X9313 app

Application Note

AN71

6608-1.0 03/20/95 TD

Xicor, Inc. • 1511 Buckeye Drive • Milpitas, CA 95035. • (408) 432-8888

AN71-1

Implement a Nonvolatile Latch with E 2

POTs

by Cliff Zitlaw, January 1993

Many applications exist for nonvolatile event detection circuits. Circuits needing to sense an over/under-voltage condition often must remember that an event occurred, even after a power-down has taken place. These applications typically use latching relays when the input is automated, or switches when manual input is appropriate. Both latching relays and switches are forms of nonvolatile memory that can often be replaced with the circuit described here.The circuit shown in Figure 1 demonstrates how the

32-tap X9313 E 2

POT can be configured to act as a single-bit nonvolatile latch. Any HIGH to LOW transition on the event detect input will change the output of the circuit to the HIGH state. This event detect input can be driven by a switch, as shown, or by the output of a transducer circuit. A manual reset is provided to allow the circuit to be returned to the LOW state.

The circuit is designed around the X9313 nonvolatile

digital potentiometer, but other Xicor E 2

POTs could be used. This device stores the “state” of the circuit

by moving the wiper from one end of the potentiometer to the other. With the wiper moved to V H , the circuit outputs a logic “1”, and when the wiper is moved to V L , a logic “0” is output. Once the wiper has been moved from one end of the resistor array to the other, the wiper position is stored in a nonvolatile E 2

PROM register. Moving the wiper is accomplished with a circuit that uses a Schmitt-triggered inverter and several discretes. The driver circuitry assures that when a HIGH to LOW transition occurs at the event detect input, the wiper of the X9313 will move from tap position 0 to tap position 31. Much of the circuitry is used to generate a one-shot pulse that allows the CS and INC pin circuitry to move the wiper through full-scale.

This provides a cost effective method of implementing a nonvolatile latching function, using a

low cost E 2

POT and a little bit of “glue” circuitry. The circuit can be especially attractive when an expensive micro-controller implementation cannot be justified.

EVENT DETECT

100 K

+5V

+5V

RESET

100 K

0.1uF

100 K

100 K

100 K

0.001uF

CS

100 K

INC

0.001uF

U/D

CS INC U/D VH VW

VL

712

3

56VH VW VL

X9313

Figure 1. Schematic for the Nonvolatile Latch