ATtiny5 datasheet(27/168 Pages) ATMEL

27 / 168 page

8127C–AVR–10/09

ATtiny4/5/9/10

8. System Control and Reset

8.1

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 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 8-1 shows the reset logic. Electrical parameters of the reset circuitry are

defined in section “System and Reset Characteristics” on page 119.

Figure 8-1.

Reset Logic

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 start up

sequence is described in “Starting from Reset” on page 20.

8.2

Reset Sources

The ATtiny4/5/9/10 have three sources of reset:

• Power-on Reset. The MCU is reset when the supply voltage is below the Power-on Reset

threshold (VPOT)

• 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

Reset Flag Register

(RSTFLR)

Delay Counters

TIMEOUT

DATA BUS

Clock

Generator

SPIKE

FILTER

Pull-up Resistor

Watchdog

Oscillator

Power-on Reset

Circuit

ATtiny5 Scheda tecnica(PDF) 27 Page - ATMEL Corporation