Re: [PATCH] vf610_adc: Fix internal temperature calculation

[Bhuvanchandra]

Hi Fugang,

On 09/27/2015 09:23 PM, Jonathan Cameron wrote:

> On 23/09/15 14:43, 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.
> >
> > 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>
> Looks fine to me, but I'll need an Ack from Fugang on this one
> as I don't know or have the part I'm afraid.

Can you please give your ACK if the patch looks fine.

Best regards,
Bhuvan

> Jonathan
> > ---
> >   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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