DeviceMotion.mm

/*
If a device has both an accelerometer and a gyroscope, Core Motion offers a device-motion service that processes raw motion data from both sensors. Device motion uses sensor fusion algorithms to refine the raw data and generate information for a device’s attitude, its unbiased rotation rate, the direction of gravity on a device, and the user-generated acceleration. An instance of the CMDeviceMotion class encapsulates all of this data. Additionally, you do not need to filter the acceleration data because device-motion separates gravity and user acceleration.[1]
[1] http://developer.apple.com/library/ios/#documentation/CoreMotion/Reference/CMDeviceMotion_Class/Reference/Reference.html#//apple_ref/doc/uid/TP40009942

*/

[_manager startDeviceMotionUpdates];

/*
the following is in a timer callback:
*/
CMAcceleration deviceMotion = [[_manager deviceMotion] userAcceleration];
double magnitude = sqrt( pow(deviceMotion.x,2) + pow(deviceMotion.y, 2) + pow(deviceMotion.z,2));
if (magnitude < USER_ACCE_THRESHOLD)
// handle as we want