System

Controller

Tamper

Switch 1

RA

SA

Q

R1

Tamper

Indicator

R2

SB

Tamper

Switch 2

RB

10

SN74HCS27

SCLS794A – FEBRUARY 2020 – REVISED JUNE 2020

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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 NOR gates are used to create an SR latch as shown in Figure 9. The additional

gate can be used for other applications, or the inputs can be grounded and the channel left unused.

The SN74HCS27 is used to drive the tamper indicator LED and provide one bit of data to the system controller.

When either 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 any of the R inputs 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 SN74HCS27 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 any S input is HIGH

–

Q output remains High until any R input 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

SN74HCS27 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 SN74HCS27 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.