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 NAND gates are used to create an active-low 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 SN74HCS00 is used to drive the tamper indicator LED and provide one bit of data to the system controller.

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

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

The inputs of this active-low SR latch can often be driven by open-drain outputs which can produce slow input

transition rates when they transition from LOW to Hi-Z. This makes the SN74HCS00 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

• All signals in the system operate at 5 V

• Avoid unstable state by not having LOW signals on both inputs

• Q output is HIGH when S is LOW

– Q output remains High until R is LOW

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

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SN74HCS00

SCLS775C – DECEMBER 2019 – REVISED DECEMBER 2021

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Product Folder Links: SN74HCS00