Rainbow Electronics ATmega128L - Manual (Page 53)

53
ATmega128(L)
2467B–09/01
Timed Sequences for
Changing the
Configuration of the
Watch Dog Timer The sequence for changing configuration differs slightly between the 3 safety levels.
Separate procedures are described for each level.
Safety Level 0 This mode is compatible with the watchdog operation found in ATmega103. The watch-
dog 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 dis-
able an enabled watchdog timer, the procedure described on
page 51 (WDE bit
description) must be followed.
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 watch dog timer. To disable an
enabled watchdog timer, and/or changing the watch dog time-out, the following proce-
dure 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.
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 watch dog time-out, the following procedure must be followed:
1. In the same operation, write a logical one to WDCE and WDE. Even though the
WDE always is set, the WDE must be written to one to start the timed sequence.
2. Within the next four clock cycles, in the same operation, write the WDP bits as
desired, but with the WDCE bit cleared. The value written to the WDE bit is
irrelevant.