Problems with ADC after calibration - STM32L010K8

[ledi001]

Hello!

i develop an outdoor temperature measurement system and read every 30 minutes the ADC values -> all works fine! ADC calibration was done one times after start up.

Now i changed the ADC calibration and calibrate every 30 minutes just before i read the ADC values. I want to do this because the temperature in the environment changes between +20°C and - 5°C within 24 hours.

But now, i got wrong results after ADC conversion.

here is my code for calibration:

void ADC_calibration(void)
{
	/* (1) Ensure that ADEN = 0 */
	/* (2) Clear ADEN */
	/* (3) Set ADCAL=1 */
	/* (4) Wait until EOCAL=1 */
	/* (5) Clear EOCAL */
	if ((ADC1->CR & ADC_CR_ADEN) != 0) /* (1) */
	{
		ADC1->CR |= ADC_CR_ADDIS; /* (2) */
	}
 
	ADC1->CR |= ADC_CR_ADCAL; /* (3) */
 
	while ((ADC1->ISR & ADC_ISR_EOCAL) == 0) /* (4) */
	{
		/* For robust implementation, add here time-out management */
		HAL_Delay(200);
	}
	ADC1->ISR |= ADC_ISR_EOCAL; /* (5) */
}

and here my code in the ADC conversion function:

void concatenate_tx_data(void)
{
	ADC_calibration();		// ADC Calibration just before ADC conversion every 30 minutes
 
	vrefint_raw_value = adcBuffer[4];			// VREFINT Rohwert
	vrefint_cal_value = (*VREFINT_CAL_ADDR);	// VREFINT Factory Calibration Value @ 3V, 25C
 
 
	/*********** calculate R NTC1 (NTC mounted on PCB) ************/
	ADCx = (double)vrefint_cal_value * (double)adcBuffer[0];
	ADCy = vrefint_raw_value * ADC_RESOLUTION;
	ADCtemp = 3000.0 * (ADCx / ADCy);
	Uadc1 = ADCtemp;
	R_NTC1 = (R2*(UCONST-Uadc1))/Uadc1;
 
...
...

Can somebody give me a hint whats going wrong?

[TDK]

The calibration procedure in the RM is slightly different than what you're doing here. May or may not be the issue.

Software calibration procedure

1. Ensure that ADEN=0 and DMAEN=0.

2. Set ADCAL=1.

3. Wait until ADCAL=0 (or until EOCAL=1). This can be handled by interrupt if the

interrupt is enabled by setting the EOCALIE bit in the ADC_IER register. The ADCAL

bit can remain set for some time even after EOCAL has been set. As a result, the

software must wait for ADCAL=0 after EOCAL=1 to be able to set ADEN=1 for next

ADC conversions.

4. The calibration factor can be read from bits 6:0 of ADC_DR or ADC_CALFACT

registers.