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