Commit 6219f432 authored Oct 19, 2015 by Bhuvanchandra DV Committed by Jonathan Cameron Oct 25, 2015

vf610_adc: Fix internal temperature calculation



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>

parent b94e2280

drivers/iio/adc/vf610_adc.c

+14 −5

Original line number	Diff line number	Diff line
		@@ -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,
		@@ -636,10 +643,12 @@ static int vf610_read_raw(struct iio_dev *indio_dev,
		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
		* 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 - 864) 1000000 / 1840;
		val = 25000 - ((int)info->value - VF610_VTEMP25_3V3)
		1000000 / VF610_TEMP_SLOPE_COEFF;

		break;
		default:
		mutex_unlock(&indio_dev->mlock);

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