Datasheet

7799D–AVR–11/10

ATmega8U2/16U2/32U2

function of the Watchdog System Reset mode. If the interrupt is not executed before the next

time-out, a System Reset will be applied.

• Bit 4 - WDCE: Watchdog Change Enable

This bit is used in timed sequences for changing WDE and prescaler bits. To clear the WDE bit,

and/or change the prescaler bits, WDCE must be set.

Once written to one, hardware will clear WDCE after four clock cycles.

• Bit 3 - WDE: Watchdog System Reset Enable

WDE is overridden by WDRF in MCUSR. This means that WDE is always set when WDRF is

set. To clear WDE, WDRF must be cleared first. This feature ensures multiple resets during con-

ditions causing failure, and a safe start-up after the failure.

• Bit 5, 2:0 - WDP[3:0]: Watchdog Timer Prescaler 3, 2, 1 and 0

The WDP3..0 bits determine the Watchdog Timer prescaling when the Watchdog Timer is run-

ning. The different prescaling values and their corresponding time-out periods are shown in

Table on page 58.

10.5.3 WDTCKD – Watchdog Timer Clock Divider Register

• Bit 7:6 - Res: Reserved bits

These bits are reserved and will always read as zero.

• Bit 5 - WDEWIFCL: Watchdog Early Warning Flag Clear Mode

When this bit has been set by software, the WDEWIF interrupt flag is not cleared by hardware

when entering the Watchdog Interrupt subroutine (it has to be cleared by software by writing a

logic one to the flag).

When cleared, the WDEWIF is cleared by hardware when executing the corresponding interrupt

handling vector.

• Bit 4 - WCLKD2 bit: Watchdog Timer Clock Divider

See “Bit 1:0 - WCLKD[1:0]: Watchdog Timer Clock Divider” on page 58.

Table 10-1. Watchdog Timer Configuration

WDTON (Fuse) WDE WDIE Mode Action on 2x Time-out

1 (unprogrammed) 0 0 Stopped None

1 (unprogrammed) 0 1 Interrupt Mode Interrupt

1 (unprogrammed) 1 0 System Reset Mode Reset

1 (unprogrammed) 1 1

Interrupt and System

Reset Mode

Interrupt, then go to

System Reset Mode

0 (programmed) x x System Reset Mode Reset

Bit 7 6 5 4 3 2 1 0

(0x62) - - WDEWIF-

WCLKD2 WDEWIF WDEWIE WCLKD1 WCLKD0 WDTCKD

Read/Write R R R/W R/W R/W R/W R/W R/W

Initial Value 0 0 0 0 0 0 0 0