Public Version
Watchdog Timers
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The watchdog timer overflow rate is expressed by: OVF_Rate = (0xFFFF FFFF - WDTn.
+ 1) *
(wd-functional clock period) * PS
With wd-functional clock period = 1 / (wd-functional clock frequency) and PS = 2
(PTV)
.
CAUTION
Internal resynchronization causes any software write to GPTn.
to have
some latency before
is updated with the programmed value:
1.5 * functional clock cycles
≤
write_WSPR_latency
≤
2.5 * functional clock
cycles
Remember to take this latency into account whenever the watchdog must be
started or stopped.
For example, for a timer clock input of 32 kHz with a prescaler ratio value of 0x1 (clock divided by 2) and
WDTi.
[5] PRE bit = 1 (clock divider enabled), the reset period is as listed in
.
Table 16-64. Reset Period Examples
Value
Reset Period
0x0000 0000
74 h 56 min
0xFFFF 0000
4 s
0xFFFF FFF0
1 ms
0xFFFF FFFF
62.5 us
CAUTION
•
Ensure that the reloaded value allows the correct operation of the
application. When a WDT is enabled, the software must periodically trigger
a reload before the counter overflows. Hence, the WDTi.
[31:0] value
must be chosen according to the ongoing activity preceding the watchdog
reload.
•
Due to design reasons, WDTi.
[31:0] = 0xFFFF FFFF is a special
case, although such a WDTi.
value is meaningless. When
is programmed with the overflow value, a triggering event
generates a reset/interrupt one functional clock cycle later, even if the WDT
is stopped.
lists the default reset periods for the WDTs.
Table 16-65. Default WDT Time Periods
WDTs
Clock Source
Default Reset Period
DEVICE/IVA2 WDTs
32 kHz
10 s
16.4.3.5 Triggering a Timer Reload
To reload the timer counter and reset the prescaler before reaching overflow, a reload command is
executed by accessing the WDT trigger register (WDTi.
) using a specific reload sequence.
The specific reload sequence is performed whenever the written value on the WDT trigger register
(WDTi.
) differs from its previous value. In this case, reload is executed in the same way as an
overflow autoreload, but without a reset pulse generation.
The timer counter is loaded with the WDT load register value (the WDTi.
[31:0] TIMER_LOAD field),
and the prescaler is reset.
2752
Timers
SWPU177N – December 2009 – Revised November 2010
Copyright © 2009–2010, Texas Instruments Incorporated
