ATmega128(L)
Figure 25. External Reset During Operation
CC
Brown-out Detection
ATmega128 has an On-chip Brown-out Detection (BOD) circuit for monitoring the VCC level dur-
ing operation by comparing it to a fixed trigger level. The trigger level for the BOD can be
selected by the fuse BODLEVEL to be 2.7V (BODLEVEL unprogrammed), or 4.0V (BODLEVEL
programmed). The trigger level has a hysteresis to ensure spike free Brown-out Detection. The
hysteresis on the detection level should be interpreted as VBOT+ = VBOT + VHYST/2 and VBOT-
BOT - VHYST/2.
=
V
The BOD circuit can be enabled/disabled by the fuse BODEN. When the BOD is enabled
(BODEN programmed), and VCC decreases to a value below the trigger level (VBOT- in Figure
26), the Brown-out Reset is immediately activated. When VCC increases above the trigger level
(VBOT+ in Figure 26), the delay counter starts the MCU after the time-out period tTOUT has
expired.
The BOD circuit will only detect a drop in VCC if the voltage stays below the trigger level for
longer than tBOD given in Table 19.
Figure 26. Brown-out Reset During Operation
V
V
BOT+
CC
V
BOT-
RESET
t
TOUT
TIME-OUT
INTERNAL
RESET
53
2467P–AVR–08/07
ATMEL [ ATMEL ]
