Atmel ATtiny25 User Manual

41
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
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 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 1 Disabled Timed sequence No limitations
Programmed 2 Enabled Always enabled Timed sequence
OSC/2K OSC/4K OSC/8K
OSC/16K OSC/32K OSC/64K
OSC/128K OSC/256K
OSC/512K
OSC/1024K
MCU RESET
WATCHDOG
PRESCALER
128 kHz
OSCILLATOR
WATCHDOG
RESET
WDP0
WDP1
WDP2
WDP3
WDE

Atmel ATtiny25 Specifications

General

Brand	Atmel
Model	ATtiny25
Category	Microcontrollers
Language	English