Posted: September 25, 2014

Contributed by: St.J. Dixon-Warren

The iPhone 6 contains a six-axis InvenSense combination gyroscope-accelerometer and a Bosch BMA280 three-axis accelerometer.  There has been some considerable debate in the blogosphere as to why Apple might have incorporated the extra accelerometer functionality. We speculate that the two devices are incorporated to improve the overall user experience, while minimizing power consumption.

We believe the InvenSense device to be the MPU-6700, based on Chipworks decoding of the package markings. A data sheet is not available for the MPU-6700; however, we expect the specifications to be similar to those published by InvenSense for the MPU-6500.

  

A comparison of the published spec’s for the BMA280 and the MPU-6500 point to several reasons why Apple might have chosen to include the additional accelerometer functionality.  Some parameters from the two datasheets are summarized below.

The InvenSense Device

The InvenSense device can operate as a six axis inertial sensor, or as either a three-axis gyroscope or a three-axis accelerometer.  It is rated to consume 3.4 mA in the six-axis mode, 3.2 mA in the gyroscope mode and 450 µA in the accelerometer normal mode.  By contrast, the Bosch device operates as a 3-axis accelerometer only and it consumes 130 µA of current in the accelerometer normal mode.  Both devices offer two low power levels of operation for the accelerometer function.  The InvenSense device actually consumes less current in its lowest power mode, with a 1 Hz update rate.

The main benefit of the InvenSense is full six-axis integration of the data by the on-chip digital motion processor (DMP). This will provide a direct benefit for gaming and other applications that need sophisticated inertial sensing capabilities. In addition the InvenSense provides significantly higher sensitivity than the Bosch device. The price however, is higher power consumption.

The Bosch Device

The Bosch device, on the other hand, operates at lower power and has much faster start up time. The Bosch device will be used in situation where full six-axis integration is not required and where lower sensitivity is acceptable, for example for purposes such as screen orientation and pedometer functionality.

The integration of two accelerometers into the iPhone 6 is another example of Apple's elegant engineering. The phone would have worked with just the InvenSense device, but since not all applications require the higher sensitivity and full six-axis integration, Apple added the Bosch device, which allows them to lower the power consumption while still providing a good user experience.

Comparison of Bosch BMA280 and InvenSense MPU-6500 Specifications

Parameter

Bosch BMA280

InvenSense MPU-6500

Units

ADC range

14

16

bit

Acceleration range

±2, ±4, ±8, ±16

±2, ±4, ±8, ±16

g

Maximum sensitivity

4096

16684

LSB/g

Temperature sensitivity

0.015

0.026

%/K

Cross axis  sensitivity

1

2

%

Nonlinearity

0.5

0.5

%

Output data rate

2000

4000

Hz

Cold start up time

3

30

ms

Sleep mode start up time

1

20

ms

Accelerometer start up time from sleep mode

1.3

20

ms

Accelerometer supply current in normal mode

130

450

µA

Accelerometer supply current in low power mode 1

6.5

7.27

µA

Accelerometer supply current in low power mode 2

66

18.65

µA

Sleep mode current

2.1

6

µA