PIC16F87-E Datasheet, PDF (143/228 Pages) Microchip Technology – 18/20/28-Pin Enhan lashMicrocontrollers with nanoWatt Technology

PIC16F87/88
15.12 Watchdog Timer (WDT)
For PIC16F87/88 devices, the WDT has been modified
from previous PIC16 devices. The new WDT is code
and functionally backward compatible with previous
PIC16 WDT modules and allows the user to have a
scaler value for the WDT and TMR0 at the same time.
In addition, the WDT time-out value can be extended to
268 seconds, using the prescaler with the postscaler
when PSA is set to ‘1’.
15.12.1 WDT OSCILLATOR
The WDT derives its time base from the 31.25 kHz
INTRC. The value of WDTCON is ‘---0 1000’ on all
Resets. This gives a nominal time base of 16.38 ms,
which is compatible with the time base generated with
previous PIC16 microcontroller versions.
Note:
When the OST is invoked, the WDT is held
in Reset because the WDT ripple counter
is used by the OST to perform the oscilla-
tor delay count. When the OST count has
expired, the WDT will begin counting (if
enabled).
A new prescaler has been added to the path between
the internal RC and the multiplexors used to select the
path for the WDT. This prescaler is 16 bits and can be
programmed to divide the internal RC by 32 to 65536,
giving the time base used for the WDT a nominal range
of 1 ms to 2.097s.
15.12.2 WDT CONTROL
The WDTEN bit is located in Configuration Word 1 and
when this bit is set, the WDT runs continuously.
The SWDTEN bit is in the WDTCON register. When the
WDTEN bit in the Configuration Word 1 register is set,
the SWDTEN bit has no effect. If WDTEN is clear, then
the SWDTEN bit can be used to enable and disable the
WDT. Setting the bit will enable it and clearing the bit
will disable it.
The PSA and PS<2:0> bits (OPTION_REG register)
have the same function as in previous versions of the
PIC16 family of microcontrollers.
FIGURE 15-8:
WATCHDOG TIMER BLOCK DIAGRAM
From TMR0 Clock Source
31.25 kHz
INTRC Clock
16-bit Programmable Prescaler WDT
WDTPS<3:0>
WDTEN from Configuration Word 1
SWDTEN from WDTCON
0
1
PSA
Postscaler
8
PS<2:0>
To TMR0
0
1
PSA
WDT Time-out
TABLE 15-5: PRESCALER/POSTSCALER BIT STATUS
Conditions
Prescaler
Postscaler (PSA = 1)
WDTEN = 0
CLRWDT command
Oscillator fail detected
Exit Sleep + System Clock = T1OSC, EXTRC, INTRC, ECIO
Exit Sleep + System Clock = XT, HS, LP
Cleared
Cleared
Cleared at end of OST Cleared at end of OST
 2005 Microchip Technology Inc.
DS30487C-page 141