Datasheet

PIC16F913/914/916/917/946
DS41250F-page 234 © 2007 Microchip Technology Inc.
16.4 Watchdog Timer (WDT)
For PIC16F91X/946, the WDT has been modified from
previous PIC16F devices. The new WDT is code and
functionally compatible with previous PIC16F WDT
modules and adds a 16-bit prescaler to the WDT. This
allows the user to have a scaled value for the WDT and
TMR0 at the same time. In addition, the WDT time-out
value can be extended to 268 seconds. WDT is cleared
under certain conditions described in Table 16-7.
16.4.1 WDT OSCILLATOR
The WDT derives its time base from the 31 kHz
LFINTOSC. The LTS bit does not reflect that the
LFINTOSC is enabled.
The value of WDTCON is ‘---0 1000’ on all Resets.
This gives a nominal time base of 16 ms, which is
compatible with the time base generated with previous
PIC16F microcontroller versions.
A new prescaler has been added to the path between
the INTOSC and the multiplexers used to select the
path for the WDT. This prescaler is 16 bits and can be
programmed to divide the INTOSC by 32 to 65536,
giving the WDT a nominal range of 1 ms to 268s.
16.4.2 WDT CONTROL
The WDTE bit is located in the Configuration Word
register. When set, the WDT runs continuously.
When the WDTE bit in the Configuration Word register
is set, the SWDTEN bit of the WDTCON register has no
effect. If WDTE 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 of the OPTION register
have the same function as in previous versions of the
PIC16F family of microcontrollers. See Section 5.0
“Timer0 Module” for more information.
FIGURE 16-9: WATCHDOG TIMER BLOCK DIAGRAM
Note: When the Oscillator Start-up Timer (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).
TABLE 16-7: WDT STATUS
Conditions WDT
WDTE = 0
Cleared
CLRWDT Command
Oscillator Fail Detected
Exit Sleep + System Clock = T1OSC, EXTRC, INTOSC, EXTCLK
Exit Sleep + System Clock = XT, HS, LP Cleared until the end of OST
31 kHz
PSA
16-bit WDT Prescaler
From TMR0 Clock Source
Prescaler
(1)
8
PS<2:0>
PSA
WDT Time-out
To T MR0
WDTPS<3:0>
WDTE from Configuration Word register
1
1
0
0
SWDTEN from WDTCON
LFINTOSC Clock
Note 1: This is the shared Timer0/WDT prescaler. See Section 5.4 “Prescaler” for more information.