RE: [PATCH] vf610_adc: Fix internal temperature calculation

[Duan Andy]

From: Jonathan Cameron <jic23@xxxxxxxxxx> Sent: Sunday, September 27, 2015 11:53 PM
> To: Bhuvanchandra DV; linux-iio@xxxxxxxxxxxxxxx
> Cc: stefan@xxxxxxxx; maitysanchayan@xxxxxxxxx; Duan Fugang-B38611;
> knaack.h@xxxxxx; lars@xxxxxxxxxx; pmeerw@xxxxxxxxxx; shawn.guo@xxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; Duan Fugang-B38611
> Subject: Re: [PATCH] vf610_adc: Fix internal temperature calculation
> 
> 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.
> 
> Jonathan

The patch is fine for me. Sorry for the late response due to vacation.

Acked-by: Fugang Duan <B38611@xxxxxxxxxxxxx>

> > ---
> >  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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