USB Simplifies RF Link Designs

Developing connected wireless applications requires two main tasks. First, your design must perform its designated function, and second, it has to communicate with the host PC, laptop, tablet, or smartphone.

When the widely popular RS-232 port was available, a simple 3-wire interface was all that was needed to connect peripherals, data links, I/O, sensors and more to a host computer. Often, hundreds or even a thousand feet of low-cost wire allowed reliable communications from point to point.

The simple Centronics-style parallel port was another easy way of getting 8-bit output chunks from a PC easily onto your interface board. Both of these interfaces have become pretty much extinct as far as being available on new equipment.

Instead, Wi-Fi, ZigBee, 6LoPan, RF4CE, Bluetooth, and other wireless protocols are drawing us into a world where connection cables are a thing of the past. While some of these standards are integrated into motherboards, smartphones, and other similar devices, others are available as plug-in modules that provide the necessary functionality for your design.

As you will want to have a universal and reliable communications scheme when developing either a stand-alone wireless application or one that is available as a plug-in module, USB may be the ideal choice, particularly since PCs and advanced microcontrollers support USB directly,

This article discusses the use of USB as an RF link and development mechanism for wireless design applications. It examines off-the-shelf USB-to-RF modules, as well as example development platforms that demonstrate wireless reference designs through USB connectivity. The latter products let you use USB connectivity to provide control and access to development platforms so you can custom tailor your own wireless solution.

All parts, modules, training, and development kits mentioned here are available on the DigiKey website.

Closing the gap

It used to be clearly understood that the cabled connection could transport data faster than a wireless connection with less cost. It was also clear a radio signal that must traverse a long distance took much more power than a cabled solution.

These distinctions are cloudier today. With hardwired USB connections limited to a few meters, and Wi-Fi, cellular, and satellite links promising worldwide reach, the old rules of thumb may be expiring. In reality, meshes, cells, and dynamically routed connections are what allow the global connectivity of handheld devices. For the design engineer, adding wireless links comes down to two main options.

The first is to implement your wireless link as a plug-in module. This has the advantage of being able to swap out just the communications module as the technology improves or updates. The second is to have wireless functionality tightly integrated into a specified device. USB is useful in both places.

Wireless USB

While not as widely selected for wireless applications as Bluetooth, Wi-Fi, or ZigBee, wireless USB is an interesting option. It simplifies the design process since you only need to become an expert in one protocol and communications standard. Wireless USB also allows you to test your design by plugging it right into a USB port.

One module certainly worth considering is the Artaflex AWP24S Wireless USB Transceiver (Figure 1). These small and compact modules use the 2.4 GHz band to achieve a range of up to 50 meters using the onboard printed antenna. With data rates of 250 Kbits/s and 1 Mbit/s, it can become a daughter module on your PCB via a small 2 x 6 2-mm-pitch connector. A 4-Mbit/s SPI port allows your host microcontroller to completely control and communicate through the wireless USB module.

Figure 1: This postage-stamp-sized micro module implements wireless USB in the 2.4 GHz band.

Operating from 2.4 to 3.6 V, the module needs no proprietary software, and its 1 to 10 µA sleep current helps preserve battery life. A nice feature is auto rate detection, which allows a host to fallback to a slower data rate if problems occur. The CYWM6934 is a similar module from Cypress with a 62.5-Kbit/s data rate.

Mesh networks

Point-to-point networks are fairly straightforward compared to mesh networks and dynamically switched, routing-based network topologies. If you need to be able to speak within a mesh network such as ZigBee, selecting a module that does all the arbitration, routing, and recovery can save a lot of design, coding, and test time.

A good example from Parallax Inc. is the 32400 XBEE USB adapter (Figure 2), which comes partially assembled and is intended to be an active part of your prototype and design. Also using 2 mm sockets and headers, the 32400 features clearly labeled signals for status and control, as well as key status LEDs while operating. Based on the FTDI chips, a free virtual COM port driver makes this look like an older style RS-232 port. You can also develop software using the free FTDI API drivers.

Figure 2: For ZigBee-style networks, the partially assembled development modules let you decide how to connect to your host micro.

While the 32400 plugs into a USB port with a cable, the Atmel ATAVRRZUSBSTICK plugs right into the USB port on most computers (Figure 3). The 2.4 GHz RF link performs all the arbitration and communications setup based on the Atmel AT86RF230 processor, and can be used as a reference design, starting point, or even standalone product to implement your wireless link.

Figure 3: The Z-Link ZigBee transceiver can plug into a standard USB port on a tablet or PC, as well as on a target handheld device.

Bluetooth

Another common RF link protocol available for use is Bluetooth, and there are ready to use products waiting for you here, as well (Figure 4). Let’s look at the Bluecore DEV-SYS-1487-1B. While not meant to be part of your product, it is instead a fully functional reference design and development kit that lets you either copy it onto your application, or develop your own using the company’s full-blown BTN-002-2B development environment. Aimed at multimedia applications, the development kit contains applications boards, dongles, interface boards, headsets, cables, a software CD, and a quick start guide.

Figure 4: USB-to-Bluetooth modules can be made very small and tough. Notice the meandering PCB antenna, which saves costs but provides a nice RF distribution pattern.

Laird Technology also provides a Bluetooth development platform you can use to get up to speed quickly. The BISDK02BI-02 supports Bluetooth, 802.11, and GPRS development using USB as a connect medium, and an RS-232 port for access to the development boards. Dongles, software, and cables are also included (Figure 5).



Figure 5: Target boards, dongles, and development platforms let you test, modify, and design your own wireless Bluetooth links using USB.

Wi-Fi

Wi-Fi is the most common high-speed wireless link in use today, and it shows no signs of slowing down. Wi-Fi, however, like many complex wireless protocols, has many layers and is not an easy specification to adhere to if you are new to the standard. In this case, a ready-to-go solution or a good reference design or development platform for a comprehensive chipset is needed to get up to speed quickly enough to get a product out the door in time.

One example is from connectBlue. The company offers its CB-OWS451I-04-B Evaluation and Development Platform (Figure 6). This setup includes Wireless LAN development boards, USB dongles, cables, software, reference designs, and documentation to let you test and evaluate, as well as develop a Wi-Fi (or Bluetooth or ZigBee) wireless link.

Figure 6: Wi-Fi test and development is simplified using USB-based kit boards. Support for high-speed UART interfaces with flow control makes it easy to interface with your design.

Supporting 802.11 b/g specifications, the dual band radios work with 2.4 and 5 GHz connections, feature high-speed (up to 1.8 Mbit/s) UARTs, include several encryption standards, and have –40° to 85°C temperature ranges.

All software, including the TCP/IP stack, is embedded within the modules, and AT style command sets allow quick and easy access to configurations. RTS/CTS flow control assures reliable data transfer, and quality of service (802.11e and WMM) support is built in.

As cellular, satellite, and other new high-speed wireless formats and protocols emerge, expect to see even more sophisticated and capable modules coming to market. USB-based modules and development kits will continue to be the fastest way to test and develop new products, and as such will be of key importance if you want to get to market quickly. For more information on the products mentioned in this article, use the links provided to access product pages on the DigiKey website.