ATMEGA8 datasheet(35/303 Pages) ATMEL

35 / 303 page

ATmega8(L)

2486M–AVR–12/03

System Control and

Reset

Resetting the AVR

During Reset, all I/O Registers are set to their initial values, and the program starts exe-

cution from the Reset Vector. If the program never enables an interrupt source, the

Interrupt Vectors are not used, and regular program code can be placed at these loca-

tions. 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 14

shows the Reset Logic. Table 15 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

CKSEL Fuses. The different selections for the delay period are presented in “Clock

Sources” on page 24.

Reset Sources

The ATmega8 has four sources of Reset:

•

Power-on Reset. The MCU is reset when the supply voltage is below the Power-on

Reset threshold (VPOT).

•

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 (VBOT) and the Brown-out Detector is enabled.

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