Overview

Digital potentiometers (usually called a "pot" in electronics speak) are a solid-state replacement for mechanically adjustable variable resistors historically used for volume, balance and tone controls in consumer electronics, among many other applications. Instead of responding to shaft rotation, digital pots change their value in microseconds based upon a received command message.

Digital pots are offered in a variety of control interfaces, including 1-wire, contact closure (up/down steps), parallel, increment/decrement, SPI (3-wire) and others. Some even include nonvolatile memory to retain their last setting when power is removed. We'll look at Microchip's MCP41xxx/42xxx family of digital pots, with an SPI interface. Other manufacturers, such as Maxim (including its Dallas Semiconductor division), have wider product ranges, and you should review their offerings if you think digital potentiometers may be useful in your particular project.

First, let's clear up the terminology. The device we are talking about is known as a potentiometer, a variable resistor, a rheostat or a volume control, among many other terms. In reality, these terms can be boiled down into two possible connection arrangements, as shown in Figure 21-1. The potentiometer arrangement uses all three connections and is usually configured as a variable voltage divider. The variable resistor or rheostat arrangement uses two connections (the wiper the variable connecting piece is connected to one end of the fixed winding, but to the external circuit only two connections are seen) and operates as a variable resistance. We'll use the term "pots" as the generic name for these devices, even...