Re: [PATCH v2] hw/sensor: Add lsm303dlhc magnetometer device

[Peter Maydell]

On Mon, 20 Sept 2021 at 15:22, Kevin Townsend <kevin.townsend@linaro.org> wrote:
> On Mon, 20 Sept 2021 at 15:52, Peter Maydell <peter.maydell@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.

OK, that's not a convention I've encountered before.

>> 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.

That's OK -- our "int" properties are int64_t. So we could easily
fit something like 10000 == 1.0000 uT, in case we might want
the extra precision in future. That would be 1,000,000 == 1 G
(assuming I haven't messed up my arithmetic ;-))

> 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.

thanks
-- PMM