Datasheet

© 2009 Microchip Technology Inc. DS39663F-page 287
PIC18F87J10 FAMILY
24.2 Watchdog Timer (WDT)
For PIC18F87J10 family devices, the WDT is driven by
the INTRC oscillator. 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.
The 4 ms period of the WDT is multiplied by a 16-bit
postscaler. Any output of the WDT postscaler is selected
by a multiplexor, controlled by the WDTPS bits in Config-
uration Register 2H. Available periods range from about
4 ms to 135 seconds (2.25 minutes depending on
voltage, temperature and WDT postscaler). The WDT
and postscaler are cleared whenever a SLEEP or
CLRWDT instruction is executed, or a clock failure
(primary or Timer1 oscillator) has occurred.
24.2.1 CONTROL REGISTER
The WDTCON register (Register 24-9) is a readable
and writable register. The SWDTEN bit enables or dis-
ables WDT operation. This allows software to override
the WDTEN Configuration bit and enable the WDT only
if it has been disabled by the Configuration bit.
FIGURE 24-1: WDT BLOCK DIAGRAM
TABLE 24-3: SUMMARY OF WATCHDOG TIMER REGISTERS
Note 1: The CLRWDT and SLEEP instructions
clear the WDT and postscaler counts
when executed.
2: When a CLRWDT instruction is executed,
the postscaler count will be cleared.
REGISTER 24-9: WDTCON: WATCHDOG TIMER CONTROL REGISTER
U-0 U-0 U-0 U-0 U-0 U-0 U-0 R/W-0
— — — — — — —SWDTEN
(1)
bit 7 bit 0
Legend:
R = Readable bit W = Writable bit U = Unimplemented bit, read as ‘0’
-n = Value at POR ‘1’ = Bit is set ‘0’ = Bit is cleared x = Bit is unknown
bit 7-1 Unimplemented: Read as ‘0’
bit 0 SWDTEN: Software Controlled Watchdog Timer Enable bit
(1)
1 = Watchdog Timer is on
0 = Watchdog Timer is off
Note 1: This bit has no effect if the Configuration bit, WDTEN, is enabled.
Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Reset Values
on page
RCON
IPEN — — RI TO PD POR BOR 54
WDTCON
— — — — — — —SWDTEN 54
Legend: — = unimplemented, read as ‘0’. Shaded cells are not used by the Watchdog Timer.
INTRC Oscillator
WDT
Wake-up from
Reset
WDT
WDT Counter
Programmable Postscaler
1:1 to 1:32,768
Enable WDT
WDTPS<3:0>
SWDTEN
CLRWDT
4
Power-Managed
Reset
All Device Resets
Sleep
INTRC Control
÷128
Modes