The Ware House

Internal Sensors Calibration

When it comes to temperature measurement, the documented factory calibration of STM32F030 family members does not provide enough information to make a valid estimation. The datasheet tells us that the value stored is the raw data of ADC conversion of the temperature sensor reading at 3.3V (±10mV) and 30℃ (±5℃).

TS_CAL1

There is only one point calibration documented and its reference temperature is known with a precision of ±5℃. That’s not enough to calculate temperature but it shows that the sensor was tested in production.

Notice that the calibration value name is TS_CAL1, some other STM32 families of chipset do have a second temperature factory calibration point TS_CAL2. Also some don’t have any factory calibration stored at all. So you have to refer to the datasheet that matches the chip you are targeting when you port your code to a new chipset.

Temperature Sensor Characteristics

The sensor linearity with temperature is at worst ±2℃. So I am curious to see how it performs over a range of temperature.

If you are so lucky that you have picked a STM32F030 chip that has the typical average slope of 4.3 mV/℃ and was calibrated in factory at exactly 3.3V and 30℃, then you could use the following formula to calculate the temperature.

T = 30 + (TS_CAL1 * 3.3 – TS_RAW * VDDA) / 4095 / 0.0043

with

VDDA = 3.3 * VREFINT_CAL / V_RAW

that gives

T = 30 + 3.3 * (TS_CAL1 – TS_RAW * V_CAL / V_RAW) / 4095 / 0.0043

If I express the average slope in raw ADC units per ℃ instead of mV/℃

5.336 = 4095 * 0.0043 / 3.3

the final formula is

T = 30 + (TS_CAL1 – TS_RAW * V_CAL / V_RAW) * 1000 / 5336

which matches the sample code for temperature computation given in the reference manual.

/* Temperature sensor calibration value address */
#define TEMP30_CAL_ADDR ((uint16_t*) ((uint32_t) 0x1FFFF7B8))
#define VDD_CALIB ((uint32_t) (3300))
#define VDD_APPLI ((uint32_t) (3000))
#define AVG_SLOPE ((uint32_t) (5336)) /* AVG_SLOPE in ADC conversion step (@3.3V)/°C
                                      multiplied by 1000 for precision on the division */
int32_t temperature; /* will contain the temperature in degrees Celsius */
temperature = ((uint32_t) *TEMP30_CAL_ADDR
            - ((uint32_t) ADC1->DR * VDD_APPLI / VDD_CALIB)) * 1000;
temperature = (temperature / AVG_SLOPE) + 30;

If I use the raw ADC readings from my last run

VDDA = 3.3 * 1525 / 1518 = 3.315V

t = 30 + (1742 – 1760 * 1525 / 1518) * 1000 / 5336 = 25.2℃

I confirm the voltage with a voltmeter (actual 3.323V versus 3.315V computed) but the temperature value seems too high even considering that this is the internal chip temperature and not the room temperature (temperature of the chip casing measured at 22.8℃).

Undocumented calibration data

For the STM32F030Fx, the 5KB space before the RAM contains the System Memory (3KB) and the option bytes.

ContentStart AddressSize
System Memory0x1FFFEC003KB
Option Bytes0x1FFFF8002KB
RAM Memory0x200000004KB

The calibration data are saved in the last 96 bytes of the System Memory, starting at address 0x1FFFF7A0. So it’s simple to dump the content of that zone and compare the values for multiple chips.

$ stm32flash -r - -S 0x1FFFF7a0:96 COM6 2>/dev/null | hexdump -C
00000000  ff ff ff ff 31 00 10 00  ff ff ff ff 14 80 20 00  |....1......... .|
00000010  13 57 33 52 31 37 31 20  ce 06 f5 05 f0 ff ff ff  |.W3R171 ........|
00000020  ff ff 27 05 ff ff ff ff  fc ff ff ff 10 00 ff ff  |................|
00000030  ff ff ff ff ff ff ff ff  ff ff ff ff ff ff ff ff  |................|
00000040  ff ff ff ff ff ff ff ff  f3 ff ff ff ff ff ff ff  |................|
00000050  ff ff ff ff ff ff ff ff  68 97 64 9b 3c c3 3f c0  |........h.d.<.?.|
00000060
LocationContentSizeF030F0x1/2/8
0x1FFFF7ACUnique ID12
0x1FFFF7B8TS_CAL12
0x1FFFF7BAVREFINT_CAL2
0x1FFFF7C2TS_CAL22
0x1FFFF7CCFlash size (KB)2
0x1FFFF7FATBD4

This is the same layout as the one documented in RM0091 Reference Manual STM32F0x1/STM32F0x2/STM32F0x8 which includes the following sample code for temperature computation.

