Datasheet

41
8024A–AVR–04/08
ATmega8HVA/16HVA
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. The instruction placed at the Reset Vector must be a JMP – Absolute
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. The circuit diagram in ”Reset Logic” on page 42 shows the reset logic. Table 12-2 on
page 54 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 voltage regulator 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 Fuses. The dif-
ferent selections for the delay period are presented in ”Clock Sources” on page 25.
11.2 Reset Sources
The ATmega8HVA/16HVA has five sources of reset:
• The Power-on Reset module generates a Power-on Reset when the Voltage Regulator starts
up.
• 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 V
REG
is below the Brown-out Reset Threshold, V
BOT
.
See “Brown-out Detection” on page 44.
• debugWIRE Reset. In On-chip Debug mode, the debugWIRE resets the MCU when giving the
Reset command.