I²C Precision Altimeter¶

The MPL3115A2 employs a MEMS pressure sensor with an I2C interface to provide accurate Pressure/Altitude and Temperature data. The sensor outputs are digitized by a high resolution 24-bit ADC. Internal processing removes compensation tasks from the host MCU system. Multiple user-programmable, power saving, interrupt and autonomous data acquisition modes are available, including programmed acquisition cycle timing, and poll-only modes. Typical active supply current is 40 uA per measurement-second for a stable 30 cm output resolution. Pressure output can be resolved with output in fractions of a Pascal, and Altitude can be resolved in fractions of a meter.

Features¶

• 1.95V to 3.6V Supply Voltage, internally regulated by LDO
• 1.6V to 3.6V Digital Interface Supply Voltage
• Fully Compensated internally
• Direct Reading, Compensated
• - - Pressure: 20-bit measurement (Pascals)
• - - Altitude: 20-bit measurement (meters)
• - - Temperature: 12-bit measurement (degrees Celsius)
• Programmable Events
• Autonomous Data Acquisition
• Resolution down to 1 ft. / 30 cm
• 32 Sample FIFO
• Ability to log data up to 12 days using the FIFO
• 1 second to 9 hour data acquisition rate
• I2C digital output interface (operates up to 400 kHz)

Motivation¶

The motivation here was to get a set of support classes together that supported the chip and could be expanded on. With this library you can extract all relevant data from the sensor. Currently the classes do not support using the FIFO or programmable interrupts. Although methods in the class are there to support adding those features.

This library was inspired by the similar library available for the Arduino written by Nathan Seidle at SparkFun. I copied some of the number crunching routines and tried to follow his style of coding for the library. Arduino users should find it easy to switch to an mbed device and port their code using this library.

Interface¶

This library was created using the mbed NXP LPC11U24. Pins p27 and p28 were used for the I²C functions. Be sure to install 1K pull-up resistors on both lines. I haven't found a way on the mbed to enable the internal pull-up resistors. Also, if you're not using the SparkFun breakout board, be sure to use the right caps on the power pin. If you don't, the jitter can cause problems.

Reference¶

• MPL3115A2 Datasheet
• MPL3115A2 Application Note AN4519
• MPL3115A2 SparkFun Breakout Board

API¶

Example¶

#include "mbed.h"
#include "MPL3115A2.h"

I2C i2c(p28, p27);       // sda, scl
Serial pc(USBTX, USBRX); // tx, rx
MPL3115A2 sensor(&i2c, &pc);

DigitalOut myled(LED1);     // Sanity check to make sure the program is working.
DigitalOut powerPin(p21);   // <-- I powered the sensor from a pin. You don't have to.

int main() {
    
    powerPin = 1;
    wait_ms(300);

    pc.printf("** MPL3115A2 SENSOR **\r\n");

    sensor.init();

    pc.printf("Who Am I: 0x%X\r\n", sensor.whoAmI());

    Altitude a;
    Temperature t;
    Pressure p;
    
    // Offsets for Dacula, GA
    sensor.setOffsetAltitude(83);
    sensor.setOffsetTemperature(20);
    sensor.setOffsetPressure(-32);
    
    while(1) 
    {
        sensor.readAltitude(&a);
        sensor.readTemperature(&t);
        
        sensor.setModeStandby();
        sensor.setModeBarometer();
        sensor.setModeActive();
        sensor.readPressure(&p);
        
        pc.printf("Altitude: %sft, Temp: %sºF, Pressure: %sPa\r\n", a.print(), t.print(), p.print());
        pc.printf("OFF_H: 0x%X, OFF_T: 0x%X, OFF_P: 0x%X\r\n", sensor.offsetAltitude(), sensor.offsetTemperature(), sensor.offsetPressure());
    
        myled = 1;
        wait(5);
        myled = 0;
        wait(5);

        sensor.setModeStandby();
        sensor.setModeAltimeter();
        sensor.setModeActive();
    }
}

Debugging¶

See Also¶

• MPL115A2 I²C MEMS barometric and temperature sensor