Cannot get valid values using ADC1 channel 11 on PC1

[zamlox]

Hello,

I'm using a Nucleo board (NUCLEO-G070RB) with STM32F103RB MCU and I try to use channel 11 of ADC1 mapped to PC1 to get values from a humidity sensor. I noticed that the values are invalid and it's not because of the sensor. 

I created a sample application and connected PC1 to VCC but the values are about 2000 instead of 4095. When I connect to GND I get same values instead of 0.

Here is the code:

#include "stm32f103xb.h"
 
uint32_t adcValue = 0;
 
int main(void)
{
	/* Enable GPIOC clock */
	RCC->APB2ENR |= (1U<<4);
	/* Configure PC1 to analog input */
	GPIOC->CRL &= ~(1U<<4);	// MODE1[0]: 0
	GPIOC->CRL &= ~(1U<<5);	// MODE1[1]: 0	input mode
	GPIOC->CRL &= ~(1U<<6);	// CNF1[0]: 0
	GPIOC->CRL &= ~(1U<<7);	// CNF1[1]: 0	analog mode
#if 0
	/* Enable GPIOA clock */
	RCC->APB2ENR |= (1U<<2);
	/* Configure PA1 to analog input */
	GPIOA->CRL &= ~(1U<<4);	// MODE1[0]: 0
	GPIOA->CRL &= ~(1U<<5);	// MODE1[1]: 0	input mode
	GPIOA->CRL &= ~(1U<<6);	// CNF1[0]: 0
	GPIOA->CRL &= ~(1U<<7);	// CNF1[1]: 0	analog mode
#endif
	/* Enable ADC1 clock */
	RCC->APB2ENR |= (1U<<9);
	/* Set channel 11 for ADC1 */
	ADC1->SQR3 = 11U;
#if 0
	/* Set channel 1 for ADC1 */
	ADC1->SQR3 = 1U;
#endif
	/* Set number of conversions */
	ADC1->SQR1 = 0;	// L[3:0]: 0000 -> 1 conversion
	/* Enable ADC1 */
	ADC1->CR2 |= (1U<<0);	// set ADON to 1
	/* Enable continuous mode */
	ADC1->CR2 |= (1U<<1);	// set CONT to 1
 
	/* Set start software flag */
	ADC1->CR2 |= (1U<<22);	// set SWSTART to 1
	/* Set ADON again to start conversion */
	ADC1->CR2 |= (1U<<0);	// set ADON to 1
 
	while(1)
	{
		/* Check EOC bit for end of conversion */
		while(!(ADC1->SR & 2U));
		/* Read conversion value */
		adcValue = ADC1->DR;
	}
}

When I use PA1 with channel 1 then values are correct (4095 or 0).

What can be the issue ?

Thanks.

[TDK]

> I'm using a Nucleo board (NUCLEO-G070RB) with STM32F103RB MCU 

How, exactly? They don't have compatible pinouts.

[zamlox]

I'm using following board user manual: STM32 Nucleo-64 boards (MB1360) - User manual and in this document board name is referenced as I mentioned above. I checked my board and actually is NUCLEO-F103RB.

Configuration of connectors and pins are similar with my board.

[TDK]

Regardless, you could set PC1 as an output and see that you can measure it with a multimeter to verify correct hardware pinout. Or as an input and verify that IDR reads correctly.

You should also verify the register settings are correctly set by examining them in a debugger. Technically you need a delay of a few cycles between enabling a clock and using it. This may be preventing the "ADC1->SQR3 = 11U" line from happening.

https://github.com/STMicroelectronics/STM32CubeF1/blob/f5aaa9b45492d70585ade1dac4d1e33d5531c171/Drivers/STM32F1xx_HAL_Driver/Inc/stm32f1xx_hal_rcc.h#L535

[zamlox]

Thank you for suggestions.

I configured PC1 in output mode, toggling ODR register each second and I connected a logic analyzer to the pin. Signal is flat, not as expected.

I used same code but configured for PA1 and the output is as expected.

I don't understand why same code works for PA1 (as output and input from ADC1) but not for PC1.

I verified register settings when testing PC1 in debug mode and all registers are set correctly.

I put a delay of 1ms after setting the channel but still not working.