Datasheet

42
8197C–AVR–05/11
ATtiny261A/461A/861A
8.2.2 External Reset
An External Reset is generated by a low level on the RESET
pin if enabled. Reset pulses longer
than the minimum pulse width (see “System and Reset Characteristics” on page 188) will gener-
ate a reset, even if the clock is not running. Shorter pulses are not guaranteed to generate a
reset. When the applied signal reaches the Reset Threshold Voltage – V
RST
– on its positive
edge, the delay counter starts the MCU after the Time-out period – t
TOUT
– has expired.
Figure 8-4. External Reset During Operation
8.2.3 Brown-out Detection
A Brown-out Detection (BOD) circuit monitors the V
CC
level during operation by comparing it to a
fixed trigger level. The trigger level for the BOD can be selected by the BODLEVEL Fuses. The
trigger level has a hysteresis to ensure spike free Brown-out Detection. The hysteresis on the
detection level should be interpreted as V
BOT+
= V
BOT
+ V
HYST
/2 and V
BOT-
= V
BOT
- V
HYST
/2.
When the BOD is enabled, and V
CC
decreases to a value below the trigger level (V
BOT-
in Figure
8-5), the Brown-out Reset is immediately activated. When V
CC
increases above the trigger level
(V
BOT+
in Figure 8-5), the delay counter starts the MCU after the Time-out period t
TOUT
has
expired.
The BOD circuit will only detect a drop in V
CC
if the voltage stays below the trigger level for lon-
ger than t
BOD
given in “System and Reset Characteristics” on page 188.
Figure 8-5. Brown-out Reset During Operation
CC
V
CC
RESET
TIME-OUT
INTERNAL
RESET
V
BOT-
V
BOT+
t
TOUT