
45
4378C–AVR–09/08
AT90PWM1
9.0.5
Brown-out Detection
AT90PWM1 has an On-chip Brown-out Detection (BOD) circuit for monitoring the V
CC level dur-
ing 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 + VHYST/2 and VBOT- = VBOT - VHYST/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 VCC = VBOT 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 (VBOT- in Figure 9-5), the Brown-out Reset is immediately activated. When V
CC increases above the trigger level
(V
BOT+ in Figure 9-5), the delay counter starts the MCU after the Time-out period tTOUT 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
Table 9-2.
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 Characteristic
s(1)Symbol
Parameter
Min.
Typ.
Max.
Units
V
HYST
Brown-out Detector Hysteresis
70
mV
tBOD
Min Pulse Width on Brown-out Reset
2
s