ATMEGA8 datasheet(41/303 Pages) ATMEL

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ATmega8(L)

2486M–AVR–12/03

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 42. 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 39.

To prevent unintentional disabling of the Watchdog, a special turn-off sequence must be

followed when the Watchdog is disabled. Refer to the description of the Watchdog Timer

Control Register for details.

Figure 20. Watchdog Timer

Watchdog Timer Control

• 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 43.

WATCHDOG

OSCILLATOR

Bit

–

WDCE

WDE

WDP2

WDP1

WDP0

WDTCR

Read/Write

R/W

Initial Value

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