User Manual

46
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
8271E–AVR–07/2012
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. 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 pro-
gram 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 (ATmega88A/88PA/168A/168PA/328/328P only). The circuit diagram
in Figure 11-1 on page 47 shows the reset logic. Table 29-12 on page 310 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 different selections for the delay period are presented in ”Clock
Sources” on page 27.
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.