AT90USB646 datasheet(65/456 Pages) ATMEL

65 / 456 page

7593L–AVR–09/12

AT90USB64/128

Note: If the Watchdog is accidentally enabled, for example by a runaway pointer or brown-out

condition, the device will be reset and the Watchdog Timer will stay enabled. If the code is not

set up to handle the Watchdog, this might lead to an eternal loop of time-out resets. To avoid this

situation, the application software should always clear the Watchdog System Reset Flag

(WDRF) and the WDE control bit in the initialization routine, even if the Watchdog is not in use.

The following code example shows one assembly and one C function for changing the time-out

value of the Watchdog Timer.

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.

9.8.1

WDTCSR – Watchdog Timer Control Register

Assembly code example

(1)

WDT_Prescaler_Change:

; Turn off global interrupt

cli

; Reset Watchdog Timer

wdr

; Start timed sequence

r16, WDTCSR

ori

r16, (1<<WDCE) | (1<<WDE)

out

WDTCSR, r16

; --

Got four cycles to set the new values from here -

; Set new prescaler(time-out) value = 64K cycles (~0.5 s)

ldi

r16, (1<<WDE) | (1<<WDP2) | (1<<WDP0)

out

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();

}

Bit

7654

WDIF

WDIE

WDP3

WDCE

WDE

WDP2

WDP1

WDP0

WDTCSR

Read/write

R/W

Initial value

0000

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