STMG0B1RET output pin does not pull down in Standby Mode

[Kmax18]

I am using the STMG0B1RET on a custom, battery-powered board. During normal operation pin PC4 is used as UART1 (with an external pull-up resistor). In Standby Mode pin PC4 must be LOW in order to shut down the external device. For this I convert PC4 to GPIO and configure it for pull-down in Standby Mode to achieve the 0V at the output.

It is not working. When the HAL function HAL_PWR_EnterSTANDBYMode() is called the output goes HI.

I studied several posts and discussions about this issue, but no success yet.

The below is the latest code snipped. Wakeup pin 6 is used to exit Standby Mode (that works).
This line seems to pull the output to HI.  Why?

SCB->SCR |= SCB_SCR_SLEEPDEEP_Msk;

What else can we try? Thank you!!

/* Convert UART1 IO to a GPIO */
GPIO_InitTypeDef GPIO_InitStruct = {0};
HAL_GPIO_WritePin(UART1_GPIO_Port, UART1_Pin, GPIO_PIN_RESET);

/*Configure GPIO pin : UART1_Pin */
GPIO_InitStruct.Pin = UART1_Pin;
GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
GPIO_InitStruct.Pull = GPIO_NOPULL;
GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
HAL_GPIO_Init(UART1_GPIO_Port, &GPIO_InitStruct);

PWR->PDCRC = PWR_PDCRC_PD4;   // Set pull-ups for standby modes
PWR->CR4 = PWR_CR4_WP6;       // Set wakeup pin 6 to low level
PWR->CR3 = PWR_CR3_APC | PWR_CR3_EWUP6; // Enable pin pull configurations and wakeup pins
PWR->SCR = PWR_SCR_CWUF; // Clear wakeup flags

// Configure MCU low-power mode for CPU deep sleep mode
PWR->CR1 |= PWR_LOWPOWERMODE_STANDBY;
(void)PWR->CR1; // Ensure that the previous PWR register operations have been completed

// Configure CPU core
SCB->SCR |= SCB_SCR_SLEEPDEEP_Msk; // Enable CPU deep sleep mode

// Enter low-power mode
for (;;) {
    __DSB();
    __WFI();
}

[Kmax18]

JosueWilliams (Visitor II) posted a new reply in STM32 MCUs Products on 2025-03-20 8:08 AM:

"If the STMG0B1RET output pin isn’t pulling down in Standby Mode, check if the GPIO is properly configured as an output and if it retains state in low-power mode. Also, verify if any alternate functions or pull-up resistors are affecting it. Have you tried testing it in a different low-power state?"

[Sarra.S]

Hello @Kmax18, have you been able to solve this issue? 

[Kmax18]

Hello @Sarra.S, thanks for asking. Yes I solved the issue using a NUCLEO-G071RB board and a modified and expanded version of the code example PWR/PWR_STANDBY. The code controls a GPIO output with the PWR_PDCR and PWR_PUCR registers and the associated HAL library functions. How can a post an article about this for the ST Community?

[Kmax18 · Accepted solution]

Solution:
The following code snippet shows how the output pin PC4 is pulled down (0V) in the low-power Standby Mode of the STM32G0 MCU. A NUCLEO-G071RB board was used for developing and testing the code, which is based on the ST example PWR/PWR_STANDBY. The ST example was modified and expanded. These are the main steps:

• Determine if the MCU wakes up from Standby mode then resets the appropriate flags and registers.
• Configure and set the test GPIO output pin to HIGH. (The output will later be pulled down for verification).
• Use the PWR_PDCR register for controlling the GPIO output state (pull down to LOW) in Standby Mode.
• Enter the low-power Standby mode.
• Wake up from Standby Mode by pulling the WKUP pin2 HIGH.

The output voltage before and after entering Standby Mode was measured with a voltmeter (Figure 1).

Figure 1. Pinout of the NUCLEO board.

Code snippet:

  /* Check if the system was resumed from Standby mode */ 
  if (__HAL_PWR_GET_FLAG(PWR_FLAG_SB) != RESET)
  {
    /* Clear Standby flag */
    __HAL_PWR_CLEAR_FLAG(PWR_FLAG_SB);

    /* Check and Clear the Wakeup flag */
    if (__HAL_PWR_GET_FLAG(PWR_FLAG_WUF2) != RESET)
    {
      __HAL_PWR_CLEAR_FLAG(PWR_FLAG_WUF2);
    }

    /* Wait that user release the User push-button of the NUCLEO-G071RB */
    while ( HAL_GPIO_ReadPin(USER_BUTTON_GPIO_Port, USER_BUTTON_Pin) == GPIO_PIN_RESET )
    {}

    // Disable the pull-up bit
    HAL_PWREx_DisablePullUpPullDownConfig();
  }

  /* Set the GPIO test output pin to HIGH.
   * (Below it is pulled down to LOW before entering Standby Mode.)
  */
  HAL_GPIO_WritePin(PDCR_TEST_GPIO_Port, PDCR_TEST_Pin, GPIO_PIN_SET);

  /* Insert 5 seconds delay */
  HAL_Delay(5000);

  // Configure LOW GPIO output pin for Standby Mode
  HