Saturday, December 23, 2017

Possible noisy I2C or power lines

This is not a progress report regarding Airball per se, but rather a discussion of the kinds of problems one obtains when doing homebrew PCB design and assembly without much experience.

My last post noted how apparent transients were causing the pressure sensors to freeze up, and they would thereafter not recover without removing the battery. Here I'm going to show another effect of what may be the same underlying problem: unexplained blips in the sensor readings. Note that my probe reports data in engineering units, so these are not the binary results. But the results I observe lead me to suspect noise of some sort leading to incorrect readings over I2C. Here are the 4 variables -- temperature and 3 pressures -- reported with the probe sitting on my desk:

And here is a zoomed-in chunk of the delta P beta reading showing just one data point of the jitter:

Clearly that's no way to take data. :/ Also note that while the temperature signal has some jitter, it has less of it than the pressure sensor signals.

My suspicion right now is that something is deeply wrong with my board. A trace is broken, or a solder joint is not complete, or ... something. So there are two avenues along which to proceed:
  1. Build up a new board and see if that fixes things; and
  2. Scope the I2C signals and see if I can detect undue noise.

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