System

Controller

Tamper

Switch

R

S

Q

Tamper

Indicator

9

SN74HCS02

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SCLS776 – DECEMBER 2019

Product Folder Links: SN74HCS02

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Copyright © 2019, Texas Instruments Incorporated

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. Customers should

validate and test 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. 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

Figure 9. Typical application block diagram

9.2.1 Design Requirements

•

All signals in the system operate at 5 V

•

Avoid unstable state by not having HIGH signals on both inputs

•

Q output is HIGH when S is HIGH

–

Q output remains High until R is HIGH

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 supply must be capable of sourcing current equal to the total current to be sourced by all outputs of the

only source or sink as much current as it is provided at the supply and ground pins, respectively. Be sure not to

The SN74HCS02 can drive a load with a total capacitance less than or equal to 50 pF connected to a high-

impedance CMOS input while still meeting all of the datasheet specifications. Larger capacitive loads can be

applied, however it is not recommended to exceed 70 pF.

Total power consumption can be calculated using the information provided in CMOS Power Consumption and

Cpd Calculation.