STM32C0 SDA signal on PC14

[rnicolas]

Hi,

I'm trying to configure pin PC14 to be used as SDA signal as it stated in the datasheets STM32C031C6 and STM32C031G4:

To configure the pin (and the the I2C) I am using:

void SDK_I2C_Init(SDK_I2C_HandleTypeDef *hi2c) {
  LL_I2C_InitTypeDef I2C_InitStruct = {0};
  LL_GPIO_InitTypeDef GPIO_InitStruct = {0};

  LL_RCC_SetI2CClockSource(LL_RCC_I2C1_CLKSOURCE_PCLK1);
  LL_IOP_GRP1_EnableClock(LL_IOP_GRP1_PERIPH_GPIOB);
  LL_IOP_GRP1_EnableClock(LL_IOP_GRP1_PERIPH_GPIOC);
  /**I2C1 GPIO Configuration
  PB8   ------> I2C1_SCL
  PC14   ------> I2C1_SDA
  */
  GPIO_InitStruct.Pin = LL_GPIO_PIN_8;
  GPIO_InitStruct.Mode = LL_GPIO_MODE_ALTERNATE;
  GPIO_InitStruct.Speed = LL_GPIO_SPEED_FREQ_HIGH;
  GPIO_InitStruct.OutputType = LL_GPIO_OUTPUT_OPENDRAIN;
  GPIO_InitStruct.Pull = LL_GPIO_PULL_UP;
  GPIO_InitStruct.Alternate = LL_GPIO_AF_6;
  LL_GPIO_Init(GPIOB, &GPIO_InitStruct);

  GPIO_InitStruct.Pin = LL_GPIO_PIN_14;
  GPIO_InitStruct.Mode = LL_GPIO_MODE_ALTERNATE;
  GPIO_InitStruct.Speed = LL_GPIO_SPEED_FREQ_HIGH;
  GPIO_InitStruct.OutputType = LL_GPIO_OUTPUT_OPENDRAIN;
  GPIO_InitStruct.Pull = LL_GPIO_PULL_UP;
  GPIO_InitStruct.Alternate = LL_GPIO_AF_14;
  LL_GPIO_Init(GPIOC, &GPIO_InitStruct);

  LL_APB1_GRP1_EnableClock(LL_APB1_GRP1_PERIPH_I2C1);

  I2C_InitStruct.PeripheralMode = LL_I2C_MODE_I2C;
  I2C_InitStruct.Timing = I2C_TIMING;
  I2C_InitStruct.AnalogFilter = LL_I2C_ANALOGFILTER_ENABLE;
  I2C_InitStruct.DigitalFilter = 2;
  I2C_InitStruct.OwnAddress1 = 180;
  I2C_InitStruct.TypeAcknowledge = LL_I2C_ACK;
  I2C_InitStruct.OwnAddrSize = LL_I2C_OWNADDRESS1_7BIT;
  LL_I2C_Init(I2C1, &I2C_InitStruct);
  LL_I2C_EnableAutoEndMode(I2C1);
  LL_I2C_SetOwnAddress2(I2C1, 0, LL_I2C_OWNADDRESS2_NOMASK);
  LL_I2C_DisableOwnAddress2(I2C1);
  LL_I2C_DisableGeneralCall(I2C1);
  LL_I2C_EnableClockStretching(I2C1);

  LL_I2C_Disable(I2C1);
  LL_I2C_SetTiming(I2C1, I2C_TIMING);
  LL_I2C_Enable(I2C1);
}

 which is almost the same code that we can find here.

If I configure another pin to work as SDA (PB9 or PB7) the system works perfectly and I can communicate using the I2C protocol.

Is there any configuration that I am missing for that specific pin?

[rnicolas]

By communication I mean reading data from the I2C device. When I configure other pins I see I2C peripheral working fine, but when I configure PC14, I see it is not working. But let me do a recap what I have seen until now.

[Andrew Neil]

---

@rnicolas wrote:

By communication I mean reading data from the I2C device..

---

The question was, "what do you mean by no communication?"

That could mean:

• Communication fails
• No activity at all
• etc.

Again, have you used a scope to see what's actually happening on the wires?

[rnicolas]

First of all, thank you all of you guys for pointing several things.

It was really helpful to discover the pin PC14 in nucleo board is not connected, but I need to do some rework there to get it working, as pointed by @Andrew Neil . Since I have already tested the code with other pins and I proved it is working, I just switch to the second board (the proprietary board with 4k7 external pull-ups) that has the PC14 exposed. In that board I analyzed the value of FLASH_OPTR which is 11111111111111111111111010101010, so I can see HSE_NOT_REMAPPED is disabled according to the datasheet (1: disabled, 0: enabled). That means I should be able to use PC14 as a GPIO.

So something else is happening, and after a morning of debugging I see the following, when this code is executed:

GPIO_InitStruct.Pin = LL_GPIO_PIN_8;
GPIO_InitStruct.Mode = LL_GPIO_MODE_ALTERNATE;
GPIO_InitStruct.Speed = LL_GPIO_SPEED_FREQ_HIGH;
GPIO_InitStruct.OutputType = LL_GPIO_OUTPUT_OPENDRAIN;
GPIO_InitStruct.Pull = LL_GPIO_PULL_NO;
GPIO_InitStruct.Alternate = LL_GPIO_AF_6;
LL_GPIO_Init(GPIOB, &GPIO_InitStruct);

The application ends in here:

Default_Handler:
Infinite_Loop:
  b Infinite_Loop

but if I debug step by step, the code seems to run fine, but I cannot let the code run autonomously because whenever I let it then run autonomously it ends in the Infinite_Loop.

[TDK]

> but I cannot let the code run autonomously because whenever I let it then run autonomously it ends in the Infinite_Loop.

Surely this is a bug which should be fixed first, not avoided.

Look at VECTACTIVE bit in SCB->ICSR to understand what interrupt is active.

[rnicolas · Accepted solution]

Hey everyone, I just wanted to take a moment to say thank you for all the insights and suggestions!

After hours of debugging, I finally got my I2C communication working with sensor. The issue? A hidden EXTI pin configuration buried in some project files that I wasn't even considering.

I kept reading through my code again and again, and eventually, I realized that the EXTI settings were conflicting with my I2C pinout. Once I removed those calls—boom! Everything started working perfectly.

This was a great reminder that sometimes the problem isn't where you're looking—it’s hiding in plain sight.

Big thanks to everyone who shared their knowledge and helped me troubleshoot this! Your support made all the difference.