Living up to its name, the Universal Serial Bus is supplanting the once-popular RS-232 bus as the standard serial interface in general-purpose microcontrollers. The capability of integrated USB is also increasing as more devices incorporate both host and On- the-Go (OTG) capability to augment the endpoint-device interfaces of earlier microcontrollers. Recent product introductions show that the USB is making inroads into more specialized devices as well, targeting applications such as automotive, industrial and ultralow-power consumer products.

Product introductions at last month's Embedded Systems Conference exemplified the ardor with which microcontroller vendors are embracing USB. Two ARM suppliers, Atmel and Luminary Micro, have added advanced USB capabilities to their product families. The Atmel SAM9R64 microcontroller, based on the ARM-9 core, targets design of endpoint devices operating in USB's high-speed (480-Mbit/second) mode. The Luminary Micro Stellaris family, based on the ARM Cortex-M3 core, added 20 members containing USB functionality, with many offering both host controller and OTG functionality. Such functionality also appeared in new members of the PIC32 family from Microchip Technology, as well as a dozen 16-bit MCUs in Microchip's new PIC23FJ256GI family.

Additional products are waiting offstage for introduction in the next few months, aiming to bring USB functionality to application areas where it has not been practical until now. Kevin Belnap, Texas Instruments' MPS430 product marketing manager, said TI "will be announcing product shortly to address the ultralow-power consumer space, and USB will be part of it."

Similarly, Microchip is on the verge of releasing a line of USB-enabled 8-bit microcontrollers that target ultralow cost, with volume pricing expected to fall below a dollar.

The role of USB in embedded control seems destined to continue expanding. NEC Electronics America, which offers a range of microcontrollers with USB functionality, sees USB going into gaming, industrial automation and building management.

USB functionality in microcontrollers is on the rise, with high-speed, host and On-the-Go support now appearing. Source: Microchip Technology

Interest is also growing in bringing USB to the automotive market, according to David Stone, director of marketing for NEC's automotive business unit. "There is interest in using USB to allow consumers to download their own audio files from a USB stick into the auto's entertainment systems," said Stone. "Discussions are also starting on using wireless USB for video download from a server in the home to the car in the garage, and for distribution within the car."

A number of factors are driving this adoption of USB within the microcontroller community. One is the increasing use of PCs, especially laptops, as the interface and control unit for embedded systems. "If the application is to connect to a PC," said Ray Shin, senior engineering manager for NEC's multipurpose MCU strategic business unit, "it has to have USB. Laptops are no longer providing RS-232." TI's Belnap agreed, noting, "Computers have taken off their serial ports, so to interface equipment to a PC for data download or configuration, the equipment needs USB."

But even embedded applications that do not need a PC interface are adopting USB. In its release announcing the Stellaris family members, Luminary Micro pointed out that the industrial market is interested in the standardized connections and hot-swap capability of USB as well as its ability to provide power to instrumentation. The ability of a USB interface to send data to memory through a DMA interface without CPU intervention is another compelling feature. Industrial systems are already using USB as the interface to flash memory sticks to download configuration data to industrial systems as well as to retrieve logged data without PC involvement.

The speed and utility of the USB interface does come at a price, however: increased design complexity. "USB is a fairly complex protocol," said TI's Belnap, "and developers need the right tools to be in place in order to get up to speed in using it." Belnap noted that microcontroller vendors are having to address the needs of both beginning and experienced users of USB with software, tools and reference designs.

A host of new applications, such as this security key, are increasing demand for USB connectivity in microcontrollers. Source: Atmel Corp.

Terry West, marketing manager for high-performance MCUs at Microchip, agreed, noting that the addition of USB to the microcontroller can have a major impact on software in particular. "The USB host stack alone is more than 100 kbytes of binary code," said West.

USB is also proving to be challenging when designing for new application spaces, such as ultralow-power consumer products, because USB was developed for the PC environment. "Ultralow-power systems are typically operating from a single AAA battery or a coin cell," said Belnap, "so they are running at a lower voltage than the USB interface. That has to be accounted for. Also, the USB interface needs to be powered down when not connected, to conserve battery life."

Fortunately, microcontroller vendors are addressing software complexity and other design issues for their customers by providing full software support for their on-chip USB peripherals, including drivers, class drivers and stacks for host, device and OTG functionality. The vendor may develop the software in-house, as with NEC and Microchip, or work with a third-party provider, as with Express Logic's USBX support for Luminary Micro's Stellaris family.

However they go about it, the support microcontroller vendors provide for USB development is extensive. "Customers are always asking us how we can save them money," said NEC's Shin, "so we have to provide a full solution." Typical USB vendor support includes software source code, reference designs, and development boards and tool sets.

Richard A. Quinnell (RichQuinnell@att.net) is a contributing technical editor based in Gardiner, Wash.