Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 07 Oct 2015 23:16:10 -0700 | From | Stefan Agner <> | Subject | Re: [PATCH] vf610_adc: Fix internal temperature calculation |
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Hi Bhuvan,
On 2015-09-23 06:43, 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.
I am not entirely happy with the message. The proposed changes seem not to alter the offset which would explain a ~20°C difference... Where do this 20°C come from?
-- 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@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.dv@toradex.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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