AT90S/LS2333 and AT90S/LS4433
21
Figure 26. MCU Start-Up, RESET Controlled Externally
External Reset
An external reset is generated by a low level on the RESET pin. Reset pulses longer than 50 ns 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 timer starts the MCU after the Time-out period t
TOUT
has
expired.
Figure 27. External Reset During Operation
Brown-Out Detection
AT90S2333/4433 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,
in 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).
VCC
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
