Datasheet
41
ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
8.4 External Reset
An external reset is generated by a low level on the RESET pin. Reset pulses longer than the minimum pulse width (see
Table 8-1 on page 40) will generate a reset, even if the clock is not running. Shorter pulses are not guaranteed to generate a
reset. When the applied signal reaches the reset threshold voltage – V
RST
– on its positive edge, the delay counter starts the
MCU after the time-out period – t
TOUT
–
has expired. The external reset can be disabled by the RSTDISBL fuse, see
Table 25-6 on page 244.
Figure 8-4. External Reset During Operation
8.5 Brown-out Detection
Atmel
®
ATmega48/88/168 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.
t
TOUT
RESET
V
CC
INTERNAL
RESET
TIME-OUT
V
RST
Table 8-2. BODLEVEL Fuse Coding
(1)
BODLEVEL 2..0 Fuses Min V
BOT
Typ V
BOT
Max V
BOT
Unit
111 BOD Disabled
110 1.7
(2)
1.8 2.0
(2)
V101 2.5
(2)
2.7 2.9
(2)
100 4.1
(2)
4.3 4.5
(2)
011
Reserved
010
001
000
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 guarantees 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 = 110 and BODLEVEL = 101 for Atmel ATmega48V/88V/168V, and
BODLEVEL = 101 and BODLEVEL = 101 for Atmel ATmega48/88/168.
2. Min/max values applicable for Atmel ATmega48.
Table 8-3. Brown-out Characteristics
Parameter Symbol Min Typ Max Unit
Brown-out detector hysteresis V
HYST
50 mV
Min pulse width on brown-out reset t
BOD
ns