PIC16 datasheet(90/418 Pages) MICROCHIP

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PIC16(L)F1508/9

DS40001609C-page 90

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9.1

Independent Clock Source

The WDT derives its time base from the 31 kHz

LFINTOSC internal oscillator. Time intervals in this

chapter are based on a nominal interval of 1 ms. See

Section 29.0 “Electrical Specifications” for the

LFINTOSC 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 SWDTEN bit of the

WDTCON register.

WDT protection is unchanged by Sleep. See Table 9-1

for more details.

TABLE 9-1:

WDT OPERATING MODES

9.3

Time-Out Period

The WDTPS bits of the WDTCON 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

Clearing the WDT

The WDT is cleared when any of the following condi-

tions occur:

•Any Reset

• CLRWDT instruction is executed

• Device enters Sleep

• Device wakes up from Sleep

• Oscillator fail

• WDT is disabled

• Oscillator Start-up Timer (OST) is running

See Table 9-2 for more information.

9.5

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 5.0 “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.

WDTE<1:0>

SWDTEN

Device

Mode

WDT

Mode

XActive

Awake

Active

Sleep

Disabled

XActive

Disabled

TABLE 9-2:

WDT CLEARING CONDITIONS

Conditions

WDT

WDTE<1:0> = 00

Cleared

WDTE<1:0> = 01 and SWDTEN = 0

WDTE<1:0> = 10 and enter Sleep

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

Change INTOSC divider (IRCF bits)

Unaffected

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