WMS7204

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7. FUNCTIONAL DESCRIPTION

The WMS7204 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 WMS7204 is controlled by a serial SPI interface that allows setting tap register values as well as

storing data in the nonvolatile memory.

The WMS7204 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. Each channel can be independently

set to either buffer On or Off.

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 WMS7204 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

WMS7204 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 WMS7204:

•

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 commands.

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 each

channel to store 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.