From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: From: Bhuvanchandra Subject: Re: [PATCH] vf610_adc: Fix internal temperature calculation To: Stefan Agner References: <1443015807-17764-1-git-send-email-bhuvanchandra.dv@toradex.com> <0b977c21c414aef0c250755c9d772ead@agner.ch> CC: , , , , , , , , , Message-ID: <561C7C93.4090608@toradex.com> Date: Tue, 13 Oct 2015 09:07:55 +0530 MIME-Version: 1.0 In-Reply-To: <0b977c21c414aef0c250755c9d772ead@agner.ch> Content-Type: text/plain; charset="utf-8"; format=flowed List-ID: 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 >> --- >> 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);