ATmega328P datasheet(54/448 Pages) ATMEL

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8161D–AVR–10/09

ATmega48PA/88PA/168PA/328P

10.9

10.9.1

MCUSR – MCU Status Register

The MCU Status Register provides information on which reset source caused an MCU reset.

• Bit 7..4: Res: Reserved Bits

These bits are unused bits in the ATmega48PA/88PA/168PA/328P, and will always read as

zero.

• Bit 3 – WDRF: Watchdog System Reset Flag

This bit is set if a Watchdog System Reset occurs. The bit is reset by a Power-on Reset, or by

writing a logic zero to the flag.

• Bit 2 – BORF: Brown-out Reset Flag

This bit is set if a Brown-out Reset occurs. The bit is reset by a Power-on Reset, or by writing a

logic zero to the flag.

• Bit 1 – EXTRF: External Reset Flag

This bit is set if an External Reset occurs. The bit is reset by a Power-on Reset, or by writing a

logic zero to the flag.

• Bit 0 – PORF: Power-on Reset Flag

This bit is set if a Power-on Reset occurs. The bit is reset only by writing a logic zero to the flag.

To make use of the Reset Flags to identify a reset condition, the user should read and then

Reset the MCUSR as early as possible in the program. If the register is cleared before another

reset occurs, the source of the reset can be found by examining the Reset Flags.

10.9.2

WDTCSR – Watchdog Timer Control Register

• Bit 7 - WDIF: Watchdog Interrupt Flag

This bit is set when a time-out occurs in the Watchdog Timer and the Watchdog Timer is config-

ured for interrupt. WDIF is cleared by hardware when executing the corresponding interrupt

handling vector. Alternatively, WDIF is cleared by writing a logic one to the flag. When the I-bit in

SREG and WDIE are set, the Watchdog Time-out Interrupt is executed.

• Bit 6 - WDIE: Watchdog Interrupt Enable

When this bit is written to one and the I-bit in the Status Register is set, the Watchdog Interrupt is

enabled. If WDE is cleared in combination with this setting, the Watchdog Timer is in Interrupt

Mode, and the corresponding interrupt is executed if time-out in the Watchdog Timer occurs. If

WDE is set, the Watchdog Timer is in Interrupt and System Reset Mode. The first time-out in the

Bit

76543210

0x35 (0x55)

––––

WDRF

BORF

EXTRF

PORF

MCUSR

Read/Write

RRRR

R/W

Initial Value

0000

See Bit Description

Bit

654

(0x60)

WDIF

WDIE

WDP3

WDCE

WDE

WDP2

WDP1

WDP0

WDTCSR

Read/Write

R/W

Initial Value

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