PIC18F2331/2431/4331/4431
23.2 Watchdog Timer (WDT)
Note 1: The CLRWDT and SLEEP instructions
clear the WDT and postscaler counts
when executed.
For PIC18F2331/2431/4331/4431 devices, the WDT is
driven by the INTRC source. When the WDT is
enabled, the clock source is also enabled. The nominal
WDT period is 4 ms and has the same stability as the
INTRC oscillator.
2: Changing the setting of the IRCF bits
(OSCCON<6:4>) clears the WDT and
postscaler counts.
The 4 ms period of the WDT is multiplied by a 16-bit
postscaler. Any output of the WDT postscaler is
3: When a CLRWDTinstruction is executed,
the postscaler count will be cleared.
selected by
a multiplexer, controlled by bits in
4: If WINEN = 0, then CLRWDTmust be exe-
cuted only when WDTW = 1; otherwise, a
device Reset will result.
Configuration Register 2H (see Register 23-3).
Available periods range from 4 ms to 131.072 seconds
(2.18 minutes). The WDT and postscaler are cleared
when any of the following events occur: execute a
SLEEP or CLRWDT instruction, the IRCF bits
(OSCCON<6:4>) are changed or a clock failure has
occurred (see Section 23.4.1 “FSCM and the
Watchdog Timer”).
23.2.1
CONTROL REGISTER
Register 23-15 shows the WDTCON register. This is a
readable and writable register. The SWDTEN bit allows
software to enable or disable the WDT, but only if the
Configuration bit has disabled the WDT. The WDTW bit
is a read-only bit that indicates when the WDT count is
in the fourth quadrant (i.e., when the 8-bit WDT value is
b’11000000’ or greater).
Adjustments to the internal oscillator clock period using
the OSCTUNE register also affect the period of the
WDT by the same factor. For example, if the INTRC
period is increased by 3%, then the WDT period is
increased by 3%.
FIGURE 23-1:
WDT BLOCK DIAGRAM
Enable WDT
SWDTEN
INTRC Control
WDT Counter
125
WDTEN
Wake-up
from Sleep
INTRC Source
Change on IRCF bits
CLRWDT
WDT
Reset
Reset
Programmable Postscaler
1:1 to 1:32,768
All Device Resets
WDT
4
WDTPS<3:0>
Sleep
DS39616D-page 274
2010 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
