On Tue, 30 May 2023 11:20:59 +0300 Matti Vaittinen <mazziesaccount@xxxxxxxxx> wrote: > Hi dee Ho peeps, > > Once again I am trying to learn from more experienced sensor folks :) (Someone get Matti some nice simple devices to add support for - he needs a rest!) :) > > I am currently working (also) on ROHM BM1390 pressure sensor. The sensor > can measure pressure ranging 300 - 1300 hPa. Measurement times can be > around 20 - 160 mS depending on the amount of samples being averaged. > There is also an IIR filter included in sensor with 3 different settings > titled as "weak", "middle" and "strong". Unfortunately the exact maths > is not explained. Gah. We all love this sort of dumbing down in datasheets. There is a picture though... Hmm. Anyone remember enough about filter design to tell me if you can go from a 63% point on a step response (in time domain) to a 3DB in frequency? I'd go with probably not but it's been far too many years since I did any of this so I could be wrong. I'd kick this into the long grass - pressure sensors that aren't the type that you connect to a pipe tend to be for measuring slow changes, so I'd just set it to the middle value and not worry about making it configurable. Can revisit later. > Furthermore, the sensor can be configured to store > maximum of 4 measured pressure samples in a HW FIFO. > > The sensor can also measure temperature and error for temperature > measurement is told to be +/- 2C when temperature is in a range of 25 C > - 85 C. On the other hand, operating temperature for the device is said > to be -40 C - 85 C. Helpful to have a sensor with no defined precision in the range in which the device is meant to operate. *sniggers* I wonder if it's a typo as the operating range matches the storage range which is unusual. > > Now, the temperature measurements are not stored in a FIFO. > > Here comes my question - what do you see as the typical use-cases for > such a sensor? Or, to be more exact, do you think "quick" changes in > temperature are expected to be measured with this type of sensor? My understanding of pressure sensors is a bit weak so the following might not be accurate... No. Temp stuff on pressure sensors is normally there for temperature compensation algorithms. > I am > asking this because I would like to support using the hardware FIFO and > I am wondering if reading the temperature at FIFO flush and populating > also the temperature channel values corresponding to all (up to 4) > pressure measurements would be the best approach? > > Other options is to not support using FIFO when temperature is scanned. A third is only let the temperature be read from sysfs. That's fairly common in devices like this where the temp sensor isn't expected to be read as often as the others and maps to what we can actually control. (scan_index == -1) IIRC some of the IMUs do this as again temp not expected to change much, but it if has radically different stable values it will affect precision calibration. > > I have a gut feeling that the users who measure pressure are often also > interested in getting the temperatures. Hence I would like to allow > getting both. "Cheating" by using the same temperature value (measured > when FIFO is flushed) should be Ok if temperature is not changing > rapidly as even with the slowest measurement speed collecting 4 samples > should finish in well under a second - and mentioned accuracy for the > temperature sensor is such that small temperature changes are probably > going unnoticed anyways. Fifo normally only of interest at high rates anyway, not that it makes much practical difference when implementing the driver. > > Here I would love to have an opinion from more experienced IIO > developers/users - and potentially a hint how similar sensors have been > handled previously. (I would assume not storing some slowly changing > values in a HW FIFO is quite common because HW FIFOs won't come without > a cost). I don't really mind if you either go with 1 - fill it copies of same temperature. 2 - only have temp via sysfs (this I know we've done a bunch of times) The mess of switch between fifo and non fifo depending on whether the temp sensor is in the scan strikes me as too fiddly to make sense. Jonathan > > Yours, > -- Matti > > Oh, in case someone wants to see the data sheet: > https://fscdn.rohm.com/en/products/databook/datasheet/ic/sensor/pressure/bm1390glv-z-e.pdf > >
