ATtiny25 datasheet(41/196 Pages) ATMEL

41 / 196 page

7598H–AVR–07/09

ATtiny25/45/85

ADC is used. To reduce power consumption in Power-down mode, the user can avoid the three

conditions above to ensure that the reference is turned off before entering Power-down mode.

8.9

Watchdog Timer

The Watchdog Timer is clocked from an On-chip Oscillator which runs at 128 kHz. By controlling

the Watchdog Timer prescaler, the Watchdog Reset interval can be adjusted as shown in Table

8-7 on page 43. The WDR – Watchdog Reset – instruction resets the Watchdog Timer. The

Watchdog Timer is also reset when it is disabled and when a Chip Reset occurs. Ten different

clock cycle periods can be selected to determine the reset period. If the reset period expires

without another Watchdog Reset, the ATtiny25/45/85 resets and executes from the Reset Vec-

tor. For timing details on the Watchdog Reset, refer to Table 8-7 on page 43.

The Wathdog Timer can also be configured to generate an interrupt instead of a reset. This can

be very helpful when using the Watchdog to wake-up from Power-down.

To prevent unintentional disabling of the Watchdog or unintentional change of time-out period,

two different safety levels are selected by the fuse WDTON as shown in Table 8-5. Refer to

“Timed Sequences for Changing the Configuration of the Watchdog Timer” on page 44 for

details.

Figure 8-7.

Watchdog Timer

Table 8-4.

Internal Voltage Reference Characteristics

Symbol

Parameter

Condition

Min

Typ

Max

Units

VBG

Bandgap reference voltage

VCC = 1.1V / 2.7V,

A = 25°C

1.0

1.1

1.2

tBG

Bandgap reference start-up time

VCC = 2.7V,

A = 25°C

µs

IBG

Bandgap reference current

consumption

CC = 2.7V,

TA = 25°C

µA

Table 8-5.

WDT Configuration as a Function of the Fuse Settings of WDTON

WDTON

Safety

Level

WDT Initial

State

How to Disable the

WDT

How to Change

Time-out

Unprogrammed

Disabled

Timed sequence

No limitations

Programmed

Enabled

Always enabled

Timed sequence

MCU RESET

WATCHDOG

PRESCALER

128 kHz

OSCILLATOR

WATCHDOG

RESET

WDP0

WDP1

WDP2

WDP3

WDE

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