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Subject[PATCH v2] vf610_adc: Fix internal temperature calculation
Date
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 |

Acked-by: Fugang Duan <B38611@freescale.com>

Signed-off-by: Bhuvanchandra DV <bhuvanchandra.dv@toradex.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);
--
2.6.1


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