ATMEGA8_08 Datasheet, PDF (43/308 Pages) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

ATmega8(L)
Watchdog Timer
The Watchdog Timer is clocked from a separate On-chip Oscillator which runs at 1 MHz. This is
the typical value at V_CC = 5V. See characterization data for typical values at other V_CC levels. By
controlling the Watchdog Timer prescaler, the Watchdog Reset interval can be adjusted as
shown in Table 17 on page 44. The WDR – Watchdog Reset – instruction resets the Watchdog
Timer. The Watchdog Timer is also reset when it is disabled and when a Chip Reset occurs.
Eight different clock cycle periods can be selected to determine the reset period. If the reset
period expires without another Watchdog Reset, the ATmega8 resets and executes from the
Reset Vector. For timing details on the Watchdog Reset, refer to page 41.
To prevent unintentional disabling of the Watchdog, a special turn-off sequence must be fol-
lowed when the Watchdog is disabled. Refer to the description of the Watchdog Timer Control
Register for details.
Figure 20. Watchdog Timer
WATCHDOG
OSCILLATOR
Watchdog Timer
Control Register –
WDTCR
Bit
7
6
5
4
3
2
1
0
–
–
–
WDCE
WDE
WDP2 WDP1 WDP0 WDTCR
Read/Write
R
R
R
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
• Bits 7..5 – Res: Reserved Bits
These bits are reserved bits in the ATmega8 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. In Safety Level 1 and 2, this bit
must also be set when changing the prescaler bits. See the Code Examples on page 45.
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