PIC16LF18854 Datasheet, PDF (162/668 Pages) Microchip Technology – C Compiler Optimized RISC Architecture

PIC16(L)F18856/76
9.1 Independent Clock Source
The WDT can derive its time base from either the 31
kHz LFINTOSC or 31.25 kHz MFINTOSC internal
oscillators, depending on the value of either the
WDTCCS<2:0> Configuration bits or the WDTCS<2:0>
bits of WDTCON1. Time intervals in this chapter are
based on a minimum nominal interval of 1 ms. See
Section 37.0 “Electrical Specifications” for
LFINTOSC and MFINTOSC tolerances.
9.2 WDT Operating Modes
The Watchdog Timer module has four operating modes
controlled by the WDTE<1:0> bits in Configuration
Words. See Table 9-1.
9.2.1 WDT IS ALWAYS ON
When the WDTE bits of Configuration Words are set to
‘11’, the WDT is always on.
WDT protection is active during Sleep.
9.2.2 WDT IS OFF IN SLEEP
When the WDTE bits of Configuration Words are set to
‘10’, the WDT is on, except in Sleep.
WDT protection is not active during Sleep.
9.2.3 WDT CONTROLLED BY SOFTWARE
When the WDTE bits of Configuration Words are set to
‘01’, the WDT is controlled by the SEN bit of the
WDTCON0 register.
WDT protection is unchanged by Sleep. See Table 9-1
for more details.
TABLE 9-1: WDT OPERATING MODES
WDTE<1:0>
SEN
Device WDT
Mode Mode
11
X
X
Active
Awake Active
10
X
Sleep Disabled
1
X
Active
01
0
X Disabled
00
X
X Disabled
9.3 Time-Out Period
The WDTPS bits of the WDTCON0 register set the
time-out period from 1 ms to 256 seconds (nominal).
After a Reset, the default time-out period is two
seconds.
9.4 Watchdog Window
The Watchdog Timer has an optional Windowed mode
that is controlled by the WDTCWS<2:0> Configuration
bits and WINDOW<2:0> bits of the WDTCON1 register.
In the Windowed mode, the CLRWDT instruction must
occur within the allowed window of the WDT period.
Any CLRWDT instruction that occurs outside of this win-
dow will trigger a window violation and will cause a
WDT Reset, similar to a WDT time out. See Figure 9-2
for an example.
The window size is controlled by the WDTCWS<2:0>
Configuration bits, or the WINDOW<2:0> bits of
WDTCON1, if WDTCWS<2:0> = 111.
In the event of a window violation, a Reset will be
generated and the WDTWV bit of the PCON register
will be cleared. This bit is set by a POR or can be set in
firmware.
9.5 Clearing the WDT
The WDT is cleared when any of the following
conditions occur:
• Any Reset
• Valid CLRWDT instruction is executed
• Device enters Sleep
• Device wakes up from Sleep
• WDT is disabled
• Oscillator Start-up Timer (OST) is running
• Any write to the WDTCON0 or WDTCON1 registers
9.5.1
CLRWDT CONSIDERATIONS
(WINDOWED MODE)
When in Windowed mode, the WDT must be armed
before a CLRWDT instruction will clear the timer. This is
performed by reading the WDTCON0 register. Execut-
ing a CLRWDT instruction without performing such an
arming action will trigger a window violation.
See Table 9-2 for more information.
9.6 Operation During Sleep
When the device enters Sleep, the WDT is cleared. If
the WDT is enabled during Sleep, the WDT resumes
counting. When the device exits Sleep, the WDT is
cleared again.
The WDT remains clear until the OST, if enabled, com-
pletes. See Section 6.0 “Oscillator Module (with Fail-
Safe Clock Monitor)” for more information on the OST.
When a WDT time-out occurs while the device is in
Sleep, no Reset is generated. Instead, the device
wakes up and resumes operation. The TO and PD bits
in the STATUS register are changed to indicate the
event. The RWDT bit in the PCON register can also be
used. See Section 3.0 “Memory Organization” for
more information.
DS40001824A-page 162
Preliminary
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