[PATCH 4.2.y-ckt 002/305] vf610_adc: Fix internal temperature calculation

[Kamal Mostafa]

4.2.8-ckt2 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

From: Bhuvanchandra DV <bhuvanchandra.dv@xxxxxxxxxxx>

commit 6219f432ec037317a77c40910da12a626c34af1c upstream.

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 25ÂC 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@xxxx
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@xxxxxxxxxxx>
Acked-by: Fugang Duan <B38611@xxxxxxxxxxxxx>
Signed-off-by: Jonathan Cameron <jic23@xxxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
---
 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 819632b..fe8cf11 100644
--- a/drivers/iio/adc/vf610_adc.c
+++ b/drivers/iio/adc/vf610_adc.c
@@ -98,6 +98,13 @@
 #define VF610_ADC_CALF			0x2
 #define VF610_ADC_TIMEOUT		msecs_to_jiffies(100)
 
+/* V at 25ÂC of 696 mV */
+#define VF610_VTEMP25_3V0		950
+/* V at 25ÂC 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,
@@ -569,11 +576,13 @@ 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/ÂC and
-			* V at 25ÂC 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/ÂC
+			 * and VREFH_ADC at 3.3V, V at 25ÂC of 699 mV
+			 */
+			*val = 25000 - ((int)info->value - VF610_VTEMP25_3V3) *
+					1000000 / VF610_TEMP_SLOPE_COEFF;
+
 			break;
 		default:
 			mutex_unlock(&indio_dev->mlock);
-- 
1.9.1