ATtiny87-15XZ datasheet(49/282 Pages) ATMEL

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7728D–AVR–07/09

ATtiny87/ATtiny167

System Control and Reset

6.1

Reset

6.1.1

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 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. The circuit diagram in Figure 6-1 shows the reset circuit. Tables in Section 22.5

“RESET Characteristics” on page 246 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 Section 4.2 “Clock

Sources” on page 25.

6.1.2

Reset Sources

The ATtiny87/167 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 Vcc is below the Brown-out

Reset threshold (V

BOT) and the Brown-out Detector is enabled.

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