ATtiny261A datasheet(40/292 Pages) ATMEL

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8197A–AVR–10/09

ATtiny261A/461A/861A

8.2

Reset Sources

The ATtiny261A/461A/861A has four 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.

• Brown-out Reset. The MCU is reset when the supply voltage VCC is below the Brown-out

Reset threshold (VBOT) and the Brown-out Detector is enabled.

8.2.1

Power-on Reset

A Power-on Reset (POR) pulse is generated by an On-chip detection circuit. The detection level

is defined in “System and Reset Characteristics” on page 187. The POR is activated whenever

VCC 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.

A Power-on Reset (POR) circuit ensures that the device is reset from Power-on. Reaching the

Power-on Reset threshold voltage invokes the delay counter, which determines how long the

device is kept in RESET after VCC rise. The RESET signal is activated again, without any delay,

when VCC decreases below the detection level.

Figure 8-2.

MCU Start-up, RESET Tied to VCC

Figure 8-3.

MCU Start-up, RESET Extended Externally

RESET

TIME-OUT

INTERNAL

RESET

TOUT

POT

RST

RESET

TIME-OUT

INTERNAL

RESET

TOUT

POT

RST

VCC

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