Electronic Design

Accelerometers Shake Up The Old Ways To Play

Is it time to think outside the box or just give it a shake? Movement- or gesture-oriented input isn’t new. Try shaking a cell phone like Apple’s iPhone. Matched with the right application, like a game, juggling the phone may produce the effect you want. Then again, it may not.

These little tricks are brought to you by accelerometers. Microelectromechanical systems (MEMS) technology has made these devices small and inexpensive. They’re used to shut down falling hard disks before they hit the floor, stabilize cameras, and support cellphone navigation interfaces, among other apps.

Unfortunately, the original accelerometers required quite a bit of external support to be useful, starting with analog inputs and ending with some interesting software to recognize tips from taps.

App notes are great, but getting analog accelerometers working is a challenge. Of course, wait long enough and everything goes digital. This is where products like Freescale’s MMA7660FC threeaxis accelerometer with an I2C interface come in.

This tiny device weighs 1.5 g and comes in a 3.3- by 3.0- by 0.9-mm dual flat no-lead package. It provides auto wake/sleep, configurable tilt orientation, and gesture detection. Also, its lowpower modes consume 0.4 µA (off), 2 µA (standby), and 47 µA (active). It costs $1.39 in quantity.

SHAKE, ROCK, AND ROLL
The MMA7660FC’s I2C interface isn’t just a digital interface to an array of analog converters. Instead, it provides an array of options that offload the host. Even a six-pin, 8-bit micro can easily handle it, leading to low-cost solutions. The accelerometer’s range of features and modes can lead to different user interfaces as well.

For example, it can sample at a rate from 1 to 120 samples/s. Power usage isn’t linear, but the least amount of power is used at 1 sample/s. A pair of AA batteries can drive the MMA7660FC for 80,000 hours. Power-on can use a low sample rate with an application switching to a higher rate once everything is up and running.

This type of low-power operation gets interesting when tied to other low-power systems such as E-Ink displays and ZigBee microcontrollers. Actions such as a display change or a wireless message may only be needed when movement is detected.

The chip can be programmed to periodically check for changes and then wake up the host. It can be used to perform tricks like detecting when a device is flipped over. Now you can create a device that turns itself on, or off, with a flip. This can replace one or more mechanical switches, often providing a more reliable and possibly more intuitive user interface.

Accelerometers can sense a number of actions, from the orientation of a digital picture frame to shake, rattle, and roll for games. The quality of sensor data limits the kinds of movements that can be detected. For instance, the MMA7660FC uses 6-bit analog-to-digital converters (ADCs) internally that suit movement detection but not dead reckoning, which would require 12-bit ADCs. Of course, designers face cost and power tradeoffs with higher-resolution systems.

PLAYING WITH ACCELEROMETERS
It’s also easier getting started with digital sensors these days. The RD3803MMA7660FC Evaluation Kit does I2C one better with an intelligent USB interface (see the figure). A Freescale USB-based microcontroller provides access to the MMA7660FC hosted on a postage-stamp sized dual inline package (DIP) board.

The kit comes with a sample application that shows off the range of features, from low-power modes to motion detection. Easy to use, it shows the orientation of the device as if it were a small, handheld unit. It’s a very nice tool for understanding the tradeoffs between lower power, low sample rates, and higher sample rates that consume more power. It also allows for experimentation with response time to actions such as taps, twists, and shakes.

Pricing for the MMA7660FC starts at $1.39. The eval kit costs $119. The accelerometer DIP board, which is available for $35, provides an easy way to incorporate the chip into a prototype.

So what new ways to play can you come up with for under $2? Drop me an e-mail if you have something for our Ideas for Design section.

FREESCALE SEMICONDUCTOR
www.freescale.com

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