AT80SND1C-ROTIL datasheet(60/210 Pages) ATMEL

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AT8xC51SND1C

4109E–8051–06/03

Watchdog Operation

After reset, the WDT is disabled. The WDT is enabled by writing the sequence 1Eh and

E1h into the WDTRST register. As soon as it is enabled, there is no way except the chip

reset to disable it. If it is not cleared using the previous sequence, the WDT overflows

and forces a chip reset. This overflow generates a high level 96 oscillator periods pulse

on the RST pin to globally reset the application (refer to Section “Power Management”,

page 46).

The WDT time-out period can be adjusted using WTO2:0 bits located in the WDTPRG

sents the decimal value of WTO2:0 bits. Table 66 reports the time-out period depending

on the WDT frequency.

Figure 40. WDT Time-Out Formula

Notes:

1. These frequencies are achieved in X1 mode or in X2 mode when WTX2 = 1:

WDT = FOSC ÷ 2.

2. These frequencies are achieved in X2 mode when WTX2 = 0: FWDT = FOSC.

WDT Behavior during Idle and

Power-down Modes

Operation of the WDT during power reduction modes deserves special attention.

The WDT continues to count while the AT8xC51SND1C is in Idle mode. This means

that you must dedicate some internal or external hardware to service the WDT during

Idle mode. One approach is to use a peripheral Timer to generate an interrupt request

when the Timer overflows. The interrupt service routine then clears the WDT, reloads

the peripheral Timer for the next service period and puts the AT8xC51SND1C back into

Idle mode.

The Power-down mode stops all phase clocks. This causes the WDT to stop counting

and to hold its count. The WDT resumes counting from where it left off if the Power-

down mode is terminated by INT0, INT1 or keyboard interrupt. To ensure that the WDT

does not overflow shortly after exiting the Power-down mode, it is recommended to clear

the WDT just before entering Power-down mode.

The WDT is cleared and disabled if the Power-down mode is terminated by a reset.

Table 66. WDT Time-Out Computation

WTO2

WTO1

WTO0

WDT (ms)

6 MHz

(1)

8 MHz

(1)

10 MHz

(1)

12 MHz

(2)

16 MHz

(2)

20 MHz

(2)

16.38

12.28

9.83

8.19

6.14

4.92

32.77

24.57

19.66

16.38

12.28

9.83

65.54

49.14

39.32

32.77

24.57

19.66

131.07

98.28

78.64

65.54

49.14

39.32

262.14

196.56

157.29

131.07

98.28

78.64

524.29

393.1

314.57

262.14

196.56

157.29

1049

786.24

629.15

524.29

393.12

314.57

2097

1572

1258

1049

786.24

629.15

WDT

TO=

WDT

⋅ ((214 ⋅ 2WTOval) – 1)

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