AN3956

Sensors

Freescale Semiconductor

19

Inertial MMA7361L

The DEMOAPEXSENSOR has a quick implementation of the 3 axis selectable low-g accelerometer. Notably the demo board

implements the tilt function of the accelerometer and Free Fall detection. The Quick Start Guide shows the modes to select either

function.

TILT ANGLE ON THE DEMOAPEXSENSOR

The ADC counts are converted to tilt angles using the equation below:

Where:

Vout = Analog output of accelerometer

Voff = Offset voltage of accelerometer

V/g = Sensitivity

1.0g = Earth’s gravity

θ = Tilt Angle

Note: MMA7361 was set to a g-level of 1.5 g.

Offset voltage is typically 1.65 V @ 3.3 V biasing supply

FREE FALL DETECT ON THE DEMOAPEXSENSOR

Free Fall detect is the situation where all three axes of the inertial sensor converge towards the offset or ‘0g’ range. This is

also given as an output on the MMA7361L as the 0g-Detect pin. ‘1’ or logic high is the output for a detected Free Fall. Note that

this is also for Free Fall in a linear fashion such that the board is not spinning or in rotation as it falls. The DEMOAPEXSENSOR

emits a buzzer noise and a flashing red LED as the Free Fall detect occurs.

Vout

Voff

ΔV

Δg

-------- 1.0g Sin

θ

⋅⋅

⎝⎠

⎛⎞

+

=

(eq. 4)