System Controller

Tamper

Switch 1

RA

SA

Q

R1

Tamper

Indicator

R2

SB

Tamper

Switch 2

RB

9

SN74HCS10

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SCLS789A – JANUARY 2020 – REVISED MAY 2020

Product Folder Links: SN74HCS10

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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 3-input NAND gates are used to create an active-low SR latch as shown in Figure 9. The

additional gate can be used elsewhere in the system, or the inputs can be grounded and left unused.

The SN74HCS10 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 SN74HCS10 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 LOW signals on both R and S inputs

•

Q output is HIGH when any S input is LOW

–

Q output remains HIGH until any R input 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

SN74HCS10 plus the maximum supply current, ICC, listed in the Electrical Characteristics. The logic device can

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

exceed the maximum total current through GND or VCC listed in the Absolute Maximum Ratings.

The SN74HCS10 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.