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:
ΔV
Δg
⎛
⎝
⎞
⋅ 1.0g ⋅ Sinθ
-------
Vout = Voff +
⎠
(eq. 4)
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.
AN3956
Sensors
Freescale Semiconductor
19
FREESCALE [ Freescale ]
