Re: [PATCH] vf610_adc: Fix internal temperature calculation

[Jonathan Cameron]

On 08/10/15 03:01, Duan Andy wrote:
> 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>
Applied to the fixes-togreg branch of iio.git and marked for stable.
Thanks,
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 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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