| Message ID | 20200416140917.8087-2-linus.walleij@linaro.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [1/2,v2] iio: magnetometer: ak8974: Break out measurement | expand |
Hi Linus, 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? > 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@web.de> > Cc: Stephan Gerhold <stephan@gerhold.net> > Cc: Michał Mirosław <mirq-linux@rere.qmqm.pl> > Signed-off-by: Linus Walleij <linus.walleij@linaro.org> > --- > 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. > + */ 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). 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? 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. > + *val = 24 * 2; > + *val2 = U16_MAX; > + ret = IIO_VAL_FRACTIONAL; > + break; > } > out_err_read: > return ret; > @@ -667,7 +679,8 @@ static const struct iio_chan_spec_ext_info ak8974_ext_info[] = { > .type = IIO_MAGN, \ > .modified = 1, \ > .channel2 = IIO_MOD_##axis, \ > - .info_mask_separate = BIT(IIO_CHAN_INFO_RAW), \ > + .info_mask_separate = BIT(IIO_CHAN_INFO_RAW) | \ > + BIT(IIO_CHAN_INFO_SCALE), \ > .ext_info = ak8974_ext_info, \ > .address = index, \ > .scan_index = index, \ > -- > 2.21.1 >
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
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@web.de> > Cc: Stephan Gerhold <stephan@gerhold.net> > Cc: Michał Mirosław <mirq-linux@rere.qmqm.pl> > Signed-off-by: Linus Walleij <linus.walleij@linaro.org> > --- > 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. Best Regards, Michał Mirosław
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@web.de> > > Cc: Stephan Gerhold <stephan@gerhold.net> > > Cc: Michał Mirosław <mirq-linux@rere.qmqm.pl> > > Signed-off-by: Linus Walleij <linus.walleij@linaro.org> > > --- > > 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
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; } out_err_read: return ret; @@ -667,7 +679,8 @@ static const struct iio_chan_spec_ext_info ak8974_ext_info[] = { .type = IIO_MAGN, \ .modified = 1, \ .channel2 = IIO_MOD_##axis, \ - .info_mask_separate = BIT(IIO_CHAN_INFO_RAW), \ + .info_mask_separate = BIT(IIO_CHAN_INFO_RAW) | \ + BIT(IIO_CHAN_INFO_SCALE), \ .ext_info = ak8974_ext_info, \ .address = index, \ .scan_index = index, \
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@web.de> Cc: Stephan Gerhold <stephan@gerhold.net> Cc: Michał Mirosław <mirq-linux@rere.qmqm.pl> Signed-off-by: Linus Walleij <linus.walleij@linaro.org> --- ChangeLog v1->v2: - Split out the measurement refactoring. --- drivers/iio/magnetometer/ak8974.c | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-)