Datasheet

53
7734Q–AVR–02/12
AT90PWM81/161
7.1.5 Brown-out Detection
AT90PWM81/161 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 Voltageand BODLEVEL = 101 for High Operating
Voltage.
2. Values are guidelines only.
Notes: 1. Values are guidelines only.
When V
CC
decreases to a value below the trigger level (V
BOT-
in Figure 7-5 on page 54), the
Brown-out Reset is immediately activated. When V
CC
increases above the trigger level (V
BOT+
in
Figure 7-5 on page 54), 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 7-3.
Table 7-2. BODLEVEL fuse coding
(1)(2)
.
BODLEVEL 2..0 fuses Min V
BOT
Typ V
BOT
Max V
BOT
Units
111 Forbidden, BOD must be enabled
110 1.8
V
101 (default configuration) 2.7
100 3.9 4.3 4.6
011 2.3
010 2.2
001 1.9
000 2.5 2.0 2.9
Table 7-3. Brown-out characteristics
(1)
.
Symbol Parameter Minimum Typical Maximum Units
V
HYST
Brown-out detector hysteresis 70 mV
t
BOD
Minimum pulse width on brown-out reset 2 µs