Atmel AT90PWM2, Руководство пользователя

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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 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 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 9-3 .
Table 9-2. BODLEVEL Fuse Coding
(1)(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 Characteristics
(1)
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