PIC12F508_09 datasheet(52/110 Pages) MICROCHIP

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PIC12F508/509/16F505

DS41236E-page 52

7.5

Device Reset Timer (DRT)

On the PIC12F508/509/16F505 devices, the DRT runs

any time the device is powered up. DRT runs from

Reset and varies based on oscillator selection and

Reset type (see Table 7-6).

The DRT operates on an internal RC oscillator. The

processor is kept in Reset as long as the DRT is active.

The DRT delay allows VDD to rise above VDD min. and

for the oscillator to stabilize.

Oscillator circuits based on crystals or ceramic resona-

tors require a certain time after power-up to establish a

stable oscillation. The on-chip DRT keeps the devices in

a Reset condition for approximately 18 ms after MCLR

has

reached

logic

high

(VIH MCLR)

level.

Programming (GP3/RB3)/MCLR/VPP as MCLR and

using an external RC network connected to the MCLR

input is not required in most cases. This allows savings

in cost-sensitive and/or space restricted applications, as

well as allowing the use of the (GP3/RB3)/MCLR/VPP

pin as a general purpose input.

The Device Reset Time delays will vary from chip-to-

chip due to VDD, temperature and process variation.

See AC parameters for details.

The DRT will also be triggered upon a Watchdog Timer

time-out from Sleep. This is particularly important for

applications using the WDT to wake from Sleep mode

automatically.

Reset sources are POR, MCLR, WDT time-out and

wake-up on pin change. See Section 7.9.2 “Wake-up

from Sleep”, Notes 1, 2 and 3.

7.6

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

external RC oscillator of the (GP5/RB5)/OSC1/CLKIN

pin and the internal 4 MHz oscillator. This means that

the WDT will run even if the main processor clock has

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.

The

WDT

can

permanently

disabled

programming the configuration WDTE as a ‘0’ (see

Section 7.1 “Configuration Bits”). Refer to the

PIC12F508/509/16F505 Programming Specifications

to determine how to access the Configuration Word.

TABLE 7-6:

DRT (DEVICE RESET TIMER

PERIOD)

7.6.1

WDT PERIOD

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

writing to the OPTION register. Thus, a time-out period

of a nominal 2.3 seconds can be realized. These

periods vary with temperature, VDD and part-to-part

process variations (see DC specs).

Under worst case conditions (VDD = Min., Temperature

= Max., max. WDT prescaler), it may take several

seconds before a WDT time-out occurs.

7.6.2

WDT PROGRAMMING

CONSIDERATIONS

The CLRWDT instruction clears the WDT and the

postscaler, if assigned to the WDT, and prevents it from

timing out and generating a device Reset.

The SLEEP instruction resets the WDT and the post-

scaler, if assigned to the WDT. This gives the maximum

Sleep time before a WDT wake-up Reset.

Oscillator

Configuration

POR Reset

Subsequent

Resets

INTOSC, EXTRC

18 ms (typical)

μs (typical)

HS(1), XT, LP

18 ms (typical)

EC(1)

18 ms (typical)

μs (typical)

Note 1:

PIC16F505 only.

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