How to convert calibration values for temperature sensor from 3.3V to 2.5V?

[DFuchs]

Hi,

i have a STM32H743 with an external voltage reference with 2.5V and want to use the internal temperature sensor.

When i use the calibration values from flash i converted them by ((TS_CALx *33.0)/25.0).

The measured value at room temperature seems to be good. I put our prototype in a conditioning cabinet at 70°C and the measured temperaure was 70°C too. I expected something like 85°C.

Do i need measure new calibration values or is the there a formula to transfer the calibration values from 3.3V to 2.5V?

Kind regards

Daniel

[TDK]

Look at the code in __LL_ADC_CALC_TEMPERATURE and use that calculation:

/**
 * @brief Helper macro to calculate the temperature (unit: degree Celsius)
 * from ADC conversion data of internal temperature sensor.
 * @note Computation is using temperature sensor calibration values
 * stored in system memory for each device during production.
 * @note Calculation formula:
 * Temperature = ((TS_ADC_DATA - TS_CAL1)
 * * (TS_CAL2_TEMP - TS_CAL1_TEMP))
 * / (TS_CAL2 - TS_CAL1) + TS_CAL1_TEMP
 * with TS_ADC_DATA = temperature sensor raw data measured by ADC
 * Avg_Slope = (TS_CAL2 - TS_CAL1)
 * / (TS_CAL2_TEMP - TS_CAL1_TEMP)
 * TS_CAL1 = equivalent TS_ADC_DATA at temperature
 * TEMP_DEGC_CAL1 (calibrated in factory)
 * TS_CAL2 = equivalent TS_ADC_DATA at temperature
 * TEMP_DEGC_CAL2 (calibrated in factory)
 * Caution: Calculation relevancy under reserve that calibration
 * parameters are correct (address and data).
 * To calculate temperature using temperature sensor
 * datasheet typical values (generic values less, therefore
 * less accurate than calibrated values),
 * use helper macro @ref __LL_ADC_CALC_TEMPERATURE_TYP_PARAMS().
 * @note As calculation input, the analog reference voltage (Vref+) must be
 * defined as it impacts the ADC LSB equivalent voltage.
 * @note Analog reference voltage (Vref+) must be either known from
 * user board environment or can be calculated using ADC measurement
 * and ADC helper macro @ref __LL_ADC_CALC_VREFANALOG_VOLTAGE().
 * @note On this STM32 serie, calibration data of temperature sensor
 * corresponds to a resolution of 16 bits,
 * this is the recommended ADC resolution to convert voltage of
 * temperature sensor.
 * Otherwise, this macro performs the processing to scale
 * ADC conversion data to 16 bits.
 * @param __VREFANALOG_VOLTAGE__ Analog reference voltage (unit: mV)
 * @param __TEMPSENSOR_ADC_DATA__ ADC conversion data of internal
 * temperature sensor (unit: digital value).
 * @param __ADC_RESOLUTION__ ADC resolution at which internal temperature
 * sensor voltage has been measured.
 * This parameter can be one of the following values:
 * @arg @ref LL_ADC_RESOLUTION_16B
 * @arg @ref LL_ADC_RESOLUTION_14B
 * @arg @ref LL_ADC_RESOLUTION_12B
 * @arg @ref LL_ADC_RESOLUTION_10B
 * @arg @ref LL_ADC_RESOLUTION_8B
 * @retval Temperature (unit: degree Celsius)
 */
#define __LL_ADC_CALC_TEMPERATURE(__VREFANALOG_VOLTAGE__,\
 __TEMPSENSOR_ADC_DATA__,\
 __ADC_RESOLUTION__) \
 (((( ((int32_t)((__LL_ADC_CONVERT_DATA_RESOLUTION((__TEMPSENSOR_ADC_DATA__), \
 (__ADC_RESOLUTION__), \
 LL_ADC_RESOLUTION_16B) \
 * (__VREFANALOG_VOLTAGE__)) \
 / TEMPSENSOR_CAL_VREFANALOG) \
 - (int32_t) *TEMPSENSOR_CAL1_ADDR) \
 ) * (int32_t)(TEMPSENSOR_CAL2_TEMP - TEMPSENSOR_CAL1_TEMP) \
 ) / (int32_t)((int32_t)*TEMPSENSOR_CAL2_ADDR - (int32_t)*TEMPSENSOR_CAL1_ADDR) \
 ) + TEMPSENSOR_CAL1_TEMP \
 )

