linux-kernel - Re: [PATCH] vf610_adc: Fix internal temperature calculation

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Date:	Wed, 07 Oct 2015 23:16:10 -0700
From:	Stefan Agner <stefan@...er.ch>
To:	Bhuvanchandra DV <bhuvanchandra.dv@...adex.com>
Cc:	linux-iio@...r.kernel.org, jic23@...nel.org,
	maitysanchayan@...il.com, B38611@...escale.com, knaack.h@....de,
	lars@...afoo.de, pmeerw@...erw.net, shawn.guo@...aro.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vf610_adc: Fix internal temperature calculation

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@...p25
> 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@...p25:
> ADCR@...p25 = (ADCR@Vdd * V@...P25 * 10) / VDDconv
> 
> ADCR@Vdd for 12-Bit ADC = 4095
> VDDconv = VREFH_ADC * 10
> 
> VREFH_ADC@...V
> ADCR@...p25 = (4095 * .699 * 10) / 33
> ADCR@...p25 ~= 867
> 
> | VREFH_ADC | V@...P25 | VDDconv | ADCR@...p25 |
> |   3.0V    | 0.696mV  |    30   |     950     |
> |   3.3V    | 0.699mV  |    33   |     867     |
> 
> Signed-off-by: Bhuvanchandra DV <bhuvanchandra.dv@...adex.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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