Re: [lm-sensors] [PATCH] hwmon: sht15: Fix sht15_calc_temp

[Jonathan Cameron <jic23@cam.ac.uk>]

On 03/29/10 17:23, Jerome Oufella wrote:
> 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 ?

If it works it is fine with me, but please add some documentation so that any
one coming to the code in the future can understand why you aren't using
what the datasheet lists.

Given it would be useful to provide everything you have listed here, I'd
be inclined to add a file in Documentation/hwmon supported by a comment inline
with the code.  If you have any details on date of manufacture of the part you
tested etc or any comments from the manufacturer it would be useful to put
those in as well.

Jonathan

> 
> 
> 
> ==> 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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