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vf610_adc: Fix internal temperature calculation

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Calculate ADCR_VTEMP25 using VTEMP25 at VREFH_ADC 3V3. Existing
calculations consider the typical values provided in datasheet.
Those typical values are valid for VREFH_ADC at 3.0V. VTEMP25
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.

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>
Acked-by: Fugang Duan <B38611@freescale.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
This commit is contained in:
Bhuvanchandra DV 2015-10-19 21:24:36 +05:30 committed by Jonathan Cameron
parent b94e22805a
commit 6219f432ec
1 changed files with 14 additions and 5 deletions
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19
drivers/iio/adc/vf610_adc.c
View file

@ -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,
@ -635,11 +642,13 @@ 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);