PIC16F946 Datasheet, PDF (208/274 Pages) Microchip Technology – 64-Pin Flash-Based, 8-Bit CMOS Microcontrollers with LCD Driver and nanoWatt Technology

PIC16F946
16.6 Watchdog Timer (WDT)
For PIC16F946, 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.
16.6.1 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.
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).
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.
16.6.2 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 (WDTCON<0>) 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 (OPTION_REG) 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
From TMR0 Clock Source
0
16-bit WDT Prescaler
1
Prescaler⁽¹⁾
8
31 kHz
LFINTOSC Clock
WDTPS<3:0>
PSA
0
1
WDTE from Configuration Word register
SWDTEN from WDTCON
WDT Time-out
Note 1: This is the shared Timer0/WDT prescaler. See Section 5.4 “Prescaler” for more information.
PS<2:0>
To TMR0
PSA
TABLE 16-7: WDT STATUS
Conditions
WDTE = 0
CLRWDT Command
Oscillator Fail Detected
Exit Sleep + System Clock = T1OSC, EXTRC, INTOSC, EXTCLK
Exit Sleep + System Clock = XT, HS, LP
WDT
Cleared
Cleared until the end of OST
DS41265A-page 206
Preliminary
© 2005 Microchip Technology Inc.