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