Datasheet

PIC16F5X
DS41213D-page 38 © 2007 Microchip Technology Inc.
8.2 Watchdog Timer (WDT)
The Watchdog Timer (WDT) is a free running on-chip
RC oscillator which does not require any external
components. This RC oscillator is separate from the
RC oscillator of the OSC1/CLKIN pin. That means that
the WDT will run even if the clock on the OSC1/CLKIN
and OSC2/CLKOUT pins have been stopped, for
example, by execution of a SLEEP instruction. During
normal operation or Sleep, a WDT Reset or Wake-up
Reset generates a device Reset.
The TO
bit (STATUS<4>) will be cleared upon a
Watchdog Timer Reset (Section 3.3 “STATUS
Register”).
The WDT can be permanently disabled by program-
ming the Configuration bit WDTE as a ‘0’ (Section 8.1
“Configuration Bits”). Refer to the PIC16F54 and
PIC16F57 Programming Specifications to determine
how to access the Configuration Word. These
documents can be found on the Microchip web site at
www.microchip.com.
8.2.1 WDT PERIOD
An 8-bit counter is available as a prescaler for the
Timer0 module (Section 7.2 “Prescaler”), or as a
postscaler for the Watchdog Timer (WDT), respec-
tively. For simplicity, this counter is being referred to as
“prescaler” throughout this data sheet.
The PSA and PS<2:0> bits (OPTION<3:0>) determine
prescaler assignment and prescale ratio (Section 3.4
“Option Register”).
The WDT has a nominal time-out period of 18 ms (with
no prescaler). If a longer time-out period is desired, a
prescaler with a division ratio of up to 1:128 can be
assigned to the WDT (under software control) by writ-
ing to the Option register. Thus time-out, a period of a
nominal 2.3 seconds, can be realized. These periods
vary with temperature, V
DD and part-to-part process
variations (see Device Characterization).
Under worst case conditions (VDD = Min., Temperature
= Max., WDT prescaler = 1:128), it may take several
seconds before a WDT time-out occurs.
8.2.2 WDT PROGRAMMING
CONSIDERATIONS
The CLRWDT instruction clears the WDT and the
prescaler, if assigned to the WDT, and prevents it from
timing out and generating a device Reset.
The SLEEP instruction resets the WDT and the
prescaler, if assigned to the WDT. This gives the
maximum Sleep time before a WDT Wake-up Reset.
FIGURE 8-1: WATCHDOG TIMER
BLOCK DIAGRAM
TABLE 8-1: SUMMARY OF REGISTERS ASSOCIATED WITH THE WATCHDOG TIMER
Note: The prescaler may be used by either the
Timer0 module or the WDT, but not both.
Thus, a prescaler assignment for the
Timer0 module means that there is no
prescaler for the WDT, and vice-versa.
M
U
X
Note 1: T0CS, T0SE, PSA, PS<2:0> are bits in the
Option register.
Prescaler
Watchdog
Timer
WDTE
PSA
(1)
1
0
8-to-1
MUX
To TMR0
MUX
PSA
(1)
WDT Time-out
01
From TMR0 Clock Source
PS<2:0>
(1
)
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Value on
Power-on
Reset
Value on
MCLR and
WDT Reset
N/A OPTION — — T0CS T0SE PSA PS2 PS1 PS0 --11 1111 --11 1111
Legend: Shaded cells not used by Watchdog Timer, - = unimplemented, read as ‘0’, u = unchanged