47
4317I–AVR–01/08
AT90PWM2/3/2B/3B
Figure 9-1.
Reset Logic
Notes:
1. Values are guidelines only..
2. The Power-on Reset will not work unless the supply voltage has been below V
POT
(falling)
9.0.3
Power-on Reset
A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level
is defined in
. The POR is activated whenever V
CC
is below the detection level. The
POR circuit can be used to trigger the start-up Reset, as well as to detect a failure in supply
voltage.
A Power-on Reset (POR) circuit ensures that the device is reset from Power-on. Reaching the
Power-on Reset threshold voltage invokes the delay counter, which determines how long the
device is kept in RESET after V
CC
rise. The RESET signal is activated again, without any delay,
when V
CC
decreases below the detection level.
Table 9-1.
Reset Characteristics
Symbol
Parameter
Condition
Min.
Typ.
Max.
Units
V
POT
Power-on Reset Threshold
Voltage (rising)
1.4
2.3
V
Power-on Reset Threshold
Voltage (falling)
1.3
2.3
V
V
RST
RESET Pin Threshold Voltage
0.2Vcc
0.85Vcc
V
t
RST
Minimum pulse width on RESET
Pin
400
ns
MCU Status
Register (MCUSR)
Brown-out
Reset Circuit
BODLEVEL [2..0]
Delay Counters
CKSEL[3:0]
CK
TIMEOUT
WDRF
BORF
EXTRF
PORF
DATA BUS
Clock
Generator
Spike
Filter
Pull-up Resistor
Watchdog
Oscillator
SUT[1:0]
Power-on Reset
Circuit