10.9 Register Description
10.9.1
MCUSR – MCU Status Register
The MCU Status Register provides information on which reset source caused an MCU reset.
Bit
7
–
6
–
5
–
4
–
3
2
1
0
0x35 (0x55)
Read/Write
Initial Value
WDRF
R/W
BORF
R/W
EXTRF
R/W
PORF
R/W
MCUSR
R
0
R
0
R
0
R
0
See Bit Description
• Bit 7..4: Res: Reserved Bits
These bits are unused bits in the ATmega48/88/168, 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
(0x60)
7
6
5
WDP3
R/W
0
4
WDCE
R/W
0
3
2
WDP2
R/W
0
1
WDP1
R/W
0
0
WDP0
R/W
0
WDIF
WDIE
WDE
R/W
X
WDTCSR
Read/Write
Initial Value
R/W
0
R/W
0
• 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.
54
ATmega48/88/168
2545M–AVR–09/07
ATMEL [ ATMEL ]
