atmega103 ATMEL Corporation, atmega103 Datasheet - Page 29

• Manufacturer Part Number: atmega103
• Description: Atmega103 8-bit With 128k Bytes Of In-system Programmable Flash
• Manufacturer: ATMEL Corporation

Watchdog Reset
MCU Status Register –
MCUSR
0945I–AVR–02/07
When the Watchdog times out, it will generate a short reset pulse of 1 XTAL cycle dura-
tion. On the falling edge of this pulse, the delay timer starts counting the Time-out period
t
Figure 27. Watchdog Reset during Operation
The MCU Status Register provides information on which reset source caused an MCU
reset.
• Bits 7..2 – Res: Reserved Bits
These bits are reserved bits in the ATmega103(L) and always read as zero.
• Bit 1 – EXTRF: External Reset Flag
After a Power-on Reset, this bit is undefined (X). It will be set by an external reset. A
Watchdog reset will leave this bit unchanged.
• Bit 0 – PORF: Power-on Reset Flag
This bit is set by a Power-on Reset. A Watchdog Reset or an External Reset will leave
this bit unchanged.
To summarize, Table 7 shows the value of these two bits after the three modes of reset:
Table 7. PORF and EXTRF Values after Reset
To make use of these bits to identify a reset condition, the user software should clear
both the PORF and EXTRF bits as early as possible in the program. Checking the
PORF and EXTRF values is done before the bits are cleared. If the bit is cleared before
TOUT
Bit
$34 ($54)
Read/Write
Initial Value
INTERNAL
Reset Source
Power-on Reset
External Reset
Watchdog Reset
TIME-OUT
TIME-OUT
RESET
RESET
RESET
. Refer to page 55 for details on operation of the Watchdog.
WDT
VCC
R
7
–
0
R
6
–
0
R
5
–
0
1 XTAL Cycle
t
TOUT
R
4
–
0
unchanged
undefined
EXTRF
1
R
3
–
0
R
2
–
0
ATmega103(L)
EXTRF
See bit description
R/W
1
unchanged
unchanged
PORF
PORF
R/W
0
1
MCUSR
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