Embedding wireless into a microcontroller-based design can be an intimidating experience for the designer.

By Steven Bible and Tyler Smith, RF Products Div., Microchip Technology, Inc.

Wireless is fast becoming a feature of choice for many products. The ability to cut the wires and mobilize a device is a compelling feature to microcontroller users. For the intrepid design engineer, adding wireless to a new or existing product can be a daunting exercise. This article covers the common questions and answers that come up when embedding wireless into a microcontroller-based design.
1. How will radio regulations affect my product?
Often, customers ask the question, "What frequency or modulation should I use for my XYZ product?" We simply reply that technology can do many things, but it is
Figure 1. Microchip's MRF24J40MA certified IEEE 802.15.4 module helps keep development costs low.
what the radio regulations will "permit" you to do that dictates your decision on frequency, modulation and output power. Regulations differ from country to country, and they are administered by government agencies. In the United States, that agency is the Federal Communications Commission (FCC). These government agencies publish regulations and certify radio devices for operation in their respective countries.

If you plan to sell your wireless product in multiple countries, you have to keep in mind that each country in which an "intentional radiator" (a legal term that means RF transmitter) is sold requires some sort of regulatory certification. This certification requires that design engineers be familiar with radio regulations and how they specify a device's operation. A fair warning - the radio regulations are written for lawyers, not engineers. Trying to ascertain technical specifications from these regulations can cause a bit of frustration. Even seasoned RF engineers must work closely with compliance test houses for the most current regulatory information and consulting.

Table 1. lists the costs for radio certification in the U.S., Canada and Europe. Keep in mind that these costs are for first-time successes. If any retesting is required, additional fees are levied. Remember to book the certification house in advance.

In the United States, the FCC allows for "modular certification." This allows companies to create an RF module, certify it for use in the U.S. and free the integrator of the module from any further intentional-radiator testing. This is a great boost to the design engineer, as FCC certification becomes one less thing they have to worry about.
2. Should I design to a standard or a proprietary wireless protocol?
Standards help define the frequency, modulation and protocol that wireless devices employ to communicate. This eases the burden of designing your own wireless protocol from scratch. Standards

click to enlarge
also give you more choices in device selection and potential second sources. They may also be important for your product, if it is to be interoperable with other manufacturers' products. If interoperability is required, then choosing a standard protocol is a must.

However, standards can be burdensome. The standards organizations require you to be a member, charging both initiation and annual fees. Then there is the cost of interoperability testing and certification, per product. These fees quickly add up and increase the overall cost of the product.

Creating your own protocol can ease the standards burden; however, this necessitates engineering time that must be devoted to the task. Many companies offer simple to complex protocols under some licensing terms. Licensing a proprietary protocol may alleviate the cost of starting from scratch and provide an established foundation from which to design.
3. What test equipment do I need?
RF test equipment are unique tools. They allow you to see and measure the radio waves while evaluating the performance of your wireless design.

A spectrum analyzer is a mandatory piece of equipment. It displays frequency versus power. The frequency range should be 10 times the fundamental frequency in order to view the 10th harmonic of the device's operating frequency. This is important because the FCC certification process will look all the way up to the 10th harmonic.

A Vector Network Analyzer (VNA) measures the amplitude and phase characteristics of wireless networks. It is a very useful instrument for measuring the characteristics of filters and antennas. The VNA makes short work of matching circuit impedances a very common practice in RF. The frequency range of your VNA should be twice the fundamental frequency, so that the second harmonic frequencies can be analyzed. Harmonics must be suppressed in order to pass radio regulations.

If the radio design involves complex modulation signals (e.g. OFDM, QPSK), you should seriously consider using a signal analyzer or a signal-analysis function in the spectrum analyzer. You should also consider a method to create complex signals from the signal generator. Naturally, this adds to the overall cost of your test equipment.

Table 2. lists the approximate prices of basic RF test equipment. However, there are many features, accessories and support equipment surrounding this basic list of test equipment.
4. How do I choose a microcontroller?
Wireless protocols require some processing power. Simple protocols can use low-cost, less powerful microcontrollers. More complicated protocols require more powerful microcontrollers with lots of program memory and RAM.

click to enlarge
Choosing a microcontroller depends on where you are in the development of your wireless solution. You may have already chosen the microcontroller and need a wireless solution, or you might need to both decide on a wireless solution and select the appropriate microcontroller.

If you have already decided on a microcontroller, you'll have to look at its available resources and then choose a wireless protocol to fit it. If the protocol exceeds the microcontroller's resources, you can look into a wireless device that has hardware assist functions to ease the microcontroller processing burden. You could also consider a wireless module that handles the protocol internally, thus relieving the host microcontroller from wireless protocol duties.

If you are trying to decide on a wireless protocol, then that decision may be dictated by cost and, therefore, the size of microcontroller. There are many wireless protocols to choose from. The choice depends on the application, and then the microcontroller size follows as a result. Starting by choosing the right wireless protocol for your application will then help dictate the appropriate microcontroller.
5. Is there a module in my future?
This is a question you should ask yourself if the previous questions seem daunting. Wireless modules offer a ready-built, cost-effective and quick wireless solution. Some modules are completely self-contained; that is, they embed not only the wireless radio but also a microcontroller pre-programmed with a wireless protocol. Other modules contain the RF solution only, allowing you to select the best microcontroller for the application.

As mentioned in the first answer, modules can receive FCC approval and certification. This relieves the radio certification task from the project, thereby reducing risk and saving time and money.

Modules allow the designer to get a product up and running quickly, whether it is proof of concept or full production. And, modules can offer a cost-effective solution for just about any design.

Hopefully, the answers to these five questions will help you get started in developing wireless solutions for your microcontroller-based applications.

Steven Bible is applications engineering manager in the RF Products Division of Microchip Technology, Inc. Tyler Smith is marketing manager of Microchip's RF Products Division.