Rainbow-electronics ATtiny861_V Bedienungsanleitung PDF herunterladen (Seite 39)

2588B–AVR–11/06

ATtiny261/461/861

9. System Control and Reset

9.0.1 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 9-1 shows the reset logic. ”System and Reset Character-

istics” on page 189 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 dif-

ferent selections for the delay period are presented in ”Clock Sources” on page 26.

9.0.2 Reset Sources

The ATtiny261/461/861 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 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

is below the Brown-out Reset

threshold (V

BOT

) and the Brown-out Detector is enabled.

Figure 9-1. Reset Logic

MCU Status

Brown-out

Reset Circuit

BODLEVEL [1..0]

Delay Counters

CKSEL[1:0]

TIMEOUT

WDRF

BORF

EXTRF

PORF

DATA BU S

Clock

Generator

SPIKE

FILTER

Pull-up Resistor

Watchdog

Oscillator

SUT

[

1:0

]

Power-on Reset

Circuit

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