/* Temperature sensor calibration value address */
#define TEMP110_CAL_ADDR ((uint16_t*) ((uint32_t) 0x1FFFF7C2))
#define TEMP30_CAL_ADDR ((uint16_t*) ((uint32_t) 0x1FFFF7B8))
#define VDD_CALIB ((uint16_t) (330))
#define VDD_APPLI ((uint16_t) (300))
int32_t temperature; /* will contain the temperature in degrees Celsius */
temperature = ((int32_t) ADC1->DR * VDD_APPLI / VDD_CALIB)
            - (int32_t) *TEMP30_CAL_ADDR;
temperature *= (int32_t)(110 - 30);
temperature /= (int32_t)(*TEMP110_CAL_ADDR - *TEMP30_CAL_ADDR);
temperature += 30;

Factoring in the actual measured voltage, this gives

T = 30 + (TS_CAL1 – TS_RAW * V_CAL / V_RAW) * 80 / (TS_CAL1 – TS_CAL2)

If I use the raw ADC readings from my last run

TSCAL2_SLOPE = (1742 - 1319) / 80 = 5.287 ADC step/
             = 3.3 * 5.287 / 4095 = 4.26 mV/℃

t = 30 + (1742 – 1760 * 1525 / 1518) * 80 / (1742 – 1319) = 25.1℃

Which is only .1℃ lower than the previous result based on the more generic formula.

Securing stable ADC readings

Before cranking up the temperature, it is worth to check how the ADC readings behave at a stable temperature. Are they steady or do they fluctuate?

The temperature affects the clocks, the power conversion and the sensors response linearity. Inevitably, some fluctuation will appear during the calibration as I increase or drop the temperature. If at stable room temperature there is already strong fluctuations, the cause of those fluctuations need to be identified and fixed before any calibration can be done.

● ADC Sampling Clock

The ADC sampling clock is maximum 14MHz and can take HSI14, PCLK/2 or PCLK/4 as a source. In my experience, ADC readings done with HSI14 are too noisy while I can secure stable readings with PCLK/2 as input with a PLL output between 16 and 28MHz using either HSI or an 8MHz HSE clock.

I didn’t investigate further what is the issue with HSI14 as I am not doing any advanced use of the ADC at this stage.

● ADC temperature channel startup time

Both reference voltage and temperature sensor characteristics give a minimum ADC sampling time of 4µs. With an ADC sampling frequency at 14MHz, this means 56 ADC sampling clock cycles.

A corresponding value needs to be set in the ADC Sampling Time Register according to the ADC sampling clock frequency (ADC_SMPR, 6 => 71.5 ADC sampling clock cycles). Picking up the maximum value of 7 (17.1µs or 239.5 ADC sampling clock cycle) is fine for a calibration application.

● Warm up

The internal temperature sensor measures the chip temperature, so there is a warm up time after a cold start during which the temperature will settle. Accordingly when the external temperature changes, there will be some delay before the effects of the changes are fully reflected in the measurements.

To minimize the effect of heat dissipation, I will avoid using the PLL and select HSI as main CPU clock. I will also turn off the peripherals when not in use, which means switching off the ADC peripheral clock between readings. My code already yield() when it is waiting for the next sampling time, going to stand by mode on the other hand is not something I want to do at this stage.

● Power supply

All ADC conversions are relative to VDDA value. It is worth to understand how the board hardware follows ST Micro recommended design and to make use of a reliable power source. This also means the reference voltage readings need to be tuned before the temperature ones.

Calibration Application

At this stage I have two open questions:

  • Can I rely on the factory calibrations or should I make my own?
  • How to make a second point calibration for the temperature sensor?

I am using a DS18B20 as a reference while tracking the minimum and maximum values acquired so far. Because the DS18B20 and the STM32F030 have a different latency in picking up temperature changes, they won’t be strictly aligned at a given time if the temperature keeps changing.

