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>
>> ---
>> 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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