Nuts and Volts - August 2015

The BMP085 — produced by Bosch Sensortec — is just such a device. This eight- pin marvel is exceedingly accurate, yet priced to be within easy reach of the most frugal DIYer. Communication with the chip is by way of the popular I2C bus, meaning that we can concentrate almost exclusively on manipulating the data it sends, rather than fussing over how to receive the data in the first place. As icing on the cake, the BMP085 also contains an integral thermometer.

Not only is knowing the temperature useful to us humans, but the value detected also automatically figures into the barometric pressure measurement.

If you’re a weather junkie like me who’s always wanted a decent barometer which doesn’t break the bank, then read on! In this article, you’ll learn how to interface the BMP085 to most any PIC microcontroller. After covering the electrical requirements, we’ll unravel the somewhat nasty software aspects (don’t worry — all of the ugly mathematics and tricky data type manipulations have been taken care of for you). With what you learn here, you’ll be all set to design a customized barometer and start making your own weather forecasts.

One final thing before we tuck into the details. Using the BMP085 with an Arduino is a breeze since software libraries already exist for it, but going with a microcontroller chip instead has a number of advantages. The PIC16F1825 that I used costs about one-tenth of an Arduino. Moreover, the required program code runs half as long.

Finally, with a PIC you get total control of the design at a much lower level. When you throw in the fact that we’ll be using a free compiler to create the firmware, we’re talking a complete high performance barometer for about ten smackers!

Hooking It Up

The BMP085 is a tiny device (about a quarter inch square) intended for surface-mount construction. I don’t know about you, but my eyes and hands are way too old to deal with such things. Fortunately, the component is readily available already soldered to a breakout board from a number of suppliers. See Figures 1 and 2 which show the front and back.

This nifty affair features standard

0.1” pin spacing, meaning that it’s easy to socket on a circuit board or try out on a solderless breadboard. I got mine for some six bucks from Amazon, but if you shop around you’ll find it for even less on eBay. Just so you know, you might also bump into the BMP180 (and breakout board) which is a newer version of the thing. The hardware and software described in this article are compatible with either.

Apart from making the chip more conducive to hand construction, the breakout board sports two other features to really help things along. First, you need to know that the BMP085 is a 3.3V device, but frequently (as with the circuit described herein) you’ll want to use it with a 5V microcontroller. Conveniently, the breakout board comes loaded with a regulator and appropriate decoupling capacitors to derive the required lower voltage.

It was mentioned above that communication with the chip proper is by means of the relatively straightforward I2C bus, which is a two-wire affair. If you’ve played with this protocol at all, then you’ll already know that the two lines comprising it — called SCL and SDA — each require a pull-up resistor. These, too, are provided on the breakout board.

Now, one thing might worry you at this point. If the microcontroller is powered by 5V and the BMP085 by 3.3V, are we cruising for a bruising on the I2C bus with incompatible clock and data voltages? Not at all. One of the excellent features of this protocol is that the bus lines are configured around open-drain FETs. Instead of forcing pins high (to a possibly hazardous voltage for the BMP085), instead we’re only safely bringing them low through those pull-up resistors just pointed out.