AN3956

Sensors

Freescale Semiconductor

9

Altimetry

1. Alti/Baro

Press

2. Waterlevel

UPorDN

3. Inertial

4. Additional

‘1’

1. Altimetry

Press

a. MPXM2102A

UPorDN

b. MPL115A2

c. Experimental

‘a’

Raw Value

Altitude

0x017c1d

2(LP) value

Pressure

0x017c1f

Select the MPXM2102A,

option ‘a’ for higher resolution,

amplified pressure readings

for Altimetry.

Note the way that the output is displayed. The Raw value of the

24-bit ADC value is displayed on the left. The bottom left has the

raw value after it passes two low pass filters in software. This value

is mores stable but takes longer to update.

On the right the Altitude is displayed. This is the pressure

converted to altitude in (m). The Pressure corresponding to the

ADC value is shown in the bottom right.

The board can be placed on the table. Note the Altitude

measurement as it stabilizes. Then raise it above your head as a

‘1m’ increase. The value should go up by 1m. Place on the table

again. It should drop to the original value. Place on the floor, it will

again decrement 1m. Note that ambient pressure from A/C units

can affect this value drastically.

Barometer Compensated Pressure Output

1. Altimetry

Press

a. MPXM2102A

UPorDN

b. MPL115A2

c. Experimental

‘b’

MPL115A2

RawValues

Press ADC

74c0

0467

Temp ADC

6b40

0428

ENTER for Coefficients

6 Coefficients

ao = 3dc4

c11 = f8a0

b1 = bd7a

c12 = 2flc

b2 = c299

c22 = 0dc0

Compensated Pressure

Altitude = 365 m

The values for the MPL115A

are shown here. To the left are

the Pressure and Temp ADC

values in their raw form in

HEX. To the right of them are

the shifted values displayed in

decimal.

Values of the Coefficients are

shown here. These are used

for the next step in calculating

the compensated Pressure.

Compensated Pressure is

displayed. This is with an

accuracy of ±1kPa.

Press ENT to continue to the

Alarm section.

ALARM System

Enter your Threshold

0.50

kPa

Enter for Activation

ALARM sounds when

Threshold is reached

Press.

ALARM

97.324

Continued from the MPL115A

compensated Barometer

section above. Press UP and

DN to set the Alarm Threshold

in increments of 0.25kPa.

Press ENT to activate the

Alarm

Alarm is now Active. Note the

outputs of the current Pressure

in kPa, and the Alarm

Threshold point. Once

crossed, the red LED will flash

as the buzzer sounds. Try

activating by pressurizing the

MPL115A. This can be done

with a plastic syringe with

rubber tubing at the end to

make an air tight fit.

ENT

ENT

ENT

ENT

ENT

ENT

ENT