48
8271D–AVR–05/11
ATmega48A/PA/88A/PA/168A/PA/328/P
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 Vector. For ATmega168A/168PA/328/328P the instruction placed at the Reset
Vector must be a JMP – Absolute Jump – instruction to the reset handling routine. For the
ATmega 48A/48PA and ATmega88A/88PA, 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 sec-
tion while the Interrupt Vectors are in the Boot section or vice versa
(ATmega88A/88PA/168A/168PA/328/328P only). The circuit diagram in Figure 11-1 on page 49
shows the reset logic. Table 29-12 on page 324 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 28.
11.2 Reset Sources
The ATmega48A/PA/88A/PA/168A/PA/328/P 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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