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’.
15.12.2
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) have the
same function as in previous versions of the PIC16
family of microcontrollers.
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
oscillator 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 128 to 65536,
giving the time base used for the WDT a nominal range
of 1 ms to 2.097s.
FIGURE 15-8:
WATCHDOG TIMER BLOCK DIAGRAM
From TMR0 Clock Source
0
Postscaler
16-bit Programmable Prescaler WDT
1
8
PSA
PS<2:0>
TO TMR0
0
1
PSA
31.25 kHz
INTRC Clock
WDTPS<3:0>
WDTEN from Configuration Word
SWDTEN from WDTCON
WDT Time-out
2003 Microchip Technology Inc.
Preliminary
DS30487B-page 141
MICROCHIP [ MICROCHIP TECHNOLOGY ]
