TC1796
System Units (Vol. 1 of 2)
Watchdog Timer
User’s Manual 16-20 V2.0, 2007-07
WDT, V2.0
Note: In Prewarning Mode, the device does not need to wait for the end of the Time-out
Period and the reset. After having saved required state in the NMI routine,
software can execute a software reset to shorten the time. However, the state of
the Watchdog Status Register should also be saved in this case, since the error
flags contained in it will be cleared due to the software reset (this is not the case
if the Watchdog reset is awaited).
16.4.7.2 Normal Period
The duration of Normal Mode can be varied by two parameters, the input clock and the
reload value.
The system clock, f
SYS
, can be divided by either 256 or 16384. WDT_SR.WDTIS selects
the input clock divider. The default value of WDTIS after reset is 0, corresponding to a
frequency of f
SYS
/16384.
When the WDT is serviced in Normal Mode, it is reloaded with the 16-bit reload value,
WDT_CON0.WDTREL.
The WDT Period can be varied over a wide range with these two parameters. Again,
since there is no synchronization of the clock divider to the mode transitions of the
Watchdog, the next clock pulse, incrementing the counter, may come after one clock
divider period, or immediately after the counter was reloaded. Thus, it is recommended
that the reload value is programmed to a value that results to one clock pulse more than
the required period. Using a reload value of FFFF
H
could, therefore, lead to an
immediate overflow of the timer. Thus, the examples given in Table 16-9 are only shown
with a maximum reload value of FFFE
H
.
Table 16-9 Timer Periods in Normal Mode
WDTIS Reload
Value
Min./
Max.
Period Example
@ f
SYS
= 75 MHz
0 0000
H
min. 65535 × 16384/f
SYS
= 1073725440/f
SYS
14.3 s
max. 65536 × 16384/f
SYS
= 1073741824/f
SYS
14.3 s
FFFE
H
min. 1 × 16384/f
SYS
= 16384/f
SYS
218 µs
max. 2 × 16384/f
SYS
= 32768/f
SYS
437 µs
1 0000
H
min. 65535 × 256/f
SYS
= 16776960/f
SYS
224 ms
max. 65536 × 256/f
SYS
= 16777216/f
SYS
224 ms
FFFE
H
min. 1 × 256/f
SYS
= 256/f
SYS
3.5 µs
max. 2 × 256/f
SYS
= 512/f
SYS
6.8 µs
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