Manual

25
AT90S/LS4433
1042G–AVR–09/02
Brown-out Detection AT90S4433 has an On-chip Brown-out Detection (BOD) circuit for monitoring the V
CC
level during the operation. The power supply must be decoupled with a 47 nF to 100 nF
capacitor if the BOD function is used. The BOD circuit can be enabled/disabled by the
fuse BODEN. When BODEN is enabled (BODEN programmed), and V
CC
decreases to a
value below the trigger level, the Brown-out Reset is immediately activated. When V
CC
increases above the trigger level, the Brown-out Reset is deactivated after a delay. The
delay is defined by the user in the same way as the delay of POR signal (see Table 5).
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 of 50 mV to ensure spike-free Brown-out Detection.
The BOD circuit will only detect a drop in V
CC
if the voltage stays below the trigger level
for longer than 3 µs for trigger level 4.0V, 7 µs for trigger level 2.7V (typical values).
Figure 28. Brown-out Reset during Operation
Watchdog Reset When the Watchdog times out, it will generate a short reset pulse of one XTAL cycle
duration. On the falling edge of this pulse, the delay timer starts counting the Time-out
period (t
TOUT
). See page 43 for details on operation of the Watchdog.
Figure 29. Watchdog Reset during Operation
VCC
RESET
TIME-OUT
INTERNAL
RESET
V
BOT-
V
BOT+
t
TOUT