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PIC16F8X
1998 Microchip Technology Inc.
DS30430C-page 51
8.11
Watchdog Timer (WDT)
The Watchdog Timer is a free running on-chip RC
oscillator
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. If the device is in SLEEP
mode, a WDT Wake-up causes the device to wake-up
and continue with normal operation. The WDT can be
permanently disabled by programming configuration bit
WDTE as a ’0’ (Section 8.1).
8.11.1
WDT PERIOD
The WDT has a nominal time-out period of 18 ms, (with
no
prescaler).
The
time-out
periods
vary
with
temperature, VDD and process variations from part to
part (see DC specs). If longer time-out periods are
desired, a prescaler with a division ratio of up to 1:128
can be assigned to the WDT under software control by
writing to the OPTION_REG register. Thus, time-out
periods up to 2.3 seconds can be realized.
The CLRWDT and SLEEP instructions clear the WDT
and the postscaler (if assigned to the WDT) and pre-
vent it from timing out and generating a device
RESET condition.
The TO bit in the STATUS register will be cleared upon
a WDT time-out.
8.11.2
WDT PROGRAMMING CONSIDERATIONS
It should also be taken into account that under worst
case conditions (VDD = Min., Temperature = Max., max.
WDT prescaler) it may take several seconds before a
WDT time-out occurs.
FIGURE 8-18: WATCHDOG TIMER BLOCK DIAGRAM
TABLE 8-7
SUMMARY OF REGISTERS ASSOCIATED WITH THE WATCHDOG TIMER
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Value on
Power-on
Reset
Value on all
other resets
2007h
Config. bits
(2)
PWRTE(1)
WDTE
FOSC1
FOSC0
(2)
81h
OPTION_
REG
RBPU
INTEDG
T0CS
T0SE
PSA
PS2
PS1
PS0
1111 1111
Legend: x = unknown. Shaded cells are not used by the WDT.
Note 1:
See Figure 8-1 and Figure 8-2 for operation of the PWRTE bit.
2:
See Figure 8-1, Figure 8-2 and Section 8.13 for operation of the Code and Data protection bits.
From TMR0 Clock Source
(Figure 6-6)
To TMR0 (Figure 6-6)
Postscaler
WDT Timer
M
U
X
PSA
8 - to -1 MUX
PSA
WDT
Time-out
1
0
1
WDT
Enable Bit
PS2:PS0
8
MUX
Note: PSA and PS2:PS0 are bits in the OPTION_REG register.
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