ATMEGA8_14 Datasheet, PDF (40/331 Pages) ATMEL Corporation – High-performance, Low-power Atmel

External Reset
ATmega8(L)
An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the
minimum pulse width (see Table 15 on page 38) 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.
Figure 17. External Reset During Operation
CC
Brown-out Detection
ATmega8 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 fuse BODLEVEL to be 2.7V (BODLEVEL unprogrammed), or 4.0V (BODLEVEL pro-
grammed). 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.
The BOD circuit can be enabled/disabled by the fuse BODEN. When the BOD is enabled
(BODEN programmed), and V_CC decreases to a value below the trigger level (V_BOT- in Figure
18), the Brown-out Reset is immediately activated. When V_CC increases above the trigger level
(V_BOT+ in Figure 18), 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 15 on page 38.
Figure 18. Brown-out Reset During Operation
V_CC
V_BOT-
V_BOT+
RESET
TIME-OUT
t_TOUT
INTERNAL
RESET
40
2486AA–AVR–02/2013