44
7810C–AVR–10/12
Atmel ATmega328P [Preliminary]
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. For the ATmega328P, the instruction placed at the Reset Vector must be
an RJMP – Relative 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 sec-
tion while the Interrupt Vectors are in the Boot section. The circuit diagram in
Figure 10-1 onreset 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 ATmega328P has four 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 System Reset. The MCU is reset when the Watchdog Timer period expires and the
Watchdog System Reset mode 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.