MCUs with Serial Connectivity Make an Ideal Junction Box for Wearable Computers

The evolving push toward wearable devices is transforming individuals into their own data centers, complete with flash drives, mobile PCs, sensor arrays, medical devices, and more. Various technologies are competing for input, output, connectivity, and capability in wearable designs. For example, TFTs, virtual vision goggles, pico projectors, 3D displays, and holographic display systems all have unique benefits, but it will be unlikely that every technology will be implemented as a user interface in a single wearable system. You can say the same thing about input technologies like 3D gesture recognition, speech recognition, touch panels, keyboards, haptic feedback systems, and so on.

As a result designers of wearable computers and their associated peripherals have choices to make. Does all processing functionality live in one place, or is it distributed around us in individual nodes? Perhaps, eventually, when all these (and other) technologies are refined to the point of highest functionality and reliability at lowest cost, a single do-all device can become the popular option. Until then, count on using discrete peripherals that will not only connect to other wearable devices, but also interconnect with the web.

This article looks at the microcontrollers with features needed to create a wearable junction box for computers and PANs. Since cabling and contact points affect price and reliability, serial protocols will be used for wired network connections. Micros with rich serial connectivity can make ideal junction box communications aggregators. This is true with both wired and wireless connectivity since RF links are serial in nature. All parts and data referenced here can be found online at DigiKey’s website.

Connectivity link options

While offering many peripherals and connectivity links, historically MCUs have not always kept pace with high-speed links for data-intensive communications as they became available. For example, as early as the 1980s, Apple recognized the need for high-speed serial-bus standards for high-definition audio and video. The IEEE 1394 FireWire interface was just that, using isochronous real-time data transfers for up to 63 peripherals in a tree or daisy chain configuration.

As a predecessor to USB, it featured plug-and-play functionality, unique identifiers, and NRZ data strobe encoding techniques to reliably transfer serial data up to 400 Mbits/s in full-duplex fashion. It also performs arbitration and could use cable lengths up to 72 m. However, its 261 patents were held by 10 companies, and the licensing and royalties discouraged widespread adoption. Good luck finding any microcontrollers with full native support for FireWire. (Note: Texas Instruments does offer the TSB43AB22A Link Layer controller for IEEE 1394, but it is intended as a PCI-to-FireWire interface).

Toslink is another example of a serial protocol medium well suited for high-definition audio and video for a PAN application. The strong and flexible fiber-optic link supports data rates up to 250 Mbits/s and is immune to surrounding electronic noise. Transceivers like the Toshiba TODX2402(F) provide small-sized reliable connections, and cables are readily available as well.

While direct FireWire and Toslink interfaces are not native to microcontrollers, other ready to use serial peripherals and links are easily available. Ethernet is a well-established serial link protocol that is supported by many MCU-plus-Ethernet families with native hardware (Figure 1). Note also that basic sensors and peripherals can use simple 8-bit MCU-plus-Ethernet parts like the 8051-based W7100A from WIZnet, which also has a UART and GPIO for local buttons, switches, or sensor systems.

Figure 1: Once dedicated communications hardware is embedded on chip, even simple 8-bit processors can chain into the communications network of a PAN. This means lower-cost micros can be used for simple sensors and functions.

NXP Semiconductor offers 16/32-bit serial junction box functionality with parts like the 72 MHz ARM7-based LPC2468FBD208,551 MCU (Figure 2). As a member of the company’s LPC2400 Series, these parts feature a mix of CAN, I²C, SPI, SCI, and UART, which can allow it to connect with lower-bandwidth peripherals and sensors in a wired or wireless way. (Note that many RF transceiver chips can take data from UART, I²C, SPI, or USB.) Other family members have CAN, IrDa, Microwire, and USB OTG connectivity as well.

Figure 2: Block diagram of the NXP LPC2468FBD208,551 MCU.

A nice plus with Ethernet is its widespread support, training, and free stacks ready for integration into your source code. NXP provides a video overview of its Ethernet technology on the DigiKey website. A Product Training Module to help migrate 8- and 16-bit processors to LPC ARM processors is also available.

Another nice feature of Ethernet is that small, wearable, low-power, multiport switches and routers can be used to let peripherals communicate directly with each other without the need for intervention and monitoring from a host CPU. Very useful are the power-over-Ethernet technologies that can supply power to endpoints from a switch and router. Parts like the Linear Tech LTC4274IUHF#PBF can let these high-speed serial data links provide 25 W of power to peripherals.

As far as high-speed, self-powered, arbitrating, compact, and well-supported microcontrollers for serial connectivity are concerned, USB is at the forefront. Supported by thousands of microcontrollers in 8-bit, 16-bit, and 32-bit levels of performance, USB has several advantages when used as low- and high-speed serial links for wearable PAN processors and peripherals.

Take a look at the Cypress ARM9-based USB controllers, which support USB version 3. Parts like the CYUSB3014-BZXC support up to 5 Gbit/s data rates of USB 3.0 with up to 32 physical endpoints. An ultra-low-power core (down as little as 20 µA) makes it ideally suited for battery-powered wearable applications, and this part, like others in the EZ USB FX3 Series, also features a nice mix of UART, SPI, I²C, and I²S serial audio master transmitter functionality.

PAN peripherals requirements

All parts of our wearable computers will need power as well as signaling, and there are cases where both battery-powered stand-alone wireless and wired solutions are best. As mentioned, a display technology can benefit from a small, thin, wired high-speed serial link from a wearable host as long as it is comfortable, practical, and yes, even fashionable (Figure 3). This may also allow our display technology to be lighter in weight since power can be transferred as well as signal over a single, shielded, two-conductor link.

A heart monitor, on the other hand, must remain on our person and active 24 hours a day. It may need a hot-swappable battery pack since it must be active, even while we sleep. It will also need to be wireless since we don’t want to be wired to a transceiver while we are asleep.

Note: This wireless link does not need to be a high-speed and high-power link. Small packets can be transmitted just to alert a smartphone or tablet, or any device that has cloud connectivity. The data could be transmitted, or stored and accessed in a medical facility.

Another example is a wearable pedometer that senses and logs motion using accelerometers to determine how many miles we have run. Since we do not run in our sleep, it can sit in a charger cradle at night and store enough charge to run all day without a power cable. The relatively small amount of data gathered can use a low-baud-rate link like a UART and be transmitted through wires, optics, or RF.

Moving forward

Social trends can enhance or squash technical solutions, and the solutions that appeal to the masses will succeed. Fashionable and functional designs are emerging already (Figure 3), and new connectivity standards like JEDEC JESD204B digital serial interfaces may gain widespread acceptance. Cloud connectivity to our PANs will be cellular 3G/4G/5G, and so on. However, our local connectivity of processors, peripherals, sensors, and devices will be a combination of wired and wireless serial links. Fortunately, and as we have shown, there are several performance and feature-rich processors ready to do the heavy lifting.

Figure 3: Wearable devices need to be comfortable, functional, and fashionable.

In summary, this article has demonstrated that the parts and resources are available for designers to begin development of a wearable junction box for computers and PANs using standard serial interfaces for both wired and wireless applications. In examining the range of available micros ready to support these designs, we have provided examples of both simple, low-power data acquisition sensor MCUs as well as higher-performance multicore parts with added functionality.

To get more information about the parts discussed here, use the links provide to access product information pages on the DigiKey website.