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>
> ---
> 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?
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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