When writing a teardown article, I generally seek out the suppliers of the system's key components. From there, I discuss the various design issues with the engineers at the respective vendors. This teardown of a Microsoft Bluetooth-based keyboard was unique in that there was basically only one key component inside the system, the Broadcom Bluetooth controller.
Broadcom's stated goal with respect to keyboards (and mice) is to make the design as simple as possible. From a market perspective, Broadcom isn't looking to sell the high-end, newfangled wireless keyboards. It's more interested in flooding the market with lower-cost Bluetooth keyboards, thereby enabling a Bluetooth market. This keyboard (which also ships with a Bluetooth-enabled mouse) can be found for about $100. Broadcom would like to see that price cut down to about $60.
The industrial design for the keyboard was done at Microsoft. It replaced a model with considerably less battery life.
The Broadcom IC, the BD2040, acts as both the Bluetooth transceiver and the keyboard controller. The key-scan interface, holding the row and column information pertaining to the keys, is designed in to the chip.
"This chip is a Bluetooth controller, but we've tailored it for keyboards," said Tom Ramsthaler, Broadcom's product manager for Bluetooth chips. "We try to do everything inside the chip to bring down the cost. That's why when you pop open this keyboard, there's really not that much there."
The design process consisted of developing the interfaces for the key scan, adding an external crystal (about 35 cents) and an E2PROM (15 to 20 cents), then a handful of resistors and capacitors and an inductor. Tailoring the Bluetooth controller means adding the human-interface-device profile, part of the Bluetooth standard. That's all the Bluetooth software that's required to run the keyboard.
An alternative to the Broadcom approach would have been to employ two ICs: one for the Bluetooth connection and one for the keyboard control. Some keyboard makers have opted for this route, partly because it allows for a more exotic design. In addition to the second IC, such designs can require external flash memory for the custom firmware. But because Broadcom embeds all the necessary firm- ware, all that is needed ex- ternally is the E2PROM, which connects through a serial interface.
ÿ Broadcom supplies a reference design based on its Bluetooth wireless-keyboard controller.
A key issue for a wire- less keyboard is power consumption. Research shows that consumers want to go six to eight months without requiring a battery change. This unit, like its competitors, is powered from two AA cells. Unlike its competitors, Microsoft achieves that seven-month goal with typical use. Broadcom claims it can take the spec even further with its recently announced 2042 part.
One noticeable flaw with this keyboard: If you stop using it for a short time, it doesn't react immediately when you resume typing. That's because the keyboard drops into a sleep state when it's not used for a predetermined time. When it's fired back up, it must reestablish the Bluetooth link with the host computer.
"We've buffered the keys so that when you hit a key, it takes a second or two for the host to wake up," Ramsthaler said. "That keystroke gets through to the host, but there's a delay of about a second."
In the sleep state, the keyboard draws 10 to 20 microamps. Essentially, this allows you to walk away from your computer for a long, long time and still come back and have battery life. The designers could have chosen to keep the keyboard in its powered-up state of about 300 or 400 microamps. That would eliminate the reconnect (and the associated delay), but you'd pay a hefty battery-life penalty.
Another option keyboard designers could have tried is to employ a proprietary wireless solution, such as a 27-MHz design. This option requires a crystal, a very low-cost processor and four or five transistors, for a bill of materials ap- proaching 90 cents. While it is cheaper, some trade-offs are involved.
For example, Bluetooth is a hopping technology. So, in a classroom, every student could use a Bluetooth mouse and keyboard with no interference, while the 27-MHz system would encounter interference. You could add a layer of security to the less-expensive system, but then you'd be adding cost. By contrast, the Bluetooth system comes standard with 128-bit encryption built into the operating system.
ÿ Keyboard achieves simplicity with a single chip, the Broadcom BD2040.
A second trade-off is in the range. The 27-MHz system typically offers a range of about 3 or 4 feet, while Bluetooth easily extends to 30 feet .
By affordably putting Bluetooth onto more desktops, you open up a host of other markets. This could (and should) include printers, MP3 players, PDAs, mobile handsets and more. Broadcom's Blink software greatly simplifies the link between the PC and a handset (go to www.mobilehandsetdesignline.com and search article ID: 171202131,"Audio Blog: Bluetooth extends its usefulness" ).
Some of the cost of Bluetooth can be removed. For example, if the PC is already enabled, the keyboard can ship without a Bluetooth dongle. In a handset or PDA, the cost of the Bluetooth connection takes the place of the cradle and USB connection.
Broadcom claims that its reference designs do most of the hard work for the designer. That includes the FCC pretesting, so while you still have to go through the process yourself, Broadcom assures that it will pass if you use the reference design.
By Richard Nass, editor of EE Times sister site www.mobilehandsetdesignline.com
