ATtiny261-ESSZ datasheet(44/227 Pages) ATMEL

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7753C–AVR–07/09

ATtiny261/ATtiny461/ATtiny861

9.1

Internal Voltage Reference

ATtiny261/461/861 features an internal bandgap reference. This reference is used for Brown-out

Detection, and it can be used as an input to the Analog Comparator or the ADC.

9.1.1

Voltage Reference Enable Signals and Start-up Time

The voltage reference has a start-up time that may influence the way it should be used. The

start-up time is given in “System and Reset Characteristics” on page 190. To save power, the

reference is not always turned on. The reference is on during the following situations:

When the BOD is enabled (by programming the BODLEVEL [2..0] Fuse bits).

When the bandgap reference is connected to the Analog Comparator (by setting the

ACBG bit in ACSR).

When the ADC is enabled.

Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC, the user

must always allow the reference to start up before the output from the Analog Comparator or

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.

9.2

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

9-4 on page 48. 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 ATtiny261/461/861 resets and executes from the Reset

Vector. For timing details on the Watchdog Reset, refer to Table 9-4 on page 48.

The Watchdog 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 9-2. Refer to

“Timed Sequences for Changing the Configuration of the Watchdog Timer” on page 45 for

details.

Table 9-2.

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

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