ATmega128 datasheet(54/386 Pages) ATMEL

54 / 386 page

2467S–AVR–07/09

ATmega128

Watchdog Reset

When the Watchdog times out, it will generate a short reset pulse of 1 CK cycle duration. On the

falling edge of this pulse, the delay timer starts counting the Time-out period t

TOUT. Refer to page

55 for details on operation of the Watchdog Timer.

Figure 27. Watchdog Reset During Operation

MCU Control and

Status Register –

MCUCSR

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

MCU reset.

Note that only EXTRF and PORF are available in ATmega103 compatibility mode.

• Bit 4 – JTRF: JTAG Reset Flag

This bit is set if a reset is being caused by a logic one in the JTAG Reset Register selected by

the JTAG instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic

zero to the flag.

• Bit 3 – WDRF: Watchdog Reset Flag

This bit is set if a Watchdog 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 MCUCSR 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.

Internal Voltage

Reference

ATmega128 features an internal bandgap reference. This reference is used for Brown-out

Detection, and it can be used as an input to the Analog Comparator or the ADC. The 2.56V ref-

erence to the ADC is generated from the internal bandgap reference.

Bit

54321

JTD

–

JTRF

WDRF

BORF

EXTRF

PORF

MCUCSR

Read/Write

R/W

Initial Value

See Bit Description

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