Atmel ATmega8535L - External Reset; Brown-out Detection

39
ATmega8535(L)
2502K–AVR–10/06
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 15) 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 18. External Reset During Operation
Brown-out Detection ATmega8535 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 programmed). 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 19), the Brown-out Reset is immediately activated. When V
CC
increases
above the trigger level (V
BOT+
in Figure 19), 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 15.
Figure 19. Brown-out Reset During Operation
CC
V
CC
RESET
TIME-OUT
INTERNAL
RESET
V
BOT-
V
BOT+
t
TOUT