AN3956

Sensors

Freescale Semiconductor

15

Altimeter Example 2:

A GPS equipped smart-phone loses GPS signal as it enters a building. Knowing the approximate elevation with the GPS, the

beginning ‘zero point’ is 300 m. Entering the building the pressure sensor starts measuring the elevation, and tries to calculate

the height in meters.

Issues in this Example for Consideration:

• When entering a building from outside, a sudden rise in pressure is highly probable. This will send two different scenarios.

It could signify that the user has ‘fallen’ in altitude by several meters (such as jumping off a building or small cliff). Or they

have entered into a building with significant pressurized airflow (typical of an air-conditioned lobby with high air flow).

• Buildings vary on height per a floor. Lobbies of sky-rises have higher ceilings. Hotel accommodation floors tend to have

shorter standard heights. A restaurant located halfway or on top of the building may have a higher ceiling. The main point

is that the altitude in meters does not ideally correspond to floor location.

• Large buildings can be pressurized or not. It depends on building design. Some stadiums for sporting events are

pressurized to keep a fabric roof afloat. Tall high-rises have multiple stages of air-conditioning units. This leads to varied

pressure per a floor depending on that section’s on/off cycle, fan speed, or the air-tight nature of that section. Elevator

shafts also can equalize pressure, or create some pressure change as elevators move up and down.

• Smart use of Altimetry in tall buildings would use building information stored in a ‘smart’ GPS device so the altitude can

translate to floor level. This way location based services, emergency 911, etc. can know which floor the user is located.

MPL115A Miniature Barometer DEMOAPEXSENSOR

pressure range of 50 to 115 kPa. This narrower range is more application specific for events occurring in that altitude or in devices

requiring that pressure. In this section, the DEMOAPEXSENSOR’s implementation of the MPL115A and the information

displayed on the APEX is described.

In the Quick Start Guide section, the “Barometer Compensated Pressure Output” describes the compensated Pressure output

of the MPL115A in screen shots of the DEMOAPEXSENSOR.

The MPL115A’s Implementation is described in detail in the Application Note AN3785; How to Implement the Freescale

MPL115A Digital Barometer. Essentially in the LCD screen shots above, the Raw values of Pressure and Temperature are

displayed in Hexadecimal and Decimal format. Following this, the six coefficients MSB+LSB are shown in Hex format. The

Using eq. 2, the altitude is calculated and shown in meters.

The Pcomp value is the compensated absolute pressure value. This value unlike the analog pressure sensors does not require

any calibration trim, or offset auto zero. The Pcomp spec for the MPL115A is such that the value has an accuracy of ± 1kPa. The

advantage is implementing the sensor and having the compensated pressure readings without any additional calibration etc. on

the customer side.

As stated in AN3785, the MCU has to apply the equation below for Pcomp given that a0, b1, b2, c11, c12, c22 are coefficients

stored in MPL115A registers. Padc and Tadc are the raw ADC values of Pressure and Temperature that are clocked out of

MPL115A digitally.

Barometer Compensated Pressure Output

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

Stage 1

Stage 2

Stage 3

Raw Values of Temperature

and Pressure displayed on

LCD screen.

Display of 6 coefficients listed

on LCD screen.

Display of Compensated

Pressure output in kPa units.

Pcomp

a0

b1

cl Padc

c12 Tadc

⋅

+

⋅

+

()

+

Padc

b2

c12

+

Tadc

⋅

() Tadc

⋅

+

⋅

=

(eq. 3)