Rainbow Electronics AT90S8515 - Manual (Page 43)

43
AT90S8515
0841G–09/01
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 one 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 14.
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 reset, the Watchdog Timer should be disabled or reset
before changing the Watchdog Timer Prescale Select.
Table 14. 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 47.0 ms 15.0 ms
0 0 1 32K cycles 94.0 ms 30.0 ms
0 1 0 64K cycles 0.19 s 60.0 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