31
ATtiny20 [DATASHEET]
8235E–AVR–03/2013
The BOD circuit will only detect a drop in VCC if the voltage stays below the trigger level for longer than tBOD given in
Figure 8-6.
Brown-out Reset During Operation
8.3
Internal Voltage Reference
ATtiny20 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. The bandgap voltage varies with supply voltage and temperature.
8.3.1
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
on during the following situations:
1.
When the BOD is enabled (by programming the BODLEVEL[2:0] Fuse).
2.
When the internal 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.
8.4
Watchdog Timer
The Watchdog Timer is clocked from an on-chip oscillator, which runs at 128 kHz. See
Figure 8-7 on page 32. By
controlling the Watchdog Timer prescaler, the Watchdog Reset interval can be adjusted as shown in
Table 8-2 on page34. The WDR – Watchdog Reset – instruction resets the Watchdog Timer. The Watchdog Timer is also reset when it is
disabled and when a device 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 ATtiny20 resets and executes from the Reset Vector. For
VCC
RESET
TIME-OUT
INTERNAL
RESET
VBOT-
VBOT+
tTOUT