Rainbow Electronics ATmega128RFA1 - Manual (Page 178)

178
8266A-MCU Wireless-12/09
ATmega128RFA1
Figure 13-3. MCU Start-up, RSTN Extended Externally
V
CC
RSTN
TIME-OUT
INTERNAL
RESET
V
POT
V
RST
t
TOUT
13.2.2 External Reset
An External Reset is generated by a low level on the RSTN pin. Reset pulses longer
than the minimum pulse width (see
"System and Reset Characteristics" on page 502 )
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 13-4. Reset During Operation
DEVDD
RSTN
TIME-OUT
INTERNAL
RESET
t
TOUT
V
RST
13.2.3 Brown-out Detection
ATmega128RFA1 has an On-chip Brown-out Detection (BOD) circuit for monitoring the
EVDD 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 V
BOT+
= V
BOT
+ V
HYST
/2 and V
BOT-
= V
BOT
- V
HYST
/2.
When the BOD is enabled, and EVDD decreases to a value below the trigger level
(V
BOT-
in Figure 13-5 on page 179 ), the Brown-out Reset is immediately activated. When
EVDD increases above the trigger level (V
BOT+
in Figure 13-5 on page 179 ), the delay
counter starts the MCU after the Time-out period t
TOUT
has expired.
The BOD circuit will only detect a drop in EVDD if the voltage stays below the trigger
level for longer than t
BOD
given in "System and Reset Characteristics" on page 502 .