On Mon, 20 Sept 2021 at 15:52, Peter Maydell <peter.mayd...@linaro.org> wrote:
> >> Why "lsb" ? > >> > > > > In my head, using LSB seemed more precise since I know exactly what > value will > > be set to the registers, and exactly what I will get back when reading > versus passing > > in a float that's needs to be conveted as a 'best-fit' scenario in > 0.125°C steps? > > My question was really, "what does 'lsb' mean here"? I would usually > assume "least significant bit", but that makes no sense in this context. > > Ha, sorry. Least significant bit, yes. It gets used in sensor and IC datasheets all the time and basically means '1 bit', so if the DS says 0.125°C per LSB it means that value for 1 bit, or to multiply the integer by the 1 LSB value. Conversion factors from raw units to standard SI units are almost always indicated this way, though, such as 'LSB/Gauss', etc. Well, given that the device specifically changes the value it > shows the guest based on guest-programmable gain settings, > it does seem to me to be more natural to specify the values > in some way that represents the "real world data" that the > sensor is measuring. Ideally we would then if/when we add more > magnetometer implementations have them all use the same units, > for consistency. This is the first magnetometer we have, so this > is where we get to pick the convention. > Sounds reasonable. We have two options here: - Gauss (standard SI unit) - micro Tesla (0.01 Gauss) They've both widely used; but I think uT might be slightly more common. > > In that case, should I accept floating point inputs, however, or stick > to integers? > > When dealing with magnetometers the values can be very small, so it's > not the > > same as a temp sensor where we can provide 2300 for 23.00C. > > What sort of range and precision requirements are we talking about > here? If we can avoid having to use float that would be nice... > > Well, taking the LSM303DLHC as an example, we have 1100 LSB per Gauss at a range of +/- 1.3 Gauss, so 1 bit is: 0.0009090909091 Gauss. If we use micro Tesla (uT) we get 11 LSB per uT so the smallest value is 0.09090909091 uT ... which we could represent with something like 1000 = 1.000 uT That gives us +/- 1.3 G = +/- 130 uT = +/- 130,000, for example. This would require a 32-bit integer, though, to take into account the full range of +/-8.1 G (+/- 810 uT) = +/- 810,000. There are devices with a much wider range, like the MLX90393, which can measure up to +/- 50 mT (50,000 uT), so +/-50,000,000. That's the largest range I'm aware of personally, with +/- 1-8G (or 100-800 uT) the most common. >> > >> > + > >> > + object_property_add(obj, LSM303DLHC_MSG_PROP_TEMP, "int", > >> > + lsm303dlhc_mag_get_temperature, > >> > + lsm303dlhc_mag_set_temperature, NULL, NULL); > >> > >> What units is this in? > > > > > > LSB where 1 LSB = 0.125 C, documented elsewhere, but as per the above > > I can change this to degrees if you can clarify if this should be in > float or something > > integere based with a specific scale factor. > > Our existing temperature sensors use integer properties whose > value is "temperature in degrees C, units of 0.001 C". > Consistency with that would be best. (We should write these > conventions down somewhere. Not sure where...) > That's similar to what I propose above, based on chosing micro Tesla as the base unit, and not Gauss, so units of 0.001 uT. Kevin
