XC878CLM
Functional Description
Data Sheet
81
V1.1, 2009-08
If the WDT is not serviced before the timer overflow, a system malfunction is assumed.
As a result, the WDT NMI is triggered (assert FNMIWDT) and the reset prewarning is
entered. The prewarning period lasts for 30H count, after which the system is reset
(assert WDTRST).
The WDT has a “programmable window boundary” which disallows any refresh during
the WDT’s count-up. A refresh during this window boundary constitutes an invalid
access to the WDT, causing the reset prewarning to be entered but without triggering the
WDT NMI. The system will still be reset after the prewarning period is over. The window
boundary is from 0000H to the value obtained from the concatenation of WDTWINB and
00H.
After being serviced, the WDT continues counting up from the value (<WDTREL> * 28).
The time period for an overflow of the WDT is programmable in two ways:
The input frequency to the WDT can be selected to be either
f
PCLK/2 or fPCLK/128
The reload value WDTREL for the high byte of WDT can be programmed in register
WDTREL
The period,
P
WDT, between servicing the WDT and the next overflow can be determined
by the following formula:
(3.3)
If the Window-Boundary Refresh feature of the WDT is enabled, the period
P
WDT
between servicing the WDT and the next overflow is shortened if WDTWINB is greater
than WDTREL, see Figure 24. This period can be calculated using the same formula by
replacing WDTREL with WDTWINB. For this feature to be useful, WDTWINB cannot be
smaller than WDTREL.
PWDT
2 1WDTIN
+
6
×
()
216 WDTREL
–
28
×
()
×
fPCLK
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