Datasheet
2013 Microchip Technology Inc. DS22331A-page 101
MCP19111
17.0 WATCHDOG TIMER (WDT)
The Watchdog Timer is a free running timer. The WDT
is enabled by setting the WDTE bit of the Configuration
Word (default setting).
During normal operation, a WDT time-out generates a
device Reset. If the device is in Sleep mode, a WDT
time-out causes the device to wake-up and continue
with normal operation.
The WDT can be permanently disabled by clearing the
WDTE bit of the Configuration bit. See Section 12.1
“Configuration Word” for more information.
17.1 Watchdog Timer (WDT) Operation
During normal operation, a WDT time-out generates a
device RESET. If the device is in SLEEP mode, a WDT
time-out causes the device to wake-up and continue
with normal operation; this is known as a WDT wake-
up. The WDT can be permanently disabled by clearing
the WDTE configuration bit.
The postscaler assignment is fully under software
control and can be changed during program execution.
17.2 WDT Period
The WDT has a nominal time-out period of 18 ms (with
no prescaler). The time-out periods vary with
temperature, V
DD
and process variations from part to
part (see Table 5-4). 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 register. Thus, time-out periods
up to 2.3 seconds can be realized.
The CLRWDT and SLEEP instructions clear the WDT
and the prescaler, if assigned to the WDT, and prevent
it from timing out and generating a device Reset.
The TO
bit in the STATUS register will be cleared upon
a Watchdog Timer time-out.
17.3 WDT Programming
Considerations
Under worst-case conditions (i.e., V
DD
= Minimum,
Temperature = Maximum, Maximum WDT prescaler), it
may take several seconds before a WDT time-out
occurs.
FIGURE 17-1: WATCHDOG TIMER WITH SHARED PRESCALER BLOCK DIAGRAM
T0CKI
T0SE
pin
TMR0
Watchdog
Timer
WDT
Time-out
PS<2:0>
Data Bus
Set Flag bit T0IF
on Overflow
T0CS
Note 1: T0SE, T0CS, PSA, PS<2:0> are bits in the OPTION_REG register.
2: WDTE bit is in the Configuration Word register.
0
1
0
1
0
1
8
8
8-bit
Prescaler
0
1
F
OSC
/4
PSA
PSA
PSA
Sync
2 T
CY
WDTE