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>

1 files changed, 14 insertions, 5 deletions

diff --git a/drivers/iio/adc/vf610_adc.c b/drivers/iio/adc/vf610_adc.c
index 6bf4c20eb231..42d7c87ea5be 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,
@@ -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);