WMS7201

Publication Release Date: January 2003

- 7 -

Revision 1.1

7. FUNCTIONAL DESCRIPTION

The WMS7201 series, a family of 256-tap, nonvolatile digitally programmable potentiometers is

designed to operate as both a potentiometer or a variable resistor depending upon the output

configuration selected.

The chip can store four 9-bit words in nonvolatile memory (NVMEM0 ~ NVMEM3) and the word stored

in the NVMEM0 will be used to set the tap register values when the device is powered up.

The WMS7201 is controlled by a serial SPI interface that allows setting tap register value as well as

storing data in the nonvolatile memory.

The WMS7201 can operate as either a rheostat or as a potentiometer (voltage divider). When in the

potentiometer configuration there are two possible modes. One is without the output buffer and the

other mode is with the output buffer. Selecting the mode is done by controlling bit D8 of the data

register. D8 = 0 sets the output buffer off and D8 = 1 sets it on.

Note that this bit can only be set by loading the value to the NVMEM with instructions #5 and

then loading the TAP register with instruction #6 from NVMEM. This bit cannot be controlled by

directly writing the value to the chip when the tap register is set.

7.1.1. Rheostat Configuration

The WMS7201 acts as a two terminal resistive element in the rheostat configuration where one

terminal is either one of the end point pins of the resistor (VA and VB) and the other terminal is the

wiper (VW) pin. This configuration controls the resistance between the two terminals and the

resistance can be adjusted by sending the corresponding tap register setting commands to the

WMS7201 or loading a pre-set tap register value from nonvolatile memory NVMEM0 ~ MVMEM3.

7.1.2. Potentiometer Configuration

In potentiometer configuration an input voltage is connected to one of the end point pins (VA or VB).

The voltage on the wiper pin will be proportional to the voltage difference between VA and VB and the

wiper setting. The resistance cannot be directly measured in this configuration.

Two program modes are available for the WMS7201:

•

Direct program mode. The tap register setting can be changed either by loading a

predetermined value from an external microcontroller or by using the UP/DOWN command.

The UP and DOWN commands change the tap register setting incrementally i.e., 1 LSB at a

time. The UP and DOWN commands will not wrap around at the ends of the scale.

•

NVMEM restore mode. One of the previously stored settings can be loaded into the TR

register from the non-volatile memory. Four 9-bit non-volatile memories, are available for to

store the tap register settings. The first register, NVMEM0, stores the favorite or default tap

register setting that will be loaded into the tap register at system power up or software power

on reset operation.