49
4317I–AVR–01/08
AT90PWM2/3/2B/3B
9.0.5
Brown-out Detection
AT90PWM2/2B/3/3B 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.
Notes:
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 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 = 010 for Low Operating Voltage and BODLEVEL = 101 for High Operating Volt-
age .
2. Values are guidelines only.
Notes:
1. Values are guidelines only.
When the BOD is enabled, and V
CC
decreases to a value below the trigger level (V
BOT-
in
), the Brown-out Reset is immediately activated. When V
CC
increases above the trigger level
(V
BOT+
), 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 9-2.
BODLEVEL Fuse Coding
BODLEVEL 2..0 Fuses
Min V
BOT
Typ V
BOT
Max V
BOT
Units
111
BOD Disabled
110
4.5
V
101
2.7
V
100
4.3
V
011
4.4
V
010
4.2
V
001
2.8
V
000
2.6
V
Table 9-3.
Brown-out Characteristics
Symbol
Parameter
Min.
Typ.
Max.
Units
V
HYST
Brown-out Detector Hysteresis
70
mV
t
BOD
Min Pulse Width on Brown-out Reset
2
µs
