System

Controller

Tamper

Switch 1

R

Q

Tamper

Indicator

Tamper

Switch 3

10

SN74HCS20

SCLS791A – MARCH 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 4-input NAND gates are used to create an active-low SR latch as shown in Figure 9.

The SN74HCS20 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.

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 inputs

•

Conditions for output

–

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

The SN74HCS20 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.