A6250
Watchdog Timeout Period Description
The watchdog timeout period is divided into two parts, a
“closed" window and an “open" window (see Fig. 3) and is
defined by two parameters, T
WD
and the Open Window
Percentage (OWP).The closed window starts just after the
watchdog timer resets and is defined by T
CW
= T
WD
-
OWP(T
WD
).The open window starts after the closed time
window finishes and lasts till T
WD
+ OWP(T
WD
). The open
window time is defined by T
OW
= 2 x OWP(T
WD
).
120 ms after the timer reset. However as the time base is
±10% accurate, software must use the following calcula-
tion for servicing signal TCL during the open window:
Related to curves (Fig. 10 to Fig. 20), especially Fig. 19
and Fig. 20, the relation between T
WD
and R
EXT
could easily
be defined. Let us take an example describing the varia-
tions due to production and temperature:
1. Choice, T
WD
= 26 ms.
2. Related to Fig. 20, the coefficient (T
WD
to R
EXT
) is 1.125
where R
EXT
is in kΩ and T
WD
in ms.
3. R
EXT
(typ.) = 26 x 1.155 = 30.0 kΩ.
4. The ratio between T
WD
= 26 ms and the (TCL period)
= 25.4 ms is 0.975. Then the relation over the
Production and the full temperature range is,
TCL period = 0.975 x T
WD
0.975 x R
EXT
or
0.975 x R
EXT
, as typical value.
TCL period =
1.155
a) ±10 % while PRODUCTION value unknown for the
customer when R
EXT
≠
123 kΩ.
b) ±5 % while operating TEMPERATURE range
-40°C
≤
T
J
≤
+125°C.
5. If you fixed a TCL period = 26 ms
26 x 1.155 = 30.8 kΩ.
⇒
R
EXT
0.975
If during your production the T
WD
time can be measured
at T
J
= +25°C and the µC can adjust the TCL period,
then the TCL period range will be much larger for the
full operating temperature.
For example if T
WD
= 100 ms (actual value) and OWP = ±
20% this means the closed window lasts during first the
80 ms (T
CW
= 80 ms = 100 ms - 0.2 (100 ms)) and the
open window the next 40 ms (T
OW
= 2 x 0.2 (100 ms) = 40
ms). The watchdog can be serviced between 80 ms and
T
WD
versus V
OUTPUT
at T
J
= +25 °C
T
WD
versus R at T
J
= +25 °C
Fig. 8
Fig. 9
7
EMMICRO [ EM MICROELECTRONIC - MARIN SA ]