[DFuchs]

Thank you for your help,

but my formula seems to be mathematically equivalent.

I'm using double values for the computation. Perhaps there is a problem or TS_CAL2 isn't measured at 110°C.

Kind regards

Daniel

auto const TS_CAL1 = *reinterpret_cast<uint16_t*>(0x1FF1E820);
auto const TS_CAL2 = *reinterpret_cast<uint16_t*>(0x1FF1E840);
auto const ts_cal1_25v = (static_cast<double>(TS_CAL1) * 33.0) / 25.0;
auto const ts_cal2_25v = (static_cast<double>(TS_CAL2) * 33.0) / 25.0;
 
auto const ts_data = Adc::GetAdc3Data();
auto const temperature = (80.0 / (ts_cal2_25v - ts_cal1_25v))*(static_cast<double>(ts_data) - ts_cal1_25v) + 30.0;

[TDK]

There’s also a sample time requirement. See the datasheet.

[waclawek.jan]

Formulae look good to me.

Besides sample time requirement, there is also a required startup time.

What were the values of ts_data for room temperature and for the 70deg.C, and what are the values of TS_CAL1 and TS_CAL2?

JW

[DFuchs]

I've tested different values fo sample time -> same behaviour.

I've initialised the ADC3 and activated the temperature sensor. Initialised some other stuff and started the ADC3 conversion with channel 18 only.

Now i added 10ms busy waiting between activation of sensor and conversion start. But testing in the cabinet takes some time.

TS_CAL1 = 12309 (30°C, 3.3V reference) -> ts_cal1_25v = 16247.88

TS_CAL2 = 16476 (110°C, 3.3V reference) -> ts_cal2_25v = 21748.32

ts_data = 15937 @ start and 24°C room temperature, so internal junction temperature could be 25.5°C

ts_data = about 19000 @ 70°C ambient temperature, so internal junction temperature cannot be 70°C

Edit:

No changed with 10ms waiting.

The prototype was deactivated and the conditioning cabinet heat up all to about 70°C.

I started the prototype and after about 4 seconds i got the temperature values.

ts_data = 18348 @ start and 70°C ambient temperature. 18348 -> 60,5°C ?!

Kind regards

Daniel

[Nikita91]

4 seconds is very short.

For me there is not enough time for a self warm up, so it seems normal to measure the ambient temperature.

How long does it stabilize at 70 ° C before starting the MCU?

[DFuchs]

I waited about 30 minutes till a sensor inside the cabinet reached and holds the 70°C. So every metal and plastic part inside the cabinet should have 70°C.

When i wait more than an hour with the running prototype the internal temperture sensor measures about 19000 = 70°C. The thermocouple on the heat sink measures 78°C already. The cabinet has an fan inside so 78°C on heat sink and 70°C ambient temperature seems to fit.

Kind regards

Daniel

[DFuchs]

Hmm,

if i don't use the calibration values the result seems to me more realistic. Instead i use the 2mV/°C from the datasheet.

30°C -> 0,620V at temperature sensor -> 16253 at adc3 with 2.5V reference

70°C -> 30°C + 40°C -> 0,620V + (40°C *2mV/°C) -> 0,700V at temperature sensor -> 18350 at adc3 with 2.5V reference.

I would have a junction temperature of 82.5°C with 78°C at heat sink and 70°C ambient.