by Brent Dirks
October 16, 2013
Apple’s use of a new chip supplier for the iPhone 5s accelerometer might be behind the well-publicized issues with the device. That’s according to developer RealityCap, who dedicates an interesting blog post to the subject. In a recent teardown of the new handset by Chipworks, the accelerometer was identified as the Bosch Sensortech BMA220. The iPhone 5 version was made by STMicroelectronics. While the first key stat for accelerometers, noise density, is very similar between the two chips, there is a large difference in a second spec - zero-g offset:
The second key spec for accelerometers is the zero-g offset, or bias. This indicates the range for a roughly constant offset that will be added to every output sample of data due to manufacturing variance. This can also change over time due to mechanical stress or temperature variation. This is where we find the problem: the typical bias for the ST part is +/- 20mg, while the Bosch part lists +/-95mg. This almost 5x greater offset range is confirmed by our measurements, and is absolutely consistent with the failures being reported by users and the media. Specifically, a +/- 20mg offset range would translate to around a +/-1 degree accuracy range in tilt detection, and a +/-95mg offset translates to +/-5 degrees in tilt.RealityCap’s CEO Eagle Jones said there is a bit of good news for developers, though:
So what can developers who depend on reasonable accuracy from the accelerometer do? The good news is that a large component of the bias error in the accelerometer doesn’t change. Thus it is possible to work around the problem by incorporating a calibration procedure into apps. This procedure would ask the user to place the device in different orientations to determine the accelerometer bias. Apps can then subtract this measured bias from the data coming from the accelerometer to get a corrected reading.Earlier this month, news first broke that the measuring tools in the new handset – including the gyroscope, on-board compass, and accelerometer – were quite inaccurate when compared to the iPhone 5. Apple has yet to comment on the issues.