Atmel AVR AT90CAN128, Manual

Page: 59 / 428

59
7679H–CAN–08/08
AT90CAN32/64/128
The following code example shows one assembly and one C function for turning off the WDT.
The example assumes that interrupts are controlled (e.g. by disabling interrupts globally) so that
no interrupts will occur during execution of these functions.
Note: 1. The example code assumes that the part specific header file is included.
7.4 Timed Sequences for Changing the Configuration of the Watchdog Timer
The sequence for changing configuration differs slightly between the two safety levels. Separate
procedures are described for each level.
7.4.1 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 writ-
ten 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.
7.4.2 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:
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.
Assembly Code Example
(1)
WDT_off:
; Write logical one to WDCE and WDE
ldi
r16, (1<<WDCE)|(1<<WDE)
sts WDTCR, r16
; Turn off WDT
ldi
r16, (0<<WDE)
sts WDTCR, r16
ret
C Code Example
(1)
void WDT_off( void )
{
/* Write logical one to WDCE and WDE */
WDTCR = (1<<WDCE) | (1<<WDE);
/* Turn off WDT */
WDTCR = 0x00;
}