C8051F70X_0910 Datasheet, PDF (154/290 Pages) Silicon Laboratories – Mixed Signal ISP Flash MCU Family

C8051F70x/71x
24. Watchdog Timer
The MCU includes a programmable Watchdog Timer (WDT) running off the system clock. A WDT overflow
will force the MCU into the reset state. To prevent the reset, the WDT must be restarted by application soft-
ware before overflow. If the system experiences a software or hardware malfunction preventing the soft-
ware from restarting the WDT, the WDT will overflow and cause a reset.
Following a reset the WDT is automatically enabled and running with the default maximum time interval. If
desired the WDT can be disabled by system software or locked on to prevent accidental disabling. Once
locked, the WDT cannot be disabled until the next system reset. The state of the /RST pin is unaffected by
this reset.
The WDT consists of a 21-bit timer running from the programmed system clock. The timer measures the
period between specific writes to its control register. If this period exceeds the programmed limit, a WDT
reset is generated. The WDT can be enabled and disabled as needed in software, or can be permanently
enabled if desired. Watchdog features are controlled via the Watchdog Timer Control Register (WDTCN)
shown in SFR Definition 24.1.
24.1. Enable/Reset WDT
The watchdog timer is both enabled and reset by writing 0xA5 to the WDTCN register. The user's applica-
tion software should include periodic writes of 0xA5 to WDTCN as needed to prevent a watchdog timer
overflow. The WDT is enabled and reset as a result of any system reset.
24.2. Disable WDT
Writing 0xDE followed by 0xAD to the WDTCN register disables the WDT. The following code segment
illustrates disabling the WDT:
CLR EA
MOV WDTCN,#0DEh
MOV WDTCN,#0ADh
SETB EA
; disable all interrupts
; disable software watchdog timer
; re-enable interrupts
The writes of 0xDE and 0xAD must occur within 4 clock cycles of each other, or the disable operation is
ignored. Interrupts should be disabled during this procedure to avoid delay between the two writes.
24.3. Disable WDT Lockout
Writing 0xFF to WDTCN locks out the disable feature. Once locked out, the disable operation is ignored
until the next system reset. Writing 0xFF does not enable or reset the watchdog timer. Applications always
intending to use the watchdog should write 0xFF to WDTCN in the initialization code.
24.4. Setting WDT Interval
WDTCN.[2:0] control the watchdog timeout interval. The interval is given by the following equation:
4^(3+WDTCN[2-0]) x Tsysclk ;where Tsysclk is the system clock period.
For a 3 MHz system clock, this provides an interval range of 0.021 to 349.5 ms. WDTCN.7 must be logic 0
when setting this interval. Reading WDTCN returns the programmed interval. WDTCN.[2:0] reads 111b
after a system reset.
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