STM32 MCU - Temp sensor gives weird values

Question

I'm fooling around with a Nucleo-F103RB STM32 board and I'm trying to measure the ambient temperature using its internal sensor. I activate the ADC and get data normally via the HAL_ADC_GetValue() function (the value is around 1700 - 1800 and varies with temperature as it should) but the problem is that I get really weird numbers when converting to Celsius using the formula given in the manual, namely: $$\frac{V_{25} - V_{ADC}}{AvgSlope} + 25$$ (about 70-80 degrees when the real temperature is no more than 25).

I suspect that has to do with the way I'm converting the ADC value into volts, but I can't see something obviously wrong, so I came to ask here.

Code is here :

#include "TempMsr.h"

volatile uint32_t sensorData = 0;
volatile uint32_t temp = 0;

uint32_t getTemp(ADC_HandleTypeDef* ADCHandle)
{
    sensorData = HAL_ADC_GetValue(ADCHandle);
    temp = ((V25 * 1000 - sensorData * 0.8) / AVGSLOPE) + 25;

    //V25 and sensorData must be in mV for this formula to work, as the
    //AVGSLOPE value is given in mV/oC

    //the 0.8 multiplication comes from dividing the max ADC voltage (3.3V) with its resolution (12 bits => 4096) which is 0.8mV/ADC unit

    return temp;
}

Its header file:

#ifndef TEMPMSR_H_
#define TEMPMSR_H_

#include "stm32f1xx_hal.h"

#define AVGSLOPE 4.3
//average slope of T-V chart according to datasheet pg 79
//(min is 4 mV/C, max 4.6, default (4.3): typical)

#define V25 1.43
//voltage of temperature sensor at 25C according to datasheet pg 79 (in V)
//(min is 1.34, max is 1.52, default(1.43): typical)

uint32_t getTemp(ADC_HandleTypeDef* ADCHandle);


#endif /* TEMPMSR_H_ */

Thanks in advance!

EDIT: I just noticed in the reference manual that the sensor has a bias (of up to +- 45 degrees) due to the manufacturing process, which is different on every chip and I have neither calculated it for my particular MCU nor included it in the formula. Could this be the problem?

Answer 1

I'm not C expert, perhaps

You have to parse uint to float, then calculate, then parse again to uint, if the result has to be uint.

{
    sensorData = HAL_ADC_GetValue(ADCHandle);
    temp = (uint32_t)(((V25 * 1000.0 - (float)sensorData * 0.8) / AVGSLOPE) + 25.0);

    //V25 and sensorData must be in mV for this formula to work, as the
    //AVGSLOPE value is given in mV/oC

    //the 0.8 multiplication comes from dividing the max ADC voltage (3.3V) with its resolution (12 bits => 4096) which is 0.8mV/ADC unit

    return temp;
}

Answer 2

Having done temperature sensing on a STM32 before, I have a reason to believe that the formula is incorrect, while an older STM32F207 documentation had the correct formula.

As the slope is positive, larger measurements (ADC value or voltage) means larger temperature. So the formula should have (Vadc-V25), not (V25-Vadc).

Still your point on the offsets are valid. The V25 measurement has a +/- 90 mV tolerance which calculates to about +/- 21 degrees celcius tolerance. ST documentation has stated it is not accurate for measuring absolute temperatures without calibration, while it can be used to track changes in temperature. In practice, it has been accurate enough without calibration to control a fan to prevent a product from overheating.

Answer 3

An on-chip temperature sensor is not going to measure the temperature of the ambient air, it's going to measure the temperature of the chip it is part of. A temperature sensor built in to a microcontroller is measuring the microcontroller's temperature, which should be significantly higher than the ambient air temperature if the microcontroller is doing any work.

Answer 4

On STM32F0 and STM32L4 we are working with at the moment, the internal temperature sensor is pre calibrated during production and can also be calibrated during runtime with the internal voltage reference.

The calculation of the temperature is also different from the normal adc value to voltage calculation because it includes the calibration values.

Maybe your controller does have the same feature. The description should be available in the ADC chapter in the Reference Manual. I would say that the resolution in our application is +/- 1 degree.