Watchdog Timer Control
Register – WDTCR
Bit
7
–
6
–
5
–
4
WDCE
R/W
0
3
WDE
R/W
0
2
WDP2
R/W
0
1
WDP1
R/W
0
0
WDP0
R/W
0
WDTCR
Read/Write
Initial Value
R
0
R
0
R
0
• Bits 7..5 – Reserved Bits
These bits are reserved bits for future use.
• Bit 4 – WDCE: Watchdog Change Enable
This bit must be set when the WDE bit is written to logic zero. Otherwise, the Watchdog
will not be disabled. Once written to one, hardware will clear this bit after four clock
cycles. Refer to the description of the WDE bit for a Watchdog disable procedure. This
bit must also be set when changing the prescaler bits. See “Timed Sequences for
Changing the Configuration of the Watchdog Timer” on page 55
• Bit 3 – WDE: Watchdog Enable
When the WDE is written to logic one, the Watchdog Timer is enabled, and if the WDE is
written to logic zero, the Watchdog Timer function is disabled. WDE can only be cleared
if the WDCE bit has logic level one. To disable an enabled Watchdog Timer, the follow-
ing procedure must be followed:
1. In the same operation, write a logic one to WDCE and WDE. A logic one 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 logic 0 to WDE. This disables the
Watchdog.
In safety level 2, it is not possible to disable the Watchdog Timer, even with the algo-
rithm described above. See “Timed Sequences for Changing the Configuration of the
Watchdog Timer” on page 55
• 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
Timeout Periods are shown in Table 24.
Table 24. Watchdog Timer Prescale Select
Number of WDT
Oscillator Cycles
Typical Time-out
at VCC = 3.0V
Typical Time-out
at VCC = 5.0V
WDP2 WDP1 WDP0
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
16K cycles
32K cycles
17.1 ms
34.3 ms
68.5 ms
0.14 s
0.27 s
0.55 s
1.1 s
16.3 ms
32.5 ms
65 ms
0.13 s
0.26 s
0.52 s
1.0 s
64K cycles
128K cycles
256K cycles
512K cycles
1,024K cycles
2,048K cycles
2.2 s
2.1 s
54
AT90CAN128
4250E–CAN–12/04
ATMEL [ ATMEL ]
