On Thu, Apr 16, 2020 at 08:17:46PM +0200, Michał Mirosław 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. > > > > When I implement this the biggest axis indicates 0.59 Gauss > > which is a reasonable measurement for the earths magnetic > > which is in the range of 0.25 to 0.65 Gauss on the surface > > according to Wikipedia. > > > > Cc: Nick Reitemeyer <nick.reitemeyer@xxxxxx> > > Cc: Stephan Gerhold <stephan@xxxxxxxxxxx> > > Cc: Michał Mirosław <mirq-linux@xxxxxxxxxxxx> > > Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx> > > --- > > ChangeLog v1->v2: > > - Split out the measurement refactoring. > > --- > > drivers/iio/magnetometer/ak8974.c | 15 ++++++++++++++- > > 1 file changed, 14 insertions(+), 1 deletion(-) > > > > diff --git a/drivers/iio/magnetometer/ak8974.c b/drivers/iio/magnetometer/ak8974.c > > index 5361647b9054..effcdd93e650 100644 > > --- a/drivers/iio/magnetometer/ak8974.c > > +++ b/drivers/iio/magnetometer/ak8974.c > > @@ -603,6 +603,18 @@ static int ak8974_read_raw(struct iio_dev *indio_dev, > > *val = outval; > > ret = IIO_VAL_INT; > > break; > > + 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. > > + */ > > + *val = 24 * 2; > > + *val2 = U16_MAX; > > + ret = IIO_VAL_FRACTIONAL; > > + break; > > Hi, > > The comment seems wrong. DS mentions that the measurement values are > S16, but limited in range (-k to +k, with k = 2^13 or 2^14). So the > denominator should be 2^13 or 2^14, and numerator 2.4mT. Ok, this is what you actually did. :-) The error is in 0xffff in the comment: it should be 0x8000 (== -0x8000). After applying the limit in range it would be 0x3fff to -0x4000 (== 0xC000). I would actually use 2^14 as denominator because that's a "round" number, the difference is insignificant, and we don't really know which one is correct. (BTW, the DS mentions 0.15uT/LSB resolution, so it would not be exact 2.4mT for the max value.) Best Regards, Michał Mirosław
