On Fri, Apr 17, 2020 at 01:40:20PM +0200, Linus Walleij wrote:
> The manual for the HSCDTD008A gives us a scaling for the
> three axis as +/- 2.4mT (24 Gauss) per axis.
>
> The manual for the AMI305 and AMI306 gives us a scaling
> for the three axis as +/- 12 Gauss per axis.
>
> Tests with the HSCDTD008A sensor, cat the raw values:
> $ cat in_magn_*_raw
> raw
> 45
> 189
> -19
>
> The scaling factor in in_magn_*_scale is 0.001464843,
> which gives:
> 0.065 Gauss
> 0.277 Gauss
> -0.027 Gauss
>
> The earths magnetic field is in the range of 0.25 to 0.65
> Gauss on the surface according to Wikipedia, so these
> seem like reasonable values.
>
> Again we are guessing that the AK8974 has a 12 bit ADC,
> based on the similarity with AMI305 and AMI306.
>
> 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 v2->v3:
> - Scale the 2.4mT/24Gauss to 15 bits for the HSCDTD008A.
> - Scale the 12 Gauss to 12 bits for the AMI305/AMI306
> - Use 12 bits for the other variants.
> - Return directly in the raw read function.
> ChangeLog v1->v2:
> - Split out the measurement refactoring.
> ---
> drivers/iio/magnetometer/ak8974.c | 45 ++++++++++++++++++++++++++++++-
> 1 file changed, 44 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/iio/magnetometer/ak8974.c b/drivers/iio/magnetometer/ak8974.c
> index b8dbea119a67..f8410ac34316 100644
> --- a/drivers/iio/magnetometer/ak8974.c
> +++ b/drivers/iio/magnetometer/ak8974.c
> @@ -608,6 +608,48 @@ static int ak8974_read_raw(struct iio_dev *indio_dev,
> if (ret)
> return ret;
> return IIO_VAL_INT;
> + case IIO_CHAN_INFO_SCALE:
> + switch (ak8974->variant) {
> + case AK8974_WHOAMI_VALUE_AMI306:
> + case AK8974_WHOAMI_VALUE_AMI305:
> + /*
> + * The datasheet for AMI305 and AMI306, page 6
> + * specifies the range of the sensor to be
> + * +/- 12 Gauss.
> + */
> + *val = 12 * 2;
> + /*
> + * 12 bits are used
> + * [ -2048 .. 2047 ] (manual page 20)
> + * [ 0xf800 .. 0x07ff ]
> + */
> + *val2 = 4096;
> + return IIO_VAL_FRACTIONAL;
You can leave '* 2' out from both values. And actually, since *val2 is 2^n,
you can return IIO_VAL_FRACTIONAL_LOG2 (with *val2 = 11) and save a division
in the caller.
Best Regards,
Michał Mirosław
