Datasheet

36
ATmega8A [DATASHEET]
8159E–AVR–02/2013
11. System Control and Reset
11.1 Resetting the AVR
During Reset, all I/O Registers are set to their initial values, and the program starts execution from the Reset Vec-
tor. 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 11-1 shows the Reset Logic.
Table 26-3 on page 228 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 25.
11.2 Reset Sources
The ATmega8A 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 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 (V
BOT
)
and the Brown-out Detector is enabled.