Hi Stefan,
Sorry! for the delay.
On 10/08/2015 11:46 AM, Stefan Agner wrote:
Hi Bhuvan,
On 2015-09-23 06:43, Bhuvanchandra DV wrote:
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@Temp25
as 867 and the final temperature readings differs with ~5°C from
the external readings.
I am not entirely happy with the message. The proposed changes seem not
to alter the offset which would explain a ~20°C difference... Where do
this 20°C come from?
The ~20°C difference is observed at higher temperatures when VTEMP25
at VREDH_ADC of 3V0 is considered. Will update the comment according
to the updated VTEMP25 constant instead of the 20°C difference which
is confusing wrt the changes done here.
Best regards,
Bhuvan
--
Stefan
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...@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);
--
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