Go Analog With Microcontrollers

April 3, 2009
Most microcontrollers are paired with discrete analog components—like ADCs, amplifiers, filters, DACs, and comparators—to achieve filtering, signal conditioning, and signal conversion in systems like those that require interfaces to analog sensors or the

Microcontrollers have long been valued for their ability to perform specific functions that system designers find themselves requiring in their applications. With a glance through any microcontroller vendor’s broad product catalog, engineers can find virtually any combination of features to satisfy their design requirements. The explosion of fixed-function devices for specific applications (brushless dc motor control, for example) has been driven partially by the desire to simplify designs for customers.

With a microcontroller that has been configured for an exact application, engineers need not worry about setting up the microcontroller. Instead, they often spend time figuring out which external components will be required in addition to the microcontroller to realize the end product’s design, and then incorporating these devices into the design.

Most microcontrollers are paired with discrete analog components—analog-to-digital converters (ADCs), amplifiers, filters, digital-to-analog converters (DACs), and comparators, to name a few—to achieve filtering, signal conditioning, and signal conversion in systems like those that require interfaces to analog sensors or the filtering of noisy signals. To help alleviate the complicated design challenges often associated with discrete analog component implementation, a trend has emerged among semiconductor companies to integrate more of these functions into a microcontroller or microcontroller-like device.

The challenges that come with utilizing external analog components include difficulty in making last-minute system changes that then require a board spin, higher bill of materials (BOM) costs, utilizing devices that are overkill for the design but the “best fit” available, and requiring the expertise of specialized analog engineers to implement more complex analog systems. With the proliferation of more complicated, but low-cost, electronics, it is important to make it easy for system designers to select and implement both analog and digital functions in a quick and easy manner. A new generation of integrated, flexible microcontroller-based devices mixes familiar digital features with combined analog functionality in a programmable manner. These devices help to address these challenges while making it easier for designers to implement more complicated designs.

A Hypothetical Case

An example of the number of analog components in a system can be seen in a security system design. A typical security system will include a number of sensors—temperature, piezo-infrared (PIR), ultrasonic, and gas. While a microcontroller may be the ultimate destination for the signals, there is often a signal-conditioning circuit in front of the microcontroller to assist in cleaning up and/or amplifying the analog information coming from each sensor.

A PIR sensor requires high pass and band-pass filters, one or two amplifiers, an ADC, and a comparator. An ultrasonic sensor requires band-pass and low pass filters, an amplifier, an ADC, and a comparator. A gas (carbon monoxide) sensor requires an amplifier, a low pass filter, and a comparator. A simple temperature sensor requires a low pass filter, an ADC, and an amplifier. If it is assumed that most microcontrollers have at least a rudimentary ADC already integrated, the analog component count is still approximately 15 (Fig. 1).

System designers must pick all of the components, possibly without knowing all of the parameters, and lay out a board. Designers also must have knowledge of analog design rules or an available analog design engineer to ensure optimal performance of the system. If system requirements change after the board has been laid out and spun, designers may be required to select different analog components, lay out the board again, and do another spin.

Along with the cost of the external components, all of this redesign work includes cost, both monetary and from a time-to-market perspective, as well. Many of these costs might be eliminated if a programmable analog device were used to integrate as many of the external components as possible.

While a device with programmable analog may never be able to attain the performance of the highest-end analog components, programmable analog will provide sufficient performance for most systems. Taking the same security system example, if a designer used a programmable mixed-signal system-on-a-chip (SoC), up to 13 of the additional discrete analog functions could also be integrated, leaving just two or three simple functions external to the device (Fig. 2).

Engineers are recognizing that simplified system design, the ability to program the analog for a close fit to the system’s performance requirements, and the ability to reprogram a device instead of respinning a board are extremely valuable features for a microcontroller or programmable SoC, and these features are now being requested more often.

System designers have also realized that the same general design can be used with simple reprogramming to add or subtract functionality, instead of selecting a whole new device or redeveloping the analog front of their new system. Semiconductor vendors are also recognizing the value provided to the customer and have begun to make these types of devices available, with varying levels of integration.

As the number of systems requiring more complex analog functions grows, system designers will be under more pressure to quickly and cost-effectively release systems. With the advent of more integrated mixed-signal SoCs and microcontrollers, semiconductor vendors can help system designers achieve their goals faster and with less complexity.

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