WATCHDOG
Table 20-103. Prescaler Clock Ratio Values
WDT_WCLR[5] PRE
WDT_WCLR[4:2] PTV
Clock Divider (PS)
0
X
1
1
0
1
1
1
2
1
2
4
1
3
8
1
4
16
1
5
32
1
6
64
1
7
128
Thus the watchdog timer overflow rate is expressed as:
OVF_Rate = (FFFF FFFFh – WD 1) × (wd-functional clock period) × PS
where wd-functional clock period = 1/(wd-functional clock frequency) and PS = 2
(PTV)
CAUTION
Internal resynchronization causes some latency in any software write to
WDT_WSPR before WDT_WSPR is updated with the programmed value:
1.5 × functional clock cycles
≤
write_WDT_WSPR_latency
≤
2.5 × functional
clock cycles
Remember to consider this latency whenever the watchdog timer must be
started or stopped.
For example, for a timer clock input of 32 kHz with a prescaler ratio value of 1 (clock divided by 2) and
WDT_WCLR[5] PRE = 1 (clock divider enabled), the reset period is as listed in
Table 20-104. Reset Period Examples
WDT_WLDR Value
Reset Period
0000 0000h
74 h 56 min
FFFF 0000h
4 s
FFFF FFF0h
1 ms
FFFF FFFFh
62.5 us
CAUTION
•
Ensure that the reloaded value allows the correct operation of the
application. When a watchdog timer is enabled, software must periodically
trigger a reload before the counter overflows. Hence, the value of the
WDT_WLDR[31:0] bit field must be chosen according to the ongoing activity
preceding the watchdog reload.
•
Due to design reasons, WDT_WLDR[31:0] = FFFF FFFFh is a special case,
although such a value of WDT_WLDR is meaningless. When WDT_WLDR
is programmed with the overflow value, a triggering event generates a
reset/interrupt one functional clock cycle later, even if the watchdog timer is
stopped.
3675
SPRUH73H – October 2011 – Revised April 2013
Timers
Copyright © 2011–2013, Texas Instruments Incorporated