Datasheet

60
7593L–AVR–09/12
AT90USB64/128
Figure 9-4. External reset during operation.
9.5 Brown-out detection
Atmel AT90USB64/128 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 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
9-5 on page 61), the Brown-out Reset is immediately activated. When V
CC
increases above the
trigger level (V
BOT+
in Figure 9-5 on page 61), 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 Table 9-1 on page 58.
CC
Table 9-2. BODLEVEL fuse coding.
BODLEVEL 2..0 Fuses Min. V
BOT
Typ. V
BOT
Max. V
BOT
Units
111 BOD disabled
110
Reserved101
100
011 2.4 2.6 2.8 V
010 3.2 3.4 3.6
001 3.3 3.5 3.7
000 4.1 4.3 4.5
Table 9-3. 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
I
BOD
Brown-out detector consumption 25 µA