Re: [PATCH] vf610_adc: Fix internal temperature calculation

[Bhuvanchandra]

Hi Stefan,

Sorry! for the delay.

On 10/08/2015 11:46 AM, Stefan Agner wrote:

> Hi Bhuvan,
>
> On 2015-09-23 06:43, Bhuvanchandra DV wrote:
> > There is an observed temperature difference of ~20ÂC with the
> > internal temperature reading and the temperature measured on
> > SoC package. Existing calculations consider the typical values
> > provided in datasheet. Those typical values are valid for
> > VREFH_ADC at 3.0V. Voltage at 25ÂC is different for different
> > VREFH_ADC voltages. With VREFH_ADC at 3.3V, voltage at 25ÂC is
> > 0.699V. Hence update the VTEMP25 to 0.699V which gives ADCR@Temp25
> > as 867 and the final temperature readings differs with ~5ÂC from
> > the external readings.
> I am not entirely happy with the message. The proposed changes seem not
> to alter the offset which would explain a ~20ÂC difference... Where do
> this 20ÂC come from?

The ~20ÂC difference is observed at higher temperatures when VTEMP25
at VREDH_ADC of 3V0 is considered. Will update the comment according
to the updated VTEMP25 constant instead of the 20ÂC difference which
is confusing wrt the changes done here.

Best regards,
Bhuvan

> --
> Stefan
>
> > Formula for finding ADCR@Temp25:
> > ADCR@Temp25 = (ADCR@Vdd * V@TEMP25 * 10) / VDDconv
> >
> > ADCR@Vdd for 12-Bit ADC = 4095
> > VDDconv = VREFH_ADC * 10
> >
> > VREFH_ADC@xxxx
> > ADCR@Temp25 = (4095 * .699 * 10) / 33
> > ADCR@Temp25 ~= 867
> >
> > | VREFH_ADC | V@TEMP25 | VDDconv | ADCR@Temp25 |
> > |   3.0V    | 0.696mV  |    30   |     950     |
> > |   3.3V    | 0.699mV  |    33   |     867     |
> >
> > Signed-off-by: Bhuvanchandra DV<bhuvanchandra.dv@xxxxxxxxxxx>
> > ---
> >   drivers/iio/adc/vf610_adc.c | 19 ++++++++++++++-----
> >   1 file changed, 14 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/iio/adc/vf610_adc.c b/drivers/iio/adc/vf610_adc.c
> > index f4df2a7..e7abc13 100644
> > --- a/drivers/iio/adc/vf610_adc.c
> > +++ b/drivers/iio/adc/vf610_adc.c
> > @@ -103,6 +103,13 @@
> >   
> >   #define DEFAULT_SAMPLE_TIME		1000
> >   
> > +/* V at 25ÂC of 696 mV */
> > +#define VF610_VTEMP25_3V0		950
> > +/* V at 25ÂC of 699 mV */
> > +#define VF610_VTEMP25_3V3		867
> > +/* Typical sensor slope coefficient at all temperatures */
> > +#define VF610_TEMP_SLOPE_COEFF		1840
> > +
> >   enum clk_sel {
> >   	VF610_ADCIOC_BUSCLK_SET,
> >   	VF610_ADCIOC_ALTCLK_SET,
> > @@ -636,11 +643,13 @@ static int vf610_read_raw(struct iio_dev *indio_dev,
> >   			break;
> >   		case IIO_TEMP:
> >   			/*
> > -			* Calculate in degree Celsius times 1000
> > -			* Using sensor slope of 1.84 mV/ÂC and
> > -			* V at 25ÂC of 696 mV
> > -			*/
> > -			*val = 25000 - ((int)info->value - 864) * 1000000 / 1840;
> > +			 * Calculate in degree Celsius times 1000
> > +			 * Using the typical sensor slope of 1.84 mV/ÂC
> > +			 * and VREFH_ADC at 3.3V, V at 25ÂC of 699 mV
> > +			 */
> > +			*val = 25000 - ((int)info->value - VF610_VTEMP25_3V3) *
> > +					1000000 / VF610_TEMP_SLOPE_COEFF;
> > +
> >   			break;
> >   		default:
> >   			mutex_unlock(&indio_dev->mlock);

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