Re: [PATCH 2/2 v2] iio: magnetometer: ak8974: Provide scaling

[Linus Walleij <linus.walleij@linaro.org>]

On Thu, Apr 16, 2020 at 5:32 PM Stephan Gerhold <stephan@gerhold.net> wrote:
> On Thu, Apr 16, 2020 at 04:09:17PM +0200, Linus Walleij wrote:

> > The manual for the HSCDTD008A gives us a scaling for the
> > three axis as +/- 2.4mT per axis.
>
> I wonder if we can really assume that this applies to
> the other models (e.g. AK8974) as well?

Patches are for testing :D

I have a Ux500 reference board with the AK8974 vanilla
variant mounted so I will check with that one.

> > +     case IIO_CHAN_INFO_SCALE:
> > +             /*
> > +              * The datasheet for HSCDTF008A, page 3 specifies the
> > +              * range of the sensor as +/- 2.4 mT per axis, which corresponds
> > +              * to +/- 2400 uT = +/- 24 Gauss. So 0x7fff is 24 Gauss and
> > +              * 0xffff is -24 Gauss. To account for the one missing value if
> > +              * we multiply by 1/S16_MAX, instead multiply with 2/U16_MAX.
> > +              */
>
> I just want to note that (according to the datasheet), HSCDTD008A
> produces either 14-bit or 15-bit measurements (depending on
> the HSCDTD008A_CTRL4_RANGE bit that we set by default).

Argh OK I will fix this. I try to get an AMI datasheet as well.

> I think this isn't exposed correctly in the AK8974_AXIS_CHANNEL() macro
> (realbits is 16 instead of 15), so this might need special casing for
> hscdt008a?

Yes definately. It's a bug. I'll make a separate patch for this.

> The reason I mention this is because I think it would also affect the
> scaling that you implement here. With 15-bit output it produces values
> from +16383 (0x3fff) (= 2.4 mT?) to -16384 (0xc000) (= -2.4 mT?).
>
> So it would never reach the 0x7fff and 0xffff you mention
> in your comment.

You're right. What I need to do is put the HSCDTD008A and
AK8974 side by side and compare.

Yours,
Linus Walleij