A Python lib for reading the MPL3115A2 over the Raspberry Pi I2C bus.

MPL3115A2.py

from smbus import SMBus
from sys import exit

#I2C ADDRESS/BITS

MPL3115A2_ADDRESS = (0x60)

#REGISTERS

MPL3115A2_REGISTER_STATUS = (0x00)
MPL3115A2_REGISTER_STATUS_TDR = 0x02
MPL3115A2_REGISTER_STATUS_PDR = 0x04
MPL3115A2_REGISTER_STATUS_PTDR = 0x08

MPL3115A2_REGISTER_PRESSURE_MSB = (0x01)
MPL3115A2_REGISTER_PRESSURE_CSB = (0x02)
MPL3115A2_REGISTER_PRESSURE_LSB = (0x03)

MPL3115A2_REGISTER_TEMP_MSB = (0x04)
MPL3115A2_REGISTER_TEMP_LSB = (0x05)

MPL3115A2_REGISTER_DR_STATUS = (0x06)

MPL3115A2_OUT_P_DELTA_MSB = (0x07)
MPL3115A2_OUT_P_DELTA_CSB = (0x08)
MPL3115A2_OUT_P_DELTA_LSB = (0x09)

MPL3115A2_OUT_T_DELTA_MSB = (0x0A)
MPL3115A2_OUT_T_DELTA_LSB = (0x0B)

MPL3115A2_BAR_IN_MSB = (0x14)

MPL3115A2_WHOAMI = (0x0C)

#BITS

MPL3115A2_PT_DATA_CFG = 0x13
MPL3115A2_PT_DATA_CFG_TDEFE = 0x01
MPL3115A2_PT_DATA_CFG_PDEFE = 0x02
MPL3115A2_PT_DATA_CFG_DREM = 0x04

MPL3115A2_CTRL_REG1 = (0x26)
MPL3115A2_CTRL_REG1_SBYB = 0x01
MPL3115A2_CTRL_REG1_OST = 0x02
MPL3115A2_CTRL_REG1_RST = 0x04
MPL3115A2_CTRL_REG1_OS1 = 0x00
MPL3115A2_CTRL_REG1_OS2 = 0x08
MPL3115A2_CTRL_REG1_OS4 = 0x10
MPL3115A2_CTRL_REG1_OS8 = 0x18
MPL3115A2_CTRL_REG1_OS16 = 0x20
MPL3115A2_CTRL_REG1_OS32 = 0x28
MPL3115A2_CTRL_REG1_OS64 = 0x30
MPL3115A2_CTRL_REG1_OS128 = 0x38
MPL3115A2_CTRL_REG1_RAW = 0x40
MPL3115A2_CTRL_REG1_ALT = 0x80
MPL3115A2_CTRL_REG1_BAR = 0x00
MPL3115A2_CTRL_REG2 = (0x27)
MPL3115A2_CTRL_REG3 = (0x28)
MPL3115A2_CTRL_REG4 = (0x29)
MPL3115A2_CTRL_REG5 = (0x2A)

MPL3115A2_REGISTER_STARTCONVERSION = (0x12)


bus = SMBus(1)

whoami = bus.read_byte_data(MPL3115A2_ADDRESS, MPL3115A2_WHOAMI)
if whoami != 0xc4:
    print "Device not active."
    exit(1)

bus.write_byte_data(
    MPL3115A2_ADDRESS,
    MPL3115A2_CTRL_REG1,
    MPL3115A2_CTRL_REG1_SBYB |
    MPL3115A2_CTRL_REG1_OS128 |
    MPL3115A2_CTRL_REG1_ALT)

bus.write_byte_data(
    MPL3115A2_ADDRESS,
    MPL3115A2_PT_DATA_CFG, 
    MPL3115A2_PT_DATA_CFG_TDEFE |
    MPL3115A2_PT_DATA_CFG_PDEFE |
    MPL3115A2_PT_DATA_CFG_DREM)

def poll():
    sta = 0
    while not (sta & MPL3115A2_REGISTER_STATUS_PDR):
        sta = bus.read_byte_data(MPL3115A2_ADDRESS, MPL3115A2_REGISTER_STATUS)


def altitude():
    bus.write_byte_data(
        MPL3115A2_ADDRESS,
        MPL3115A2_CTRL_REG1, 
        MPL3115A2_CTRL_REG1_SBYB |
        MPL3115A2_CTRL_REG1_OS128 |
        MPL3115A2_CTRL_REG1_ALT)

    poll()
    
    msb, csb, lsb = bus.read_i2c_block_data(MPL3115A2_ADDRESS,MPL3115A2_REGISTER_PRESSURE_MSB,3)
    print msb, csb, lsb

    alt = ((msb<<24) | (csb<<16) | (lsb<<8)) / 65536.

    # correct sign
    if alt > (1<<15):
        alt -= 1<<16

    return alt


def pressure():
    bus.write_byte_data(
        MPL3115A2_ADDRESS,
        MPL3115A2_CTRL_REG1, 
        MPL3115A2_CTRL_REG1_SBYB |
	    MPL3115A2_CTRL_REG1_OS128 |
	    MPL3115A2_CTRL_REG1_BAR)

    poll()
    
    msb, csb, lsb = bus.read_i2c_block_data(MPL3115A2_ADDRESS,MPL3115A2_REGISTER_PRESSURE_MSB,3)
    print msb, csb, lsb

    return ((msb<<16) | (csb<<8) | lsb) / 64.

def calibrate():
    pa = int(pressure()/2)
    bus.write_i2c_block_data(MPL3115A2_ADDRESS, MPL3115A2_BAR_IN_MSB, [pa>>8 & 0xff, pa & 0xff])

I have a Sparkfun Weather Shield with the HTU21D and MPL3115A2 chips contained on it connected to my pi. I found another library for the HTU21D and it's working fine, however, I never make it past the "Device not active" part of this code. I've tried a couple other modules and get the same result. If I comment out that piece of code it runs properly and gives back values but they aren't accurate. Pressure = 47492 and Altitude = 11780 (I'm at around 600 feet). Any ideas what is going on or how to fix it?

@kronmall There's a kernel thing that you can enable which seems to help. See this page for details: http://ciaduck.blogspot.co.uk/2014/12/mpl3115a2-sensor-with-raspberry-pi.html

Setting that kernel flag stops this code outputting "Device not active", but I don't think the results I do get from it are that accurate, either. Did you get any further with it?

My code:

import MPL3115A2

#print MPL3115A2.pressure()
print MPL3115A2.altitude()

Output for pressure:

1 0 144
1026.25

Output for altitude:

95 251 208
24571.8125

A BMP280 sensor connected to the same Pi gives this output:

Temp = 21.84 *C
Pressure = 98467.00 Pa
Altitude = 240.71 m
Sealevel Pressure = 98460.00 Pa

Edit: Actually, the code on that linked blogspot page above gives more sensible output:

0xc4
Waiting for data...
Reading sensor data...
status: 0b0
Pressure and Temperature at 06/29/2017 10:28:40+0100
98287.25 Pa
18.8125°C
65.8625°F