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2552K–AVR–04/11
ATmega329/3290/649/6490
10. System Control and Reset
10.1
Resetting the AVR
During reset, all I/O Registers are set to their initial values, and the program starts execution
from the Reset Vector. The instruction placed at the Reset Vector must be a JMP – Absolute
Jump – instruction to the reset handling routine. If the program never enables an interrupt
source, the Interrupt Vectors are not used, and regular program code can be placed at these
locations. This is also the case if the Reset Vector is in the Application section while the Interrupt
Vectors are in the Boot section or vice versa. The circuit diagram in
Figure 10-1 shows the reset
reset circuitry.
The I/O ports of the AVR are immediately reset to their initial state when a reset source goes
active. This does not require any clock source to be running.
After all reset sources have gone inactive, a delay counter is invoked, stretching the internal
reset. This allows the power to reach a stable level before normal operation starts. The time-out
period of the delay counter is defined by the user through the SUT and CKSEL Fuses. The dif-
10.2
Reset Sources
The ATmega329/3290/649/6490 has five sources of reset:
Power-on Reset. The MCU is reset when the supply voltage is below the Power-on Reset
threshold (V
POT).
External Reset. The MCU is reset when a low level is present on the RESET pin for longer
than the minimum pulse length.
Watchdog Reset. The MCU is reset when the Watchdog Timer period expires and the
Watchdog is enabled.
Brown-out Reset. The MCU is reset when the supply voltage V
CC is below the Brown-out
Reset threshold (V
BOT) and the Brown-out Detector is enabled.
JTAG AVR Reset. The MCU is reset as long as there is a logic one in the Reset Register,