mpl115a2.py

#!/usr/bin/env python2.7

from smbus import SMBus
import logging
from datetime import datetime
from time import sleep
import struct

class MPL115A2(object):
    # device registers
    _MPL115A2_ADDR = 0x60
    _MPL115A2_REG_PRESSURE = 0x00
    _MPL115A2_REG_TEMP     = 0x02
    _MPL115A2_REG_COEFF_OFFSET = 0x04
    _MPL115A2_REG_STARTCONVERSION = 0x12
    _MPL115A2_CMD_STARTCONVERSION = 0x12

    def __init__(self, bus):
        self.bus = SMBus(bus)
        self.a0 = None
        self.b1 = None
        self.b2 = None
        self.c12= None
        self.pres = None
        self.temp = None
        self.__read_coeffient()

    def __read_coeffient(self):
        # read registers
        reg = []
        for i in range(8):
            v = self.bus.read_byte_data(self._MPL115A2_ADDR, 
                self._MPL115A2_REG_COEFF_OFFSET + i)
            reg.append(v)
        # convert to 4 set of signed 16bit values
        (a0, b1, b2, c12) = struct.unpack(">hhhh",
                ''.join([chr(x) for x in reg]))
        self.a0 = float(a0) / (1 << 3)
        self.b1 = float(b1) / (1 << 13)
        self.b2 = float(b2) / (1 << 14)
        self.c12 = float((c12 >> 2)) / (1 << 22)
        logging.debug("Coefficient: a0: %f, b1: %f, b2: %f, c12: %f",
                self.a0, self.b1, self.b2, self.c12)

    def read_sensor(self):
        # start conversion
        self.bus.write_i2c_block_data(self._MPL115A2_ADDR,
                self._MPL115A2_REG_STARTCONVERSION,
                [self._MPL115A2_CMD_STARTCONVERSION])
        # wait until the conversion is finished
        sleep(0.005) # wait 5ms
        # read registers
        msb = self.bus.read_byte_data(self._MPL115A2_ADDR, 
                self._MPL115A2_REG_PRESSURE+0)
        lsb = self.bus.read_byte_data(self._MPL115A2_ADDR, 
                self._MPL115A2_REG_PRESSURE+1)
        self.pres = ((msb << 8 | lsb) & 0xffc0) >> 6

        msb = self.bus.read_byte_data(self._MPL115A2_ADDR, 
                self._MPL115A2_REG_TEMP+0)
        lsb = self.bus.read_byte_data(self._MPL115A2_ADDR, 
                self._MPL115A2_REG_TEMP+1)
        self.temp = ((msb << 8 | lsb) & 0xffc0) >> 6

    def pressure(self):
        '''
        c12x2 = self.c12 * self.temp
        a1    = self.b1 + c12x2
        a1x1  = a1 * self.pres
        y1    = self.a0 + a1x1
        a2x2  = self.b2 * self.temp
        pcomp = y1 + a2x2
        '''
        pcomp = self.a0
        pcomp += (self.b1 + self.c12 * self.temp ) * self.pres
        pcomp += self.b2 * self.temp
        return (((650.0 / 1023.0) * pcomp) + 500.0) 

    def temperature(self):
        return ((self.temp - 498.0) / -5.35) + 25.0

    def value(self):
        p = self.pressure()
        t = self.temperature()
        return [p, t]

if __name__ == "__main__":
    logging.basicConfig(
        level=logging.DEBUG,
        format="%(asctime)s %(levelname)-8s %(message)s",
        datefmt="%Y-%m-%d(%a) %H:%M:%S",
        filename='/tmp/plesure.log')

    sensor = MPL115A2(1)
    while (1):
        sensor.read_sensor()
        (p, t) = sensor.value()
        logging.info("Pres: %04.2fhPa, Temp: %02.2fdeg-C" , p, t)
        sleep (10)