Datasheet

15. System Control and Reset
15.1. Resetting the AVR
During Reset, all I/O Registers are set to their initial values, and the program starts execution from the
Reset Vector. 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
the following section shows the Reset Logic. The Table in System and Reset Characteristics 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.
Related Links
System and Reset Characteristics on page 418
Clock Sources on page 53
15.2. Reset Sources
The ATmega64A has five 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.
• JTAG AVR Reset. The MCU is reset as long as there is a logic one in the Reset Register, one of the
scan chains of the JTAG system. Refer to the section IEEE 1149.1 (JTAG) Boundary-scan for
details.
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
67