OPTION_REG datasheet(101/222 Pages) MICROCHIP

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DS22331A-page 101

MCP19111

17.0

WATCHDOG TIMER (WDT)

The Watchdog Timer is a free running timer. The WDT

is enabled by setting the WDTE bit of the Configuration

Word (default setting).

During normal operation, a WDT time-out generates a

device Reset. If the device is in Sleep mode, a WDT

time-out causes the device to wake-up and continue

with normal operation.

The WDT can be permanently disabled by clearing the

WDTE bit of the Configuration bit. See Section 12.1

“Configuration Word” for more information.

17.1

Watchdog Timer (WDT) Operation

During normal operation, a WDT time-out generates a

device RESET. If the device is in SLEEP mode, a WDT

time-out causes the device to wake-up and continue

with normal operation; this is known as a WDT wake-

up. The WDT can be permanently disabled by clearing

the WDTE configuration bit.

The postscaler assignment is fully under software

control and can be changed during program execution.

17.2

WDT Period

The WDT has a nominal time-out period of 18 ms (with

prescaler).

The

time-out

periods

vary

with

temperature, VDD and process variations from part to

part (see Table 5-4). If longer time-out periods are

desired, a prescaler with a division ratio of up to 1:128

can be assigned to the WDT under software control by

writing to the OPTION register. Thus, time-out periods

up to 2.3 seconds can be realized.

The CLRWDT and SLEEP instructions clear the WDT

and the prescaler, if assigned to the WDT, and prevent

it from timing out and generating a device Reset.

The TO bit in the STATUS register will be cleared upon

a Watchdog Timer time-out.

17.3

WDT Programming

Considerations

Under worst-case conditions (i.e., VDD = Minimum,

Temperature = Maximum, Maximum WDT prescaler), it

may take several seconds before a WDT time-out

occurs.

FIGURE 17-1:

WATCHDOG TIMER WITH SHARED PRESCALER BLOCK DIAGRAM

T0CKI

T0SE

TMR0

Watchdog

Timer

WDT

Time-out

PS<2:0>

Data Bus

Set Flag bit T0IF

on Overflow

T0CS

Note 1: T0SE, T0CS, PSA, PS<2:0> are bits in the OPTION_REG register.

2: WDTE bit is in the Configuration Word register.

8-bit

Prescaler

FOSC/4

PSA

Sync

2 TCY

WDTE

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