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 SN74HCS7002-Q1 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 SN74HCS7002-Q1 ideal for the application because it has Schmitt-trigger inputs that do not have input transition rate requirements.