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SubjectRe: [PATCH] vf610_adc: Fix internal temperature calculation
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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 ~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.dv@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.

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 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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