/* adccalib.c -- ADC calibration of internal temperature sensor */

#include <stdio.h>
#include "system.h"     /* uptime, adc_vnt() */
#include "ds18b20.h"    /* ds18b20_() */

#define ABS( i) ((i < 0) ? -i : i)

static void track( short *minp, short *maxp, short val) {
    if( val < *minp)
        *minp = val ;

    if( val > *maxp)
        *maxp = val ;
}

int main( void) {
    unsigned last = 0 ;
    short calV, calC ;
    short minC, maxC ;  /* Track temperature from DS18B20 */
    short minV, maxV ;  /* Track ADC raw Vref */
    short minT, maxT ;  /* Track ADC raw Tsense */

    minC = minV = minT = 0x7FFF ;
    maxC = maxV = maxT = -32768 ;

/* Initialize ADC and fetch calibration values */
    adc_vnt( VNT_INIT, &calV, &calC) ;
    printf( "%u, %u\n", calV, calC) ;
    int tconst = 6660 * calV / calC ;

/* Initialize DS18B20 and initiate temperature conversion */
    ds18b20_init() ;
    ds18b20_resolution( 12) ;   /* Set highest resolution: 12 bits */
    ds18b20_convert() ;         /* start temperature conversion */

/* main acquisition loop, reads samples every seconds */
    for( ;;)
        if( uptime == last)
            yield() ;
        else {
            short Vsample, Csample ;

            last = uptime ;

        /* Track DS18B20 temperature readings */
            switch( ds18b20_fetch( &Csample)) {
            case DS18B20_SUCCESS:
                track( &minC, &maxC, Csample) ;
                printf( "%i.%i, %i, %i, ", Csample / 10, ABS( Csample % 10),
                                                                minC, maxC) ;
                break ;
            case DS18B20_FAIL_TOUT:
                printf( "Timeout, ") ;
                break ;
            case DS18B20_FAIL_CRC:
                printf( "CRC Error, ") ;
            }

            ds18b20_convert() ; /* start temperature conversion */

        /* Track Internal Temperature Sensor readings */
            adc_vnt( VNT_RAW, &Vsample, &Csample) ;
            track( &minV, &maxV, Vsample) ;
            track( &minT, &maxT, Csample) ;
            printf( "%i, %i, %i, %i, %i, %i, %i, %i, ",
                    calV, Vsample, minV, maxV,
                    calC, Csample, minT, maxT) ;
            Csample = 3630 - (1 + tconst * Csample / Vsample) / 2 ;
            Vsample = (660 * calV / Vsample + 1) / 2 ;
            printf( "%i.%i, %i.%i\n",   Vsample / 100, ABS( Vsample % 100),
                                        Csample / 10, ABS( Csample % 10)) ;
        }
}

The composition in Makefile

SRCS = startup.txeie.c adc.c adccalib.c ds18b20.c

Build succeeds with a warning for the baud rate calculation as I am using HSI 8MHz as main clock. This will not affect execution as the difference is small for 9600 baud.

$ make
adc.c:205:3: warning: #warning baud rate not accurate at that clock frequency [-Wcpp]
  205 | # warning baud rate not accurate at that clock frequency
      |   ^~~~~~~
f030f4.elf
   text    data     bss     dec     hex filename
   3320       0      16    3336     d08 f030f4.elf
f030f4.hex
f030f4.bin

I use PuTTY to capture a log of the output: ds18b20 (temperature, minimum, maximum), reference voltage (ADC calibration, current, min, max and voltage), temperature sensor (ADC calibration, current, min, max and temperature).

22.0, 218, 226, 1518, 1522, 1521, 1525, 1754, 1777, 1775, 1783, 3.29, 26.6
22.0, 218, 226, 1518, 1523, 1521, 1525, 1754, 1778, 1775, 1783, 3.29, 26.6
22.0, 218, 226, 1518, 1523, 1521, 1525, 1754, 1778, 1775, 1783, 3.29, 26.6
22.0, 218, 226, 1518, 1523, 1521, 1525, 1754, 1778, 1775, 1783, 3.29, 26.6
22.0, 218, 226, 1518, 1523, 1521, 1525, 1754, 1778, 1775, 1783, 3.29, 26.6
22.0, 218, 226, 1518, 1524, 1521, 1525, 1754, 1779, 1775, 1783, 3.29, 26.6
22.0, 218, 226, 1518, 1522, 1521, 1525, 1754, 1777, 1775, 1783, 3.29, 26.6

Experiments

With the operating range of temperature being -40℃ … 85℃, I can conduct two experiments: heating and cooling.

