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2545M–AVR–09/07
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 ATmega168, the instruction placed at the Reset Vector must be a
JMP – Absolute Jump – instruction to the reset handling routine. For the ATmega48 and
ATmega88, 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 (ATmega88/168 only). The circuit diagram in Figure 10-1 shows
the reset logic. Table 28-3 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 29.
10.2 Reset Sources
The ATmega48/88/168 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.
