Datasheet

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SLUS247F − APRIL 1997 − REVISED NOVEMBER 2007
8
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APPLICATION INFORMATION
CONNECTING WDO
TO RES
In order to provide design flexibility, the reset and watchdog circuits in the UCCx946 have separate outputs.
Each output independently drives high or low, depending on circuit conditions explained previously.
In some applications, it may be desirable for either the RES
or WDO to reset the microprocessor. This can be
done by connecting WDO
to RES. If the pins try to drive to different output levels, the low output level dominates.
Additional current flows from VDD to GND during these states. If the application cannot support additional
current (during fault conditions), RES
and WDO can be connected to the inputs of an OR gate whose output
is connected to the microprocessor’s reset pin.
LAYOUT CONSIDERATIONS
A 0.1-µF capacitor connected from VDD to GND is recommended to decouple the UCCx946 from switching
transients on the VDD supply rail.
Since RP and WP are precision current sources, capacitors C
RP
and C
WP
should be connected to these pins
with minimal trace length to reduce board capacitance. Care should be taken to route any traces with high
voltage potential or high speed digital signals away from these capacitors.
Resistors R1 and R2 generally have a high ohmic value, traces associated with these parts should be kept short
in order to prevent any transient producing signals from coupling into the high impedance RTH pin.
TYPICAL CHARACTERISTICS
Figure 5.
THRESHOLD RESISTANCE
vs
AMBIENT TEMPERATURE
V
RTH
− Threshold Resistance − V
T
A
− Ambient Temperature − °C
−55
1.22
1.20
−35 −15 5 25 45 65 85 105 125
1.21
1.25
1.23
1.24
1.26
V
DD
= 5 V
2
10.0
9.0
3456
9.5
11.5
10.5
11.0
12.0
Figure 6.
V
DD
− Input Voltage − V
INPUT CURRENT
vs
INPUT VOLTAGE
I
DD
− Input Current − µA