From: Jonathan Cameron <ji...@kernel.org> Sent: Sunday, September 27, 2015
11:53 PM
> To: Bhuvanchandra DV; linux-...@vger.kernel.org
> Cc: ste...@agner.ch; maitysancha...@gmail.com; Duan Fugang-B38611;
> knaac...@gmx.de; l...@metafoo.de; pme...@pmeerw.net; shawn....@linaro.org;
> linux-kernel@vger.kernel.org; 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 ~20°C 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 25°C is different for different VREFH_ADC voltages. With
> > VREFH_ADC at 3.3V, voltage at 25°C is 0.699V. Hence update the VTEMP25
> > to 0.699V which gives ADCR@Temp25 as 867 and the final temperature
> > readings differs with ~5°C 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@3.3V
> > 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...@toradex.com>
> 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 <b38...@freescale.com>
> > ---
> > 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 25°C of 696 mV */
> > +#define VF610_VTEMP25_3V0 950
> > +/* V at 25°C 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/°C and
> > - * V at 25°C 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/°C
> > + * and VREFH_ADC at 3.3V, V at 25°C 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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