ATmega169P datasheet(54/393 Pages) ATMEL

54 / 393 page

8018N–AVR–08/09

ATmega169P

10.5

10.5.1

MCUSR – MCU Status Register

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

• 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 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.

10.5.2

WDTCR – Watchdog Timer Control Register

• Bits 7:5 – Res: Reserved Bits

These bits are reserved and will always read as zero.

• Bit 4 – WDCE: Watchdog Change Enable

This bit must be set when the WDE bit is written to logic zero. Otherwise, the Watchdog will not

be disabled. Once written to one, hardware will clear this bit after four clock cycles. Refer to the

description of the WDE bit for a Watchdog disable procedure. This bit must also be set when

changing the prescaler bits. See ”Timed Sequences for Changing the Configuration of the

Watchdog Timer” on page 52.

Bit

765

43210

0x35 (0x55)

–

JTRF

WDRF

BORF

EXTRF

PORF

MCUSR

Read/Write

R/W

Initial Value

See Bit Description

Bit

765

432

(0x60)

–

WDCE

WDE

WDP2

WDP1

WDP0

WDTCR

Read/Write

R/W

Initial Value

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