  • Heat: I can use a small heater to raise the temperature to ~50℃.
  • Cold: I can put the device in a fridge (~4℃) or freezer (~-18℃).

This would allow me to make my own two point temperature calibration at ~-18℃ and ~50℃. I’ll have to do this for several chips to understand how they compare. Then I would be able to see if there is any benefit compare to using the factory calibration values.

I do the heat experiment with the calibration application I just explained. To do the cold experiment, I use a display instead of the asynchronous serial transmission of the readings.

DS18B20VrefintTSenseVmeasured
Calibration 3015261721
Calibration 1101297
Room 19.419.4153417613.287
Freezer ~18-18.515391972
Heater 401532

I exclude the temperature readings from the heater experiment as I didn’t manage to setup a stable equal distribution of the heat between the sensors.

The voltage ADC readings varies from 1532 to 1539 for a range of temperature of at least 55℃. The voltage reading at room temperature confirms that the factory voltage calibration is reliable (measured 3.287V versus calculated 3.283V).

The difference of temperature measured between the room and freezer is 37.9℃ which is nearly a third of the operation range [-40 … 85]℃. So I use those two measurements for my two point calibration.

AVG_SLOPE = (1972/1539 - 1761/1534) * 1526 / 37.9
          = 5.370 ADC_steps/℃
          = 4.327 mV/℃

This is very close to the Temperature Sensor typical characteristic of 4.3mV/℃ for the average slope.

As a reference, the average slope using the undocumented TS_CAL2 value

TSCAL2_SLOPE = (1721 - 1297) / 80
             = 5.3 ADC_steps/℃
             = 4.271 mV/℃

Now that I know the average temperature slope, I can calculate the temperature that matches the factory calibration value.

Tfactory30  = (1972 * 1526/1539 - 1721) / 5.37 - 18.5 =  25.1℃
Tfactory30  = (1761 * 1526/1534 - 1721) / 5.37 + 19.4 =  25.1℃
Tfactoty110 = (1972 * 1526/1539 - 1297) / 5.37 - 18.5 = 104.1℃
Tfactory30  = (1761 * 1526/1534 - 1297) / 5.37 + 19.4 = 104.1℃
DeltaT = 104.1 - 25.1 = 79℃

If I repeat the same experiments with another board

DS18B20VrefintTSenseVmeasured
Calibration 3015211764
Calibration 1101333
Room 23.0℃23.0150417523.340
Freezer ~18-17.715051961
AVG_SLOPE = (1961/1505 - 1752/1504) * 1521 / 40.7
          = 5.160 ADC_steps/℃
          = 4.158 mV/℃
TSCAL2_SLOPE = (1764 - 1333) / 80
             = 5.3875 ADC_steps/℃
             = 4.341 mV/℃
Tfactory30 = (1961 * 1521/1505 - 1764) / 5.16 - 17.7 = 24.5℃
Tfactory30 = (1752 * 1521/1504 - 1764) / 5.16 + 23.0 = 24.5℃
Tfactory110 = (1961 * 1521/1505 - 1333) / 5.16 - 17.7 = 108.8℃
Tfactory110 = (1752 * 1521/1504 - 1333) / 5.16 + 23.0 = 108.0℃
DeltaT = 108.8 - 24.5 = 84.3℃

Both experiments shows that the temperature calibration value TS_CAL1 and TS_CAL2 are roughly 80℃ apart.

Checkpoint

At this point I have been tinkering for a while with ADC conversion and the internal temperature sensor. I am not doing any advanced use of the ADC peripheral but the current minimal set up is stable. My main drive in investigating the internal temperature sensor is that on one hand it seems a technically reliable solution but on the other hand I can’t find a practical use of it published anywhere. By doing a two point calibration, I can have a good estimation of the average slope for the response to temperature of a specific chip. Still I cannot link exactly my measurements with the factory temperature calibration data. I don’t have clear information on the factory conditions in which those data are collected and I need more samples to draw my own conclusion.

Once I have collected calibration data of more chips, I will update this page with my findings.

I have not included here the display code I have been using in the cold experiment, I will explain that later in the display section.

Next I will cover the toolchain update that I made while working on the temperature sensors.

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