The following code example shows one assembly and one C function for changing the time-out
value of the Watchdog Timer.
Assembly Code Example(1)
WDT_Prescaler_Change:
; Turn off global interrupt
cli
; Reset Watchdog Timer
wdr
; Start timed sequence
in
r16, WDTCSR
ori
out
r16, (1<<WDCE) | (1<<WDE)
WDTCSR, r16
; -- Got four cycles to set the new values from here -
; Set new prescaler(time-out) value = 64K cycles (~0.5 s)
ldi
out
r16, (1<<WDE) | (1<<WDP2) | (1<<WDP0)
WDTCSR, r16
; -- Finished setting new values, used 2 cycles -
; Turn on global interrupt
sei
ret
C Code Example(1)
void WDT_Prescaler_Change(void)
{
__disable_interrupt();
__watchdog_reset();
/* Start timed equence */
WDTCSR |= (1<<WDCE) | (1<<WDE);
/* Set new prescaler(time-out) value = 64K cycles (~0.5 s) */
WDTCSR = (1<<WDE) | (1<<WDP2) | (1<<WDP0);
__enable_interrupt();
}
Note:
1. The example code assumes that the part specific header file is included.
Note: The Watchdog Timer should be reset before any change of the WDP bits, since a change
in the WDP bits can result in a time-out when switching to a shorter time-out period.
8.2.1
Watchdog Timer Control Register - WDTCSR
Bit
7
6
5
4
3
2
1
0
WDIF
WDIE
WDP3
R/W
0
WDCE
R/W
0
WDE
R/W
X
WDP2
R/W
0
WDP1
R/W
0
WDP0
R/W
0
WDTCSR
Read/Write
Initial Value
R/W
0
R/W
0
• Bit 7 - WDIF: Watchdog Interrupt Flag
This bit is set when a time-out occurs in the Watchdog Timer and the Watchdog Timer is config-
ured for interrupt. WDIF is cleared by hardware when executing the corresponding interrupt
66
AT90USB64/128
7593A–AVR–02/06
ATMEL [ ATMEL ]
