Datasheet

 2018 Microchip Technology Inc. Data Sheet Complete DS40002060A-page 37

ATtiny4/5/9/10

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. Electrical parameters of the reset circuitry are

defined in section “System and Reset Characteristics” on page 127.

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 start up sequence is described in “Starting from Reset” on page 30.

9.2 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 (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

9.2.1 Power-on Reset

A Power-on Reset (POR) pulse is generated by an on-chip detection circuit. The detection level is defined in section “Sys-

tem and Reset Characteristics” on page 127. The POR is activated whenever V

is below the detection level. The POR

circuit can be used to trigger the Start-up Reset, as well as to detect a failure in supply voltage.

Reset Flag Register

(RSTFLR)

Delay Counters

TIMEOUT

WDRF

EXTRF

PORF

DATA B US

Clock

Generator

SPIKE

FILTER

Pull-up Resistor

Watchdog

Oscillator

Power-on Reset

Circuit

VLM