Hi,
My conclusion is that the measurements are biased everytime.
I wrote a small test suite testing measurements at the manufacturer-supplied reference points.
As you can see the bias is substantial.
Jonathan, any objection ?
===> Test #1
Supply: 2500000uV, raw reading: 7000,
original calc_temp: 31599mC;
patched calc_temp: 30600mC
hand-made calc gives: 30600
===> Test #2
Supply: 3000000uV, raw reading: 6800,
original calc_temp: 29598mC;
patched calc_temp: 28400mC
hand-made calc gives: 28400
===> Test #3
Supply: 3500000uV, raw reading: 6400,
original calc_temp: 25598mC;
patched calc_temp: 24300mC
hand-made calc gives: 24300
===> Test #4
Supply: 4000000uV, raw reading: 6000,
original calc_temp: 21598mC;
patched calc_temp: 20200mC
hand-made calc gives: 20200
===> Test #5
Supply: 5000000uV, raw reading: 5400,
original calc_temp: 15598mC;
patched calc_temp: 13900mC
hand-made calc gives: 13900
Jerome
> I discovered two issues.
> First the previous sht15_calc_temp() loop did not iterate through the
> temppoints array since the (data->supply_uV > temppoints[i - 1].vdd)
> test is always true in this direction.
>
> Also the two-points linear interpolation function was returning
> biased
> values which I adressed using a different form of interpolation.
>
> Signed-off-by: Jerome Oufella <jerome.oufella at
> savoirfairelinux.com>
> ---
>
> drivers/hwmon/sht15.c | 14 +++++++-------
> 1 files changed, 7 insertions(+), 7 deletions(-)
>
>
> diff --git a/drivers/hwmon/sht15.c b/drivers/hwmon/sht15.c
> index 864a371..a6ad93b 100644
> --- a/drivers/hwmon/sht15.c
> +++ b/drivers/hwmon/sht15.c
> @@ -303,15 +303,15 @@ error_ret:
> static inline int sht15_calc_temp(struct sht15_data *data)
> {
> int d1 = 0;
> - int i;
> + int i, t;
>
> - for (i = 1; i < ARRAY_SIZE(temppoints); i++)
> + for (i = ARRAY_SIZE(temppoints) - 1; i > 0; i--)
> /* Find pointer to interpolate */
> - if (data->supply_uV > temppoints[i - 1].vdd) {
> - d1 = (data->supply_uV/1000 - temppoints[i - 1].vdd)
> - * (temppoints[i].d1 - temppoints[i - 1].d1)
> - / (temppoints[i].vdd - temppoints[i - 1].vdd)
> - + temppoints[i - 1].d1;
> + if (data->supply_uV >= temppoints[i - 1].vdd) {
> + t = (data->supply_uV - temppoints[i-1].vdd) /
> + ((temppoints[i].vdd - temppoints[i-1].vdd) / 10000);
> +
> + d1 = (temppoints[i].d1 * t + (10000 - t) * temppoints[i-1].d1) /
> 10000;
> break;
> }
>
> --
> 1.6.3.3
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