System control and reset, 1 resetting the avr, System control and – Rainbow Electronics ATtiny43U User Manual

Page 48: Attiny43u

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48

8048B–AVR–03/09

ATtiny43U

9.

System Control and Reset

9.1

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.

Table 20-4 on page 158

defines the electrical parameters of the reset circuitry.

Figure 9-1.

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 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 24

.

MCU Status

Register (MCUSR)

Brown-out

Reset Circuit

BODLEVEL [2..0]

Delay Counters

CKSEL[1:0]

CK

TIMEOUT

WDRF

BORF

EXTRF

PORF

DATA BUS

Clock

Generator

SPIKE

FILTER

Pull-up Resistor

Watchdog

Oscillator

SUT[1:0]

Power-on Reset

Circuit

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