ATmega16 datasheet(37/357 Pages) ATMEL

37 / 357 page

2466S–AVR–05/09

ATmega16(L)

System Control

and Reset

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. 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 Figure 15 shows the reset

logic. Table 15 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 selec-

tions for the delay period are presented in “Clock Sources” on page 25.

Reset Sources

The ATmega16 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” on page 228 for details.

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