Datasheet

M103C WDTON Safety
Level
WDT Initial
State
How to Disable
the WDT
How to
Change Time-
out
Programmed Unprogrammed 0 Disabled Timed sequence No restriction
Programmed Programmed 2 Enabled Always enabled Timed
sequence
Figure 15-7 Watchdog Timer
WATCHDOG
OS CILLATOR
15.5. Timed Sequences for Changing the Configuration of the Watchdog Timer
The sequence for changing configuration differs slightly between the three safety levels. Separate
procedures are described for each level.
15.5.1. Safety Level 0
This mode is compatible with the Watchdog operation found in ATmega103. The Watchdog Timer is
initially disabled, but can be enabled by writing the WDE bit to 1 without any restriction. The time-out
period can be changed at any time without restriction. To disable an enabled Watchdog Timer, the
procedure described in the bit description for WDE in the WDTCR on page 75 must be followed.
15.5.2. Safety Level 1
In this mode, the Watchdog Timer is initially disabled, but can be enabled by writing the WDE bit to 1
without any restriction. A timed sequence is needed when changing the Watchdog Time-out period or
disabling an enabled Watchdog Timer. To disable an enabled Watchdog Timer, and/or changing the
Watchdog Time-out, the following procedure must be followed:
1. In the same operation, write a logic one to WDCE and WDE. A logic one must be written to WDE
regardless of the previous value of the WDE bit.
2. Within the next four clock cycles, in the same operation, write the WDE and WDP bits as desired,
but with the WDCE bit cleared.
15.5.3. Safety Level 2
In this mode, the Watchdog Timer is always enabled, and the WDE bit will always read as one. A timed
sequence is needed when changing the Watchdog Time-out period. To change the Watchdog Time-out,
the following procedure must be followed:
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
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