not used by the Watchdog Timer.
Note 1: See Register 12-1 for operation of these bits.
Bit 4
CP0
T0SE
Bit 3
PWRTE⁽¹⁾
PSA
Bit 2
WDTE
PS2
Bit 1
FOSC1
PS1
Bit 0
FOSC0
PS0
 2001 Microchip Technology Inc.
DS30292C-page 131