Datasheet

15. System Control and Reset

15.1 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 an Absolute Jump instruction (JMP) to

the reset handling routine for ATmega168PA. The instruction placed at the Reset vector must be a

Relative Jump instruction (RJMP) to the reset handling routine for ATmega48PA and ATmega88PA. 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 (only for ATmega88PAand

ATmega168PA). The circuit diagram in the next section shows the reset logic.

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 the normal operation starts. The time-out period of the

delay counter is defined by the user through the SUT and CKSEL fuses. The different selections for the

delay period are presented in the System Clock and Clock Options chapter.