AT43USB325 datasheet(105/109 Pages) ATMEL

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105

3355C–USB–4/05

AT43USB325

12. Errata Sheet

Errata (All Date Codes): Missed Watchdog Timer Reset

12.1

Problem

There is a synchronization problem between the watchdog clock and the AVR clock. Even

though the clock inputs to both the watchdog timer and the AVR core are generated through the

same crystal, the two clock sources are not going through the same PLL. The AVR is clocked at

12 MHz and the watchdog timer is clocked at 1MHz. The WDR (Watchdog Reset) instruction is a

one-clock-cycle instruction. As such, when a watchdog timer reset occurs due to a WDR, the

watchdog timer may miss the reset. This happens frequently if the AVR is clocked much faster

than the watchdog timer.

12.2

Fix/Workaround

A workaround is to invoke the WDR repetitively to ensure that the watchdog timer will be able to

receive the reset signal. If the AVR runs at 12 MHz, the WDR command must be invoked four-

teen times back to back.

The following is the sample code for resetting and arming the watchdog timer, assuming the

AVR is running at 12 MHz:

asm ( "ldi r16,15\n WDR\n WDR\n WDR\n WDR\n WDR\n WDR\n WDR\n WDR\n WDR\n

WDR\n WDR\n WDR\n WDR\n out 0x21,r16 " );

To disarm and disable the watchdog, do the following:

asm ( "ldi r16,0x18\nldi r17,0x10\n\n out 0x21,r16\n out 0x21,r17 " );

Please note that if the AVR runs at 24 MHz, the WDR should be invoked twenty-six times.

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