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

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Message-ID: <1443015807-17764-1-git-send-email-bhuvanchandra.dv@toradex.com>
Date:	Wed, 23 Sep 2015 19:13:27 +0530
From:	Bhuvanchandra DV <bhuvanchandra.dv@...adex.com>
To:	<linux-iio@...r.kernel.org>, <jic23@...nel.org>
CC:	<stefan@...er.ch>, <maitysanchayan@...il.com>,
	<B38611@...escale.com>, <knaack.h@....de>, <lars@...afoo.de>,
	<pmeerw@...erw.net>, <shawn.guo@...aro.org>,
	<linux-kernel@...r.kernel.org>,
	Bhuvanchandra DV <bhuvanchandra.dv@...adex.com>
Subject: [PATCH] vf610_adc: Fix internal temperature calculation

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.

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);
-- 
2.5.0

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