stm32adcstm32f0

Calibrating STM32 ADC (VREFINT)

I'm trying to read VDDA on an STM32F042 microcontroller. I'm getting unexpected results with VDD at 3.29V. I must be missing something fundamental.

output:

VREFINT=1917; VREFINT_CAL=1524; VDDA=2623 mV
VREFINT=1885; VREFINT_CAL=1524; VDDA=2668 mV
VREFINT=1913; VREFINT_CAL=1524; VDDA=2628 mV
VREFINT=1917; VREFINT_CAL=1524; VDDA=2623 mV
VREFINT=1917; VREFINT_CAL=1524; VDDA=2623 mV

adc_test.c:

#include <stdio.h>
#include "stm32f0xx.h"

#define VREFINT_CAL_ADDR                0x1FFFF7BA  /* datasheet p. 19 */
#define VREFINT_CAL ((uint16_t*) VREFINT_CAL_ADDR)

extern void initialise_monitor_handles(void);

int main(void)
{
    RCC->APB2ENR |= RCC_APB2ENR_ADC1EN;     /* enable ADC peripheral clock */
    RCC->CR2 |= RCC_CR2_HSI14ON;            /* start ADC HSI */
    while (!(RCC->CR2 & RCC_CR2_HSI14RDY)); /* wait for completion */
    /* calibration */
    ADC1->CR |= ADC_CR_ADCAL;               /* start ADc CALibration */
    while (ADC1->CR & ADC_CR_ADCAL);        /* wait for completion */
    ADC1->CR |= ADC_CR_ADEN;                /* ADc ENable */
    while (!(ADC1->ISR & ADC_ISR_ADRDY));   /* wait for completion */
    ADC1->SMPR |= ADC_SMPR1_SMPR_0 |        /* sampling mode: longest */
      ADC_SMPR1_SMPR_1 |
      ADC_SMPR1_SMPR_2;
    /* VDD reference */
    ADC->CCR |= ADC_CCR_VREFEN;             /* VREF Enable */
    ADC1->CHSELR = ADC_CHSELR_CHSEL17;      /* CH17 = VREFINT */

    initialise_monitor_handles();           /* enable semihosting */

    while (1) {
        ADC1->CR |= ADC_CR_ADSTART;             /* start ADC conversion */
        while (!(ADC1->ISR & ADC_ISR_EOC));     /* wait for completion */
        uint32_t vdda = 3300UL * *VREFINT_CAL / ADC1->DR; /* ref. manual p. 252; constant and result in millivolts */
        printf("VREFINT=%lu; VREFINT_CAL=%lu; VDDA=%lu mV\n",
                (unsigned long)ADC1->DR,
                (unsigned long)*VREFINT_CAL,
                (unsigned long)vdda);
    }
}

Screenshot from Datasheet:

enter image description here

Screenshot from Reference Manual

note this refers to .3V, but I believe this to be a typo, as the datasheet above and the longer formula below refer to 3.3V, and .3V is below minimum operating voltage for this part

enter image description here

Solution

  • The answer (big thanks to @jasonharper) is a missing ground connection. Jason't comments on the OP are the best source of wisdom in this thread. I post a summary here so this question can have an accepted answer.

    The board went through a number of revisions and in this iteration we forgot to connect the thermal pad, which on this part is the only ground connection. The chip was getting ground through ESD diodes on pins that were connected to ground. It's surprising to me that it worked at all. I was able to increase the current to the chip by configuring grounded GPIOs as outputs and setting them low.