Datasheet
61
4341H–MP3–10/07
AT8xC51SND2C/MP3B
12.3 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 47).
The WDT time-out period can be adjusted using WTO2:0 bits located in the WDTPRG register
accordingly to the formula shown in Figure 12-3. In this formula, WTOval represents the decimal
value of WTO2:0 bits. Table 12-1 reports the time-out period depending on the WDT frequency.
Figure 12-3. WDT Time-Out Formula
Notes: 1. These frequencies are achieved in X1 mode or in X2 mode when WTX2 = 1:
F
WDT
= F
OSC
÷ 2.
2. These frequencies are achieved in X2 mode when WTX2 = 0: F
WDT
= F
OSC
.
12.3.1 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 AT8xC51SND2C 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 over-
flows. The interrupt service routine then clears the WDT, reloads the peripheral Timer for the
next service period and puts the AT8xC51SND2C 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 ter-
minated 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 12-1. WDT Time-Out Computation
WTO2 WTO1 WTO0
F
WDT
(ms)
6 MHz
(1)
8 MHz
(1)
10 MHz
(1)
12 MHz
(2)
16 MHz
(2)
20 MHz
(2)
0 0 0 16.38 12.28 9.83 8.19 6.14 4.92
0 0 1 32.77 24.57 19.66 16.38 12.28 9.83
0 1 0 65.54 49.14 39.32 32.77 24.57 19.66
0 1 1 131.07 98.28 78.64 65.54 49.14 39.32
1 0 0 262.14 196.56 157.29 131.07 98.28 78.64
1 0 1 524.29 393.1 314.57 262.14 196.56 157.29
1 1 0 1049 786.24 629.15 524.29 393.12 314.57
1 1 1 2097 1572 1258 1049 786.24 629.15
WDT
TO
=
F
WDT
6
⋅ ((
2
14
⋅
2
WTOval
) – 1)