52
2467S–AVR–07/09
ATmega128
2. V
BOT
may be below nominal minimum operating voltage for some devices. For devices where
this is the case, the device is tested down to V
CC
= V
BOT
during the production test. This guar-
antees that a Brown-out Reset will occur before V
CC
drops to a voltage where correct
operation of the microcontroller is no longer guaranteed. The test is performed using
BODLEVEL=1for ATmega128L and BODLEVEL=0 for ATmega128. BODLEVEL=1 is not
applicable forATmega128
Power-on Reset
A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level
is defined in
. The POR is activated whenever V
CC
is below the detection level. The
POR circuit can be used to trigger the Start-up Reset, as well as to detect a failure in supply
voltage.
A Power-on Reset (POR) circuit ensures that the device is reset from Power-on. Reaching the
Power-on Reset threshold voltage invokes the delay counter, which determines how long the
device is kept in RESET after V
CC
rise. The RESET signal is activated again, without any delay,
when V
CC
decreases below the detection level.
Figure 23.
MCU Start-up, RESET Tied to V
CC
.
Figure 24.
MCU Start-up, RESET Extended Externally
External Reset
An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the
minimum pulse width (see
) will generate 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.
V
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
CC
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
V
CC
