On 6/21/08, Yair Mahalalel wrote: > Hello Piclist, > > I've been an (almost) lurker on this list for a long while, but only > recently found a professional excuse to use PICs (developing new > detectors for the planned upgrade of the Large Hadron Collider at CERN, > by the way) - I attached an analog MEMS accelerometer to a 18F4553 and > built quite a decent inclinometer from them. > > I now need to build a one off enhanced version of it that will measure > acceleration at three to five locations simultaneously at much higher > precision. I got a few LIS3LV02DL digital accelerometers from ST that > seem sufficiently accurate for the job, but as the distance between the > main processor and the sensors can be as large as 2 meters, I'm not sure > what would be a robust way to connect them together. > > The accelerometers support both I2C and SPI communications, and my > initial thought was to use sufficiently heavily shielded cables (CAT-5E) > that the noise level will be acceptable, but as I understood from the > I2C specs, it is limited to 400pF total bus capacitance, which is close > to that of the cabling alone 2 meters of CAT5 cable has 400pF ? . SPI is unstandardized, so I'm even less > sure about using it in this context. As the acc. chips have no data > correction mechanisms I am quite wary of working so close to the limits - > it won't do if due to some kink or environmental change the chips will > start reporting nonsense without the main board even noticing the > difference after they're installed at the test site. > > I can think of several ways to overcome this problem - mostly having to > do with either attaching inverters/transceivers to the sensors, fitting > each with a small PIC to handle cleverer encoding schemes, or both, but > the system has to ship in a month and I wouldn't like to over-engineer > it if some common practice for handling long range transmission of > originally inter-IC protocols exists. > > My knowledge of things EE is eclectic and haphazard (as my command of > English), and I'm unused to veering off application notes and the easily > googlable, so I hope at least the problem statement is clear, and if not, > please ask ahead. > > Cheers, > Yair. > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist