Acceleration sensing USB interface


Posted on Oct 22, 2012

The TiltStick is a small acceleration sensing device in form of a USB stick. It's using a two axis acceleration sensor to measure acceleration (caused e.g. by motion and tilt). The device is emulating a USB joystick and can thus be used in conjunction with any USB equipped host (e.g. a standard PC or the Nokia N8XX family) without any special drivers. The TiltStick is the successor of the Tilt Dongle for Palm OS. Two versions of the TiltStick have been designed based on different 2D accelerometer chips. The slightly smaller one is based on the Analog Devices XL203 (the XL202 may also be used).


Acceleration sensing USB interface
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The bigger version is based on the Freescale MMA7261. The advantage of the freescale version is that a pin 3D chip exists. The hardware of the TiltStick is prepared to support the third axis. The current design V1.0 contains some design errors. Most important the USB data lines have to be swapped. This can easily be done by cutting the traces directly at the connector and replacing them with thin wires. Press and hold the calibration button (the one close to the USB connector) on the TiltStick for at least three seconds until the green LED turns off. The yellow calibration LED will then start blinking once a second to indicate that step one of the self calibration is in progress. Hold the TiltStick "flat" and press the calibration button again within 15 seconds. The yellow LED will start flashing twice each second to indicate that this was successful and that the second step of the calibration is to be performed. Tilt the TiltStick 90 degrees to the right and again press the calibration button within 15 seconds. The LED will start blinking three times each second. Tilt the TiltStick 90 degrees down and once more press the calibration button within 15 seconds. The LED will start blinking four times each second. Fortunately the USB HID drivers are included and joystick devices are detected and included into the input event subsystem. The only part missing is the handling of the actual joystick events and their forwarding...




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