Datasheet

41
2586D–AVR–02/06
ATtiny25/45/85
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 Table 9-4. To save power, the reference is not always turned on. The
reference is on during the following situations:
1. When the BOD is enabled (by programming the BODLEVEL [2..0] Fuse).
2. When the bandgap reference is connected to the Analog Comparator (by setting the
ACBG bit in ACSR).
3. 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.
Note: 1. Values are guidelines only. Actual values are TBD.
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
9-7 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 9-7 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 9-5. Refer to
”Timed Sequences for Changing the Configuration of the Watchdog Timer” on page 44 for
details.
Table 9-4. Internal Voltage Reference Characteristics
(1)
Symbol Parameter Condition Min Typ Max Units
V
BG
Bandgap reference voltage
V
CC
= 1.1V / 2.7V,
T
A
=25°C
1.0 1.1 1.2 V
t
BG
Bandgap reference start-up time
V
CC
= 2.7V,
T
A
=25°C
40 70 µs
I
BG
Bandgap reference current
consumption
V
CC
= 2.7V,
T
A
=25°C
15 µA
Table 9-5. 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 1 Disabled Timed sequence No limitations
Programmed 2 Enabled Always enabled Timed sequence