9 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification, and TI

does not warrant its accuracy or completeness. TI’s customers are responsible for determining

suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

9.1 Application Information

In this application, two 2-input NOR gates are used to create an SR latch as shown in Figure 9-1. The two

additional gates can be used for a second SR latch, or the inputs can be grounded and both channels left

unused.

The SN74HCS02 is used to drive the tamper indicator LED and provide one bit of data to the system controller.

When the tamper switch outputs HIGH, the output Q becomes HIGH. This output remains HIGH until the system

controller addresses the event and sends a HIGH signal to the R input which returns the Q output back to LOW.

The user can add a small RC to the feedback path of the NOR gates to default the output to a certain state,

which can create slow transition rates. This fact makes the SN74HCS02 ideal for the application because it has

Schmitt-trigger inputs that do not have input transition rate requirements.

9.2 Typical Application

System

Controller

Tamper

Switch

R

S

Q

Tamper

Indicator

Figure 9-1. Typical application block diagram

9.2.1 Design Requirements

9.2.1.1 Power Considerations

Ensure the desired supply voltage is within the range specified in the Recommended Operating Conditions. The

supply voltage sets the device's electrical characteristics as described in the Electrical Characteristics.

The positive voltage supply must be capable of sourcing current equal to the total current to be sourced by all

any transient current required for switching. The logic device can only source as much current as is provided by

Maximum Ratings.

The ground must be capable of sinking current equal to the total current to be sunk by all outputs of the

required for switching. The logic device can only sink as much current as can be sunk into its ground connection.

Be sure not to exceed the maximum total current through GND listed in the Absolute Maximum Ratings.

The SN74HCS02 can drive a load with a total capacitance less than or equal to 50 pF while still meeting all of

the datasheet specifications. Larger capacitive loads can be applied, however it is not recommended to exceed

50 pF.

SN74HCS02

SCLS776B – DECEMBER 2019 – REVISED JANUARY 2021

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