ATmega8(L)
System Control
and Reset
Resetting the AVR
During Reset, all I/O Registers are set to their initial values, and the program starts execution
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 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 14 on page 38 shows the Reset Logic. Table 15 on
page 38 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 selec-
tions for the delay period are presented in “Clock Sources” on page 26.
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 VCC is below the Brown-out
Reset threshold (VBOT) and the Brown-out Detector is enabled
37
2486AA–AVR–02/2013
ATMEL [ ATMEL ]
