STM32L082 - ADC and Vrefint

[JeroenIoT]

Hello,

In my setup I am using the STM32L082. It is powered with 1.8V. The 1.8V is from a buck/boost convertor which itself is is powered by a SuperCap. I would like to measure the voltage of the SuperCap voltage, this is connected to pin PA0 (using a resistor divider to lower voltage) and mosfet switch to allow voltage measurement (PA1).

So I thought a traditional readAnalog() would do the trick. Unfortunately I am experiencing some output that I cannot explain.

• VREFINT measured at a sample value of 3361 (at 12 bit resolution)
• VREFINT_CAL(from system mem): 1673 (at 3.0V with 4096 sample range)

The calibrated VRREFINT from system memory makes sense and is approx. 1.22 (V) (3/4095 * 1673 = 1.2256...V
However I do not know why measuring in my circuit the VREFIN is sampled at 3361 as it should be at approx 2780 range

Any idea? (attached some part of my code below)

  analogReadResolution(12);  // 12-bit ADC (0-4095). Default resolution is 10
  // analogReference(AR_DEFAULT); // Dummy function for backward compatibility

  pinMode(PA0, INPUT_ANALOG);
  digitalWrite(PA1, HIGH);    // Enable voltage measurement
  delay(100);                 // Allow voltage to stabilize

  // Allow voltage reference to stabilize
  delay(20);  // Increased from 10ms to 20ms for better stability

  #ifdef DEBUG_ALL
    Serial.print(F("ADC Sample Time: ")); Serial.println(ADC1->SMPR, HEX);
    Serial.print(F("ADC Clock Config: ")); Serial.println(ADC1_COMMON->CCR, HEX);
  #endif

  uint16_t vrefint_cal = *((uint16_t*)0x1FF80078);
  Serial.print(F("VREFINT_CAL(from system mem): "));
  Serial.println(vrefint_cal);

  // Aantal metingen per cyclus
  const uint8_t numSamples = 10;
  uint32_t vrefint_sum = 0;
  uint32_t pa0_sum = 0;

  // Meerdere samples nemen
  for (uint8_t i = 0; i < numSamples; i++) {
    uint16_t vrefint_raw = analogRead(ADC_CHANNEL_VREFINT);
    uint16_t pa0_raw = analogRead(PA0);

    vrefint_sum += vrefint_raw;
    pa0_sum += pa0_raw;

    #ifdef DEBUG_ALL
      Serial.print(F("Sample ")); Serial.print(i + 1);
      Serial.print(F(" - Raw ADC VREFINT: ")); Serial.println(vrefint_raw);
      Serial.print(F("Sample ")); Serial.print(i + 1);
      Serial.print(F(" - Raw ADC PA0: ")); Serial.println(pa0_raw);
    #endif

    delay(10); 
  }

  // Avergae calculation
  uint16_t vrefint_avg = vrefint_sum / numSamples;
  uint16_t pa0_avg = pa0_sum / numSamples;

[fpistm]

Hi,
ADC_CHANNEL_VREFINT is not an Arduino pin number. It is a HAL definition.
Try:
uint16_t vrefint_raw = analogRead(AVREF);

Here an example how to use internal channel:
https://github.com/stm32duino/STM32Examples/blob/main/examples/Peripherals/ADC/Internal_channels/Internal_channels.ino

[JeroenIoT]

Hi Frederic,

Thanks for this. The AVREF does the trick to get the VREFINT correctly calculated

The 2nd issue I am struggling with is to measure the PA0 voltage. The analogRead is always measured too low compared to what I measure on the pin itself (multimeter). As the pin is connected to a R divider circuit I probably have to increase the sample time (ADC_SMPR)

Is there a 'ready to use' function' to set the sample time - like e.g.
ADC1->SMPR = (0b111 << ADC_SMPR_SMP_Pos); // 160.5 cycles

Or do I need to set all registers manually (ADSTART, ADEN etc etc)

Rgds,
Jeroen

[fpistm]

No API. You can redefine this using build_opt.h:

Arduino_Core_STM32/libraries/SrcWrapper/src/stm32/analog.cpp

Line 33 in 6b6a8a6

#ifndef ADC_SAMPLINGTIME

[JeroenIoT]

That works.

