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@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 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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