MCU for wearable junction box with computer and PAN

The continued push for wearables is transforming individuals into their own data centers, including flash drives, mobile PCs, sensor arrays, medical devices, and more. Various technologies compete for input, output, connectivity, and functionality in wearable designs. For example, TFTs, virtual vision goggles, pico projectors, 3D displays, and holographic display systems all have unique advantages, but it is unlikely that each technology can be implemented as a user interface in a single wearable system. You can say the same thing about input technologies such as 3D gesture recognition, voice recognition, touch panels, keyboards, haptic feedback systems, and more.

Designers of wearable computers and their associated peripherals therefore have a choice to make. Should all processing capabilities reside in one place, or should they be distributed among nodes around us? Perhaps, eventually, when all of these (and other) technologies have been refined to the highest level of functionality and reliability at the lowest cost, a single, all-purpose device may become the popular choice. Until then, it will be easier to rely on the use of standalone peripherals that can not only connect to other wearables, but also interconnect with the network.

This article looks at microcontrollers with the functionality needed to create a wearable junction box for a computer and PAN. Serial protocols will be used for wired network connections, as wiring and contact points affect price and reliability. A microcomputer with abundant serial connections can be an ideal junction box communications aggregator. This is true for both wired and wireless connections, as RF links are serial in nature. All parts and data referenced here can be found online at Digi-Key’s website.

Connection link options

While many peripherals and connectivity links are available, in the past MCUs have not always been able to keep up with the speed of high-speed links for data-intensive communications. For example, as early as the 1980s, Apple recognized the need for a high-speed serial bus standard for high-definition audio and video. The IEEE 1394 FireWire interface does just that, using isochronous real-time data transfer for up to 63 peripherals in a tree or daisy-chain configuration.

A predecessor to USB, it featured plug-and-play capabilities, unique identifiers, and NRZ data strobe encoding, enabling reliable transmission of serial data at up to 400 Mbits/s in full-duplex mode. It also performed arbitration, allowing the use of cables up to 72 m long. However, its 261 patents are held by 10 companies, and licensing and royalties have hindered widespread adoption. Good luck finding any microcontrollers that fully natively support FireWire. (Note: Texas Instruments does offer the TSB43AB22A link layer controller for IEEE 1394, but it is intended for use as a PCI to FireWire interface).

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

While direct FireWire and Toslink interfaces are not native to microcontrollers, other readily available serial peripherals and links are readily available. Ethernet is a mature serial link protocol, and many MCU-plus-Ethernet families have native hardware to support it (Figure 1). Also note that basic sensors and peripherals can use simple 8-bit MCU-plus-Ethernet parts, such as WIZnet's 8051-based W7100A, which also has UARTs and GPIOs for local buttons, switches, or sensor systems.

Figure 1: Even a simple 8-bit processor can be linked to a PAN's communications network by embedding dedicated communications hardware into the chip. This means that low-cost micro-devices can be used for simple sensors and functions.

NXP Semiconductor offers 16/32-bit serial-block capabilities with parts such as the 72-MHz ARM7-based LPC2468FBD208,551 MCU (Figure 2). As part of the company's LPC2400 family, these parts mix CAN, I²C, SPI, SCI, and UART to interface with low-bandwidth peripherals and sensors via wired or wireless means. (Note that many RF transceiver chips can get data from UART, I²C, SPI, or USB.) Other family members also have CAN, IrDa, Microwire, and USB OTG connections.

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

One of the advantages of Ethernet is its extensive support, training, and free stacks that are ready to integrate into your source code. NXP provides a video overview of its Ethernet technology on the Digi-Key website. There is also a product training module that helps migrate 8-bit and 16-bit processors to LPC ARM processors.

Another nice feature of Ethernet is that small, wearable, low-power, multi-port switches and routers can be used to allow peripherals to communicate directly with each other without intervention and monitoring by the host CPU. Power over Ethernet is very useful, as it can provide power to endpoints from switches and routers. Parts like the Linear Technology LTC4274IUHF#PBF can allow these high-speed serial data links to provide 25 W of power to peripherals.

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

Take a look at the Cypress ARM9-based USB controllers that support USB version 3. Parts such as the CYUSB3014-BZXC support USB 3.0 data rates up to 5 Gbit/s with up to 32 physical endpoints. The ultra-low power core (as low as 20 µA) makes it ideal for battery-powered wearable applications, and like other devices in the EZ USB FX3 family, this device also features a great combination of UART, SPI, I²C, and I²S serial audio master transmitter capabilities.

PAN Peripheral Requirements

All of the components of our wearable computers require power and signal, and in some cases both battery-powered standalone wireless and wired solutions are best. As mentioned earlier, display technology can benefit from a small, thin, wired high-speed serial link to a wearable host, as long as it is comfortable, practical, and even fashionable (Figure 3). This may also make our display technology lighter weight, because power and signal can be transmitted over a single shielded two-conductor link.

On the other hand, a heart monitor must stay on us and be active 24 hours a day. It may need a hot-swappable battery pack, since it must be active, even when we sleep. It also needs to be wireless, since we don't want to be connected to a transceiver while we sleep.

Note: This wireless link does not have to be a high speed, high power link. Small data packets can be transmitted to alert a smartphone or tablet or any device with a cloud connection. The data can be transmitted, stored, and accessed within the healthcare facility.

Another example is a wearable pedometer, which uses an accelerometer to sense and record movement to determine how many miles we have run. Since we don’t run in our sleep, it can be placed in a charging cradle at night and store enough power to run all day without a power cord. The relatively small amount of data collected can be transmitted using a low baud rate link such as a UART, via wires, optical devices, or radio frequency.

Moving forward

Social trends can enhance or suppress technology solutions, and solutions that appeal to the masses will succeed. Stylish and practical designs are already emerging (Figure 3), and new connectivity standards such as the JEDEC JESD204B digital serial interface are likely to gain widespread acceptance. Cloud connectivity to our PAN will be cellular 3G/4G/5G, etc. However, local connectivity to our processors, peripherals, sensors, and devices will be a combination of wired and wireless serial links. Fortunately, as we have shown, there are several performance- and feature-rich processors that can do the heavy lifting.

In summary, this article has shown that these parts and resources are available to designers to develop wearable breakout boxes for computers and PANs for both wired and wireless applications using standard serial interfaces. In examining the range of available microcontrollers ready to support these designs, we have provided examples of simple low-power data acquisition sensor MCUs as well as high-performance multicore parts with additional features.