Another fun little i2c device acquired cheaply via eBay. It reads barometric pressure and temperature. There’s some relatively simple maths to convert the outputs to something readable.
Once again, ADAFruit provide a fantastic guide and library for using the device in their Using the BMP085 with Raspberry Pi. As in previous posts, I could have just used the ADAFruit library out of the box. Whilst I do not feel the need to re-invent the wheel and re-code from scratch, I do like to have an understanding how it works.
I drew the information from a couple of other sources:
- Maarten Damen’s Bus Pirate reading the BMP085 Temperature (site down at the time of writing this)
- Python port of John Burn’s C Code in Reading data from a Bosch BMP085 with a Raspberry Pi.
Below is the very simple schematic for connecting the device to the Pi.
importsmbusimporttimeclassBMP085(): OSS =3 def__init__(self, i2c, address): self.i2c =i2c self.address =address self.ac1 =self.readSignedWord(0xaa) self.ac2 =self.readSignedWord(0xac) self.ac3 =self.readSignedWord(0xae) self.ac4 =self.readWord(0xb0) self.ac5 =self.readWord(0xb2) self.ac6 =self.readWord(0xb4) self.b1 =self.readSignedWord(0xb6) self.b2 =self.readSignedWord(0xb8) self.mb =self.readSignedWord(0xba) self.mc =self.readSignedWord(0xbc) self.md =self.readSignedWord(0xbe) defreadWord(self, reg): msb =self.i2c.read_byte_data(self.address, reg) lsb =self.i2c.read_byte_data(self.address, reg+1) value =(msb << 8) +lsb returnvalue defreadSignedWord(self, reg): msb =self.i2c.read_byte_data(self.address, reg) lsb =self.i2c.read_byte_data(self.address, reg+1) if(msb > 127): msb =msb -256 value =(msb << 8) +lsb returnvalue defreadUT(self): self.i2c.write_byte_data(self.address, 0xf4, 0x2e) time.sleep(0.0045) ut =self.readWord(0xf6) returnut defreadTemperature(self): ut =self.readUT() x1 =((ut -self.ac6) *self.ac5) >> 15 x2 =(self.mc << 11) /(x1 +self.md) self.b5 =x1 +x2 return((self.b5 +8) >> 4) /10.0 defreadUP(self): self.i2c.write_byte_data(self.address, 0xf4, 0x34+(self.OSS << 6)) delay =(2+(3<< self.OSS)) /1000.0 time.sleep(delay) msb =self.i2c.read_byte_data(self.address, 0xf6) lsb =self.i2c.read_byte_data(self.address, 0xf7) xlsb =self.i2c.read_byte_data(self.address, 0xf8) up =(msb << 16) +(lsb << 8) +xlsb up =up >> (8-self.OSS)
returnup defreadPressure(self): up =self.readUP() b6 =self.b5 -4000 x1 =(self.b2 *(b6 *b6)>>12)>>11 x2 =(self.ac2 *b6)>>11 x3 =x1 +x2 b3 =(((self.ac1 *4+x3)<>2 x1 =(self.ac3 *b6)>>13 x2 =(self.b1 *((b6 *b6)>>12))>>16 x3 =((x1 +x2) +2)>>2 b4 =(self.ac4 *(x3 +32768))>>15 b7 =(up -b3) *(50000>>self.OSS) if(b7 < 0x80000000): p =(b7<<1)/b4 else: p =(b7/b4)<>8) *(p>>8) x1 =(x1 *3038)>>16 x2 =(-7357*p)>>16 p +=(x1 +x2 +3791)>>4 returnpi2c =smbus.SMBus(0)bmp085 =BMP085(i2c, 0x77)t =bmp085.readTemperature()p =bmp085.readPressure()print"Temperature: %.2f C"%tprint"Pressure: %.2f hPa"%(p /100)For more detail: Raspberry Pi reading BMP085 Temperature & Pressure
