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USB technology in a battery-powered IoT era

Posted: 02 Jun 2015 ?? ?Print Version ?Bookmark and Share

Keywords:universal serial bus? USB? Internet of Things? machine-to-machine? PHY?

Over the last several years, the universal serial bus (USB) standard has been adopted by designers of industrial and consumer devices as their interface of choice for enabling connectivity to other applications due to its ease-of-use, plug-and-play functionality and robustness. USB has achieved its primary goal of simplifying the way consumers control peripherals and transfer data. With more than three billion USB-enabled devices shipped into the market, USB is not only the fastest growing interface in consumer applications but has also achieved significant growth in industrial markets.

However, USB's ease-of-use, plug-and-play functionality and robustness do not come free for embedded solutions designers, especially if they are designing power-sensitive, battery-operated connected device products for the Internet of Things. For small, portable devices, adding USB as a communication interface at least doubles application current consumption and leads to devices that require much larger batteries than originally anticipated.

Upgrading from a traditional serial interface for communication to the popular USB interface often puts unfeasible restrictions on an energy budget. Often, a developer will have to choose between doubling the battery size and increasing device cost, which makes it less appealing, or cutting back on much-needed differentiating features. Let's take a look at how the USB standard has evolved from the dream of standardising all PC connections to a state-of-the-art technology that allows even small battery-powered devices for the IoT to communicate with anything.

A short history of USB
If you have ever examined the back of a desktop PC manufactured in the late 1990s, you would instantly recognise the proliferation of standards for connecting different types of hardware to your computer. Connectivity options included a 5-pin DIN, PS/2, serial port, parallel port, and maybe an SCSI ("scuzzy") port or two, and if you were a gamer, you would also have a game port on your soundcard.

The original developers of USB recognised this fragmented connectivity situation, and in 1995, they started to create one common machine-to-machine (M2M) standard that would supersede all others. In the late 1990s, when USB was first being adopted, it was initially added to PCs as just another connector to the mix. However, during the 2000s, USB really started to proliferate, and, after a series of updates, it is now one of the most widely adopted M2M interfaces. The success of the USB standard is evident by looking at your laptop or phone. Your smart phone has just one connector: USB. If you purchased your laptop after 2010, it probably has only USB connectors in addition to the display and network connectors. In addition, touchpads, keyboards and other peripherals used in today's laptops and tablets communicate with the main processor over USB.

The USB standard separates connectivity topology into devices and hosts. The host is the machine that initiates the communication and provides the power; on your desk, this is generally your laptop or desktop PC. The device is the downstream device that is connected to the host and simply replies to whatever the host asks for. On your desk, the mouse and keyboard are examples of USB devices.

The cool thing about a USB connector is that it also supplies power to the attached device, so there is no need for an external power supply to your mouse or external hard drive. The USB standard specifies that the host deliver at least 100 mA of current to the device, and, if the device is lucky, it will have 500 mA available. These power capabilities come from the original USB standard: PCs were always the host, and they were always powered through a wall socket. This USB standard requirement effectively stopped development of USB for low-power applications, as abundant mains power supplies have always been available for PC applications.

But what happens when this proven M2M interface meets today's a battery-powered world for the IoT? What is the impact when the host is also a portable device?

Impacts on today's USB hardware
In today's portable device applications, a much-used term is "power budget." The power budget dictates how much energy the device can consume and is based on battery size and the required battery life. For example, an application that has a 250 mA battery and needs a battery-life of two days (48 hours) has a power budget of approximately 5 mA. This power budget must be distributed across everything a developer wants the device to do, from sensor acquisition and processing to communication and driving displays.

As MCUs grew smaller and batteries improved over the last two or three decades, we saw an explosion of portable electronic devices ranging from handheld wind meters and oscilloscopes to digital breathalyzers and remote controls. However, with the introduction of smart phones with quad-core gigahertz processors, we now see more portable devices being introduced as additions to smart phones since manufacturers no longer have to worry about processing power or user interfaces. This market trend is driving the proliferation of inexpensive add-ons. Examples include the Kickstarter-backed Vaavud wind meter for smart phones and a breathalyzer that plugs into your iPhone. Both applications use the HiJack interface, an ad-hoc interface that works on low-end devices but is far from optimal.

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