On Sun, 10 Dec 2017 13:11:51 -0800
Matt Ranostay <mranostay@xxxxxxxxx> wrote:
> On Sun, Dec 10, 2017 at 10:24 AM, Jonathan Cameron <jic23@xxxxxxxxxx> wrote:
> > On Wed, 6 Dec 2017 18:57:58 +0200
> > Narcisa Ana Maria Vasile <narcisaanamaria12@xxxxxxxxx> wrote:
> > +CC Matt
> >> in_concentration_raw should report, according to sysfs-bus-iio documentation,
> >> a "Raw (unscaled no offset etc.) percentage reading of a substance."
> >>
> >> Modify scale to convert from ppm/ppb to percentage:
> >> 1 ppm = 0.0001%
> >> 1 ppb = 0.0000001%
> >>
> >> There is no offset needed to convert the ppm/ppb to percentage,
> >> so remove offset from IIO_CONCENTRATION (IIO_MOD_CO2) channel.
> >>
> >> Signed-off-by: Narcisa Ana Maria Vasile <narcisaanamaria12@xxxxxxxxx>
>
> Reviewed-by: Matt Ranostay <matt.ranostay@xxxxxxxxxxxx>
Applied to the fixes-togreg branch of iio.git and marked for stable
so hopefully we can avoid too much userspace breakage...
Thanks,
Jonathan
>
> >> ---
> >> Details:
> >> The sensors reports data in parts per million for CO2 and parts per
> >> billion for VOC. The current scaling and offset values were meant to
> >> convert the ppm/ppb value into a percentage value with respect to
> >> the sensor's measurement range:
> >> For example:
> >> For eCO2, possible raw values range from 400ppm to 8192ppm.
> >> If the value reported is, let's say 500ppm, then, after scaling and
> >> offset are applied, the percentage, with reference to the interval
> >> [400, 8192] is 1.28%, using this interval mapping formula:
> >> (500 - 400) * (100 - 0) / (8192 - 400) + 0. Instead, the value should
> >> be 500 * 0.0001 = 0.05% (from ppm to percentage).
> >>
> >> Note:
> >> In the docs, the in_concentration_raw value is said to be a
> >> "raw percentage reading of a substance". The raw value from the sensor
> >> is a ppm/ppb value, not a percentage value. It becomes a percentage value
> >> after applying a scale. Is this the correct behaviour (after applying scale,
> >> a percentage value is obtained) or did I misunderstood the docs?
> >
> > I 'think' you are right on this, but would like some input from
> > someone more familiar with chemical sensors in general.
> >
> > Matt, any comment on this?
> >
>
> Yes the IIO_CONCENTRATION is suppose to be in ppm/ppb units that are
> converted to percent with the scaling value.
> Basically since someday we may have a sensor that actually with a
> digit on the other the decimal point, but hopefully not really 1.28%
> CO2 since that is well into toxic zone :)
>
> - Matt
>
> > Thanks,
> >
> > Jonathan
> >>
> >> drivers/iio/chemical/ccs811.c | 13 +++----------
> >> 1 file changed, 3 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/drivers/iio/chemical/ccs811.c b/drivers/iio/chemical/ccs811.c
> >> index 97bce83..fbe2431 100644
> >> --- a/drivers/iio/chemical/ccs811.c
> >> +++ b/drivers/iio/chemical/ccs811.c
> >> @@ -96,7 +96,6 @@ struct ccs811_data {
> >> .channel2 = IIO_MOD_CO2,
> >> .modified = 1,
> >> .info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |
> >> - BIT(IIO_CHAN_INFO_OFFSET) |
> >> BIT(IIO_CHAN_INFO_SCALE),
> >> .scan_index = 0,
> >> .scan_type = {
> >> @@ -255,24 +254,18 @@ static int ccs811_read_raw(struct iio_dev *indio_dev,
> >> switch (chan->channel2) {
> >> case IIO_MOD_CO2:
> >> *val = 0;
> >> - *val2 = 12834;
> >> + *val2 = 100;
> >> return IIO_VAL_INT_PLUS_MICRO;
> >> case IIO_MOD_VOC:
> >> *val = 0;
> >> - *val2 = 84246;
> >> - return IIO_VAL_INT_PLUS_MICRO;
> >> + *val2 = 100;
> >> + return IIO_VAL_INT_PLUS_NANO;
> >> default:
> >> return -EINVAL;
> >> }
> >> default:
> >> return -EINVAL;
> >> }
> >> - case IIO_CHAN_INFO_OFFSET:
> >> - if (!(chan->type == IIO_CONCENTRATION &&
> >> - chan->channel2 == IIO_MOD_CO2))
> >> - return -EINVAL;
> >> - *val = -400;
> >> - return IIO_VAL_INT;
> >> default:
> >> return -EINVAL;
> >> }
> >> --
> >> 1.9.1
> >>
> >
> --
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