Figure 10-3. MCU Start-up, RESET Extended Externally
VPOT
VCC
VRST
RESET
tTOUT
TIME-OUT
INTERNAL
RESET
10.4 External Reset
An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the
minimum pulse width (see “System and Reset Characteristics” on page 308) 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 – VRST – on its positive edge, the
delay counter starts the MCU after the Time-out period – tTOUT – has expired. The External Reset
can be disabled by the RSTDISBL fuse, see Table 27-6 on page 288.
Figure 10-4. External Reset During Operation
CC
10.5 Brown-out Detection
ATmega48/88/168 has an On-chip Brown-out Detection (BOD) circuit for monitoring the VCC
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 VBOT+
=
V
BOT + VHYST/2 and VBOT- = VBOT - VHYST/2.When the BOD is enabled, and VCC decreases to a
value below the trigger level (VBOT- in Figure 10-5), the Brown-out Reset is immediately acti-
vated. When VCC increases above the trigger level (VBOT+ in Figure 10-5), the delay counter
starts the MCU after the Time-out period tTOUT has expired.
The BOD circuit will only detect a drop in VCC if the voltage stays below the trigger level for
longer than tBOD given in “System and Reset Characteristics” on page 308.
48
ATmega48/88/168
2545M–AVR–09/07
ATMEL [ ATMEL ]
