Rainbow Electronics AT90LS8535 - Manual (Page 50)

50
AT90S/LS8535
1041H–11/01
1. In the same operation, write a logical “1” to WDTOE and WDE. A logical “1” must
be written to WDE even though it is set to “1” before the disable operation starts.
2. Within the next four clock cycles, write a logical “0” to WDE. This disables the
Watchdog.
• Bits 2..0 – WDP2, WDP1, WDP0: Watchdog Timer Prescaler 2, 1 and 0
The WDP2, WDP1 and WDP0 bits determine the Watchdog Timer prescaling when the
Watchdog Timer is enabled. The different prescaling values and their corresponding
time-out periods are shown in Table 21.
Note: The frequency of the Watchdog oscillator is voltage-dependent as shown in the Electrical
Characteristics section.
The WDR (Watchdog Reset) instruction should always be executed before the Watchdog
Timer is enabled. This ensures that the reset period will be in accordance with the
Watchdog Timer prescale settings. If the Watchdog Timer is enabled without reset, the
Watchdog Timer may not start to count from zero.
To avoid unintentional MCU resets, the Watchdog Timer should be disabled or reset
before changing the Watchdog Timer Prescale Select.
Table 21. Watchdog Timer Prescale Select
WDP2 WDP1 WDP0
Number of WDT
Oscillator Cycles Typical Time-out
at V
CC
= 3.0V
Typical Time-out
at V
CC
= 5.0V
0 0 0 16K cycles 47 ms 15 ms
0 0 1 32K cycles 94 ms 30 ms
0 1 0 64K cycles 0.19 s 60 ms
0 1 1 128K cycles 0.38 s 0.12 s
1 0 0 256K cycles 0.75 s 0.24 s
1 0 1 512K cycles 1.5 s 0.49 s
1 1 0 1,024K cycles 3.0 s 0.97 s
1 1 1 2,048K cycles 6.0 s 1.9 s