Datasheet

60
ATmega640/1280/1281/2560/2561
2549K–AVR–01/07
Brown-out Detection ATmega640/1280/1281/2560/2561 has an On-chip Brown-out Detection (BOD) circuit
for monitoring 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 detec-
tion level should be interpreted as V
BOT+
= V
BOT
+ V
HYST
/2 and V
BOT-
= V
BOT
- V
HYST
/2.
Note: 1. 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 guarantees 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 = 110 for 4 MHz operation of
ATmega640V/1280V/1281V/2560V/2561V, BODLEVEL = 101 for 8 MHz operation of
ATmega640V/1280V/1281V/2560V/2561V and ATmega640/1280/1281, and
BODLEVEL = 100 for 16 MHz operation of ATmega640/1280/1281/2560/2561.
When the BOD is enabled, and V
CC
decreases to a value below the trigger level (V
BOT-
in Figure 28), the Brown-out Reset is immediately activated. When V
CC
increases above
the trigger level (V
BOT+
in Figure 28), 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 longer than t
BOD
given in Table 26.
Figure 28. Brown-out Reset During Operation
Table 27. BODLEVEL Fuse Coding
(1)
BODLEVEL 2:0 Fuses Min V
BOT
Typ V
BOT
Max V
BOT
Units
111 BOD Disabled
110 1.7 1.8 2.0
V101 2.5 2.7 2.9
100 4.1 4.3 4.5
011
Reserved
010
001
000
Table 28. Brown-out Characteristics
Symbol Parameter Min Typ Max Units
V
HYST
Brown-out Detector Hysteresis 50 mV
t
BOD
Min Pulse Width on Brown-out Reset ns
V
CC
RESET
TIME-OUT
INTERNAL
RESET
V
BOT-
V
BOT+
t
TOUT