AN3956

Sensors

Freescale Semiconductor

16

This is applied in the MCU code as a series of operations so the MCU does not have to apply the equation directly. The

following is a possible sequence for the calculation Pcomp, the trimmed pressure output. Input values are in bold.

c11x1 = c11 * Padc

a11 = b1 + c11x1

c12x2 = c12 * Tadc

a1 = a11 + c12x2

c22x2 = c22 * Tadc

a2 = b2 + c22x2

a1x1 = a1 * Padc

y1 = a0 + a1x1

a2x2 = a2 * Tadc

Pcomp = y1 + a2x2

Note that the Altitude measurement displayed for the MPL115A cannot be directly compared to the previous section on the

MPXM2102A. The MPL115A has an internal 10-bit ADC, with an accuracy of ±1kPa. The system set up on the

DEMOAPEXSENSOR for the MPXM2102A shows less than 1m resolution with a 24-bit ADC. The additional hardware cost,

digital and analog noise filtering, and calibration required is substantial compared to implementing the MPL115A directly to an

MCU. The value of the altitude changes constantly on the display, but it is also a raw conversion. It could be filtered, sampled in

longer intervals etc. to give the illusion of a more stable value. Here it is shown simply to get an idea of the raw conversion result.

The MPL115A can be used to measure changes in vacuum or for barometric pressure rather than the MPXM2102A.

In the Quick Start Guide there is a section to set a threshold above the current barometric pressure for an alarm. When the

alarm is set, the status LEDs will light up green. After the alarm threshold is reached, the red LED toggles on/off while a buzzer

is sounded. This can be activated via waiting for a pressure change, or applying pressure to the DEMOAPEXSENSOR. A clear

Ziploc bag or syringe with a soft applicator tip useful in activating the alarm.

Weather Station Implementation

Freescale application note, AN3914, details altimetry and barometric weather systems and also includes code examples from

the DEMOAPEXSENSOR. There are two sections in the APEX board; a simple weather station or an advanced method. The

simple method does a comparison by asking the user to input their known altitude, and comparing this pressure to that of the

MPL115A. The delta in the value is compared in the Table 7.

This is typical of a simple application and the APEX simulates a desktop barometer that is commonly bought at retailers.

A more advanced version of this calculates the pressure change by taking values over time and seeing the delta change over

a 3 hour period. This is outlined in Table 8 (AN3914).

Table 7.

Analysis

Output

dP > +0.25 kPa

Sun Symbol

-0.25 kPa < dP < 0.25 kPa

Sun/Cloud Symbol

dP < -0.25 kPa

Rain Symbol

Table 8. Advanced Weather Determination

Analysis

Output

dP/dt > 0.25 kPa/h

Quickly rising High Pressure System, not stable

0.05 kPa/h < dP/dt < 0.25 kPa/h

Slowly rising High Pressure System, stable good weather

-0.05 kPa/h < dP/dt < 0.05 kPa/h

Stable weather condition

-0.25 kPa/h < dP/dt < -0.05 kPa/h

Slowly falling Low Pressure System, stable rainy weather

dP/dt < -0.25 kPa/h

Quickly falling Low Pressure, Thunderstorm, not stable