I placed it as building flag in my platformio.ini

build_flags =   -DADC_SAMPLINGTIME=ADC_SAMPLETIME_160CYCLES_5

FYI - implemented the following (for those who might run into this issue as well STM32L082)

   // Step 1: Initialize ADC
   analogReadResolution(12);  // 12-bit ADC (0-4095)
   pinMode(PA0, INPUT_ANALOG); // Configure PA0 as an analog input
   digitalWrite(PA1, HIGH);    // Enable voltage measurement circuit
   delay(100);                 // Allow voltage to stabilize
 
   // Step 2: Read factory-calibrated VREFINT value from system memory
   uint16_t vrefint_cal = *((uint16_t*)0x1FF80078);
 
   // Step 3: Compute the actual voltage of VREFINT (factory calibration)
   float Volt_refint = (3.0f / 4095.0f) * vrefint_cal; 
 
   // Step 4: Measure the actual ADC value of VREFINT and PA0
   uint16_t vrefint_raw = analogRead(AVREF);  // Read VREFINT (ADC Channel)
   uint16_t pa0_raw = analogRead(PA0);        // Read PA0 voltage
 
   // Step 5: Compute estimated VDDA using VREFINT
   float vdd_est = (Volt_refint / vrefint_raw) * 4095.0f;
 
   // Step 6: Convert ADC PA0 reading to voltage using estimated VDDA
   float pa0_voltage = (vdd_est / 4095.0f) * pa0_raw;

Thanks again for your help Frederic!

[lohikarhu]

Hi; I'm not even a beginner, not even a newbie, so please accept my apologies if this is a stupid thing: I was looking at the library that is referred to, below, and I noticed something that looked unusual, the two lines *highlighted*, are different than all the other lines? Just wonderin' #ifndef ADC_SAMPLINGTIME #if defined(ADC_SAMPLETIME_8CYCLES_5) #define ADC_SAMPLINGTIME ADC_SAMPLETIME_8CYCLES_5; #elif *#defined(ADC_SAMPLETIME_12CYCLES)* *#define ADC_SAMPLINGTIME ADC_SAMPLETIME_12CYCLES;* #elif defined(ADC_SAMPLETIME_12CYCLES_5) Best regards from Germany Lohi Karhu

…

On Mon, Mar 10, 2025, 18:32 Frederic Pillon ***@***.***> wrote: No API. You can redefine this using build_opt.h: https://github.com/stm32duino/Arduino_Core_STM32/blob/6b6a8a6cab34fe0f3bc78a36166acc5c99810f94/libraries/SrcWrapper/src/stm32/analog.cpp#L33 — Reply to this email directly, view it on GitHub <#2676 (reply in thread)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AGBMPKWWC2FOOWSMJHGWIVD2TXEDLAVCNFSM6AAAAABYWEN7VOVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTENBVGI2DOOI> . You are receiving this because you are subscribed to this thread.Message ID: <stm32duino/Arduino_Core_STM32/repo-discussions/2676/comments/12452479 @github.com>

[fpistm]

Sorry don't understand your question. Your code is not correct and does not match the code:

Arduino_Core_STM32/libraries/SrcWrapper/src/stm32/analog.cpp

Lines 38 to 41 in db4fdcc

	#elif defined(ADC_SAMPLETIME_12CYCLES)
	#define ADC_SAMPLINGTIME ADC_SAMPLETIME_12CYCLES;
	#elif defined(ADC_SAMPLETIME_12CYCLES_5)
	#define ADC_SAMPLINGTIME ADC_SAMPLETIME_12CYCLES_5;

[lohikarhu]

Just that the "syntax", if I can call it that, of this part: #elif defined(ADC_SAMPLETIME_12CYCLES) #define ADC_SAMPLINGTIME ADC_SAMPLETIME_12CYCLES; Doesn't have the. *_5 * after the "12CYCLES" to match the format of all of the other lines, that look like this: (ADC_SAMPLETIME_12CYCLES_5) When something looks "out of place", I just wondered why...

…

On Thu, Mar 13, 2025, 16:40 Frederic Pillon ***@***.***> wrote: Sorry don't understand your question. Your code is not correct and does not match the code: https://github.com/stm32duino/Arduino_Core_STM32/blob/db4fdcc0f2b938cb8b797c50ade8b5cd886c5037/libraries/SrcWrapper/src/stm32/analog.cpp#L38-L41 — Reply to this email directly, view it on GitHub <#2676 (reply in thread)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AGBMPKVVN4OI3OATSKYOHKL2UGRFDAVCNFSM6AAAAABYWEN7VOVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTENBZGAZDKMQ> . You are receiving this because you commented.Message ID: <stm32duino/Arduino_Core_STM32/repo-discussions/2676/comments/12490252@ github.com>

[fpistm]

Have in mind the core support several stm32 series, then not all have the _5.

