ATmega328P datasheet(46/448 Pages) ATMEL

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8161D–AVR–10/09

ATmega48PA/88PA/168PA/328P

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 ATmega168PA, the instruction placed at the Reset Vector must

be a JMP – Absolute Jump – instruction to the reset handling routine. For the ATmega48PA and

ATmega88PA, 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 section while the Interrupt Vectors are in

the Boot section or vice versa (ATmega88PA/168PA only). The circuit diagram in Figure 10-1 on

page 47 shows the reset logic. Table 28-3 on page 318 defines the electrical parameters of the

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-

ferent selections for the delay period are presented in ”Clock Sources” on page 27.

10.2

Reset Sources

The ATmega48PA/88PA/168PA/328P 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.

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