STM32C0 have:

Arduino_Core_STM32/system/Drivers/STM32C0xx_HAL_Driver/Inc/stm32c0xx_hal_adc.h

Lines 757 to 767 in db4fdcc

	/** @defgroup ADC_HAL_EC_CHANNEL_SAMPLINGTIME Channel - Sampling time
	* @{
	*/
	#define ADC_SAMPLETIME_1CYCLE_5 (LL_ADC_SAMPLINGTIME_1CYCLE_5) /*!< Sampling time 1.5 ADC clock cycle */
	#define ADC_SAMPLETIME_3CYCLES_5 (LL_ADC_SAMPLINGTIME_3CYCLES_5) /*!< Sampling time 3.5 ADC clock cycles */
	#define ADC_SAMPLETIME_7CYCLES_5 (LL_ADC_SAMPLINGTIME_7CYCLES_5) /*!< Sampling time 7.5 ADC clock cycles */
	#define ADC_SAMPLETIME_12CYCLES_5 (LL_ADC_SAMPLINGTIME_12CYCLES_5) /*!< Sampling time 12.5 ADC clock cycles */
	#define ADC_SAMPLETIME_19CYCLES_5 (LL_ADC_SAMPLINGTIME_19CYCLES_5) /*!< Sampling time 19.5 ADC clock cycles */
	#define ADC_SAMPLETIME_39CYCLES_5 (LL_ADC_SAMPLINGTIME_39CYCLES_5) /*!< Sampling time 39.5 ADC clock cycles */
	#define ADC_SAMPLETIME_79CYCLES_5 (LL_ADC_SAMPLINGTIME_79CYCLES_5) /*!< Sampling time 79.5 ADC clock cycles */
	#define ADC_SAMPLETIME_160CYCLES_5 (LL_ADC_SAMPLINGTIME_160CYCLES_5) /*!< Sampling time 160.5 ADC clock cycles */

but STM32U5 not:

Arduino_Core_STM32/system/Drivers/STM32U5xx_HAL_Driver/Inc/stm32u5xx_hal_adc.h

Lines 948 to 958 in db4fdcc

	/** @defgroup ADC_HAL_EC_CHANNEL_SAMPLINGTIME Channel - Sampling time
	* @{
	*/
	#define ADC_SAMPLETIME_5CYCLES (LL_ADC_SAMPLINGTIME_5CYCLES) /*!< Sampling time 5 ADC clock cycles */
	#define ADC_SAMPLETIME_6CYCLES (LL_ADC_SAMPLINGTIME_6CYCLES) /*!< Sampling time 6 ADC clock cycles */
	#define ADC_SAMPLETIME_12CYCLES (LL_ADC_SAMPLINGTIME_12CYCLES) /*!< Sampling time 12 ADC clock cycles */
	#define ADC_SAMPLETIME_20CYCLES (LL_ADC_SAMPLINGTIME_20CYCLES) /*!< Sampling time 20 ADC clock cycles */
	#define ADC_SAMPLETIME_36CYCLES (LL_ADC_SAMPLINGTIME_36CYCLES) /*!< Sampling time 36 ADC clock cycles */
	#define ADC_SAMPLETIME_68CYCLES (LL_ADC_SAMPLINGTIME_68CYCLES) /*!< Sampling time 68 ADC clock cycles */
	#define ADC_SAMPLETIME_391CYCLES (LL_ADC_SAMPLINGTIME_391CYCLES) /*!< Sampling time 391 ADC clock cycles */
	#define ADC_SAMPLETIME_814CYCLES (LL_ADC_SAMPLINGTIME_814CYCLES) /*!< Sampling time 814 ADC clock cycles */

[lohikarhu]

Okey dokey, just seemed strange that only one line was without the _5 As I said, new to this... 40 years of system level, and even IC Development (like LP5523, STLED25) and even "bare metal" with MC6801/09, 68HC11, 68k, but nary a line of C. Thx

…

On Thu, Mar 13, 2025, 17:57 Frederic Pillon ***@***.***> wrote: Have in mind the core support several stm32 series, then not all have the _5. STM32C0 have: https://github.com/stm32duino/Arduino_Core_STM32/blob/db4fdcc0f2b938cb8b797c50ade8b5cd886c5037/system/Drivers/STM32C0xx_HAL_Driver/Inc/stm32c0xx_hal_adc.h#L757-L767 but STM32U5 not: https://github.com/stm32duino/Arduino_Core_STM32/blob/db4fdcc0f2b938cb8b797c50ade8b5cd886c5037/system/Drivers/STM32U5xx_HAL_Driver/Inc/stm32u5xx_hal_adc.h#L948-L958 — Reply to this email directly, view it on GitHub <#2676 (reply in thread)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AGBMPKWLLU2NN3HYVDKMZ3L2UG2G5AVCNFSM6AAAAABYWEN7VOVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTENBZGEZTCOA> . You are receiving this because you commented.Message ID: <stm32duino/Arduino_Core_STM32/repo-discussions/2676/comments/12491318@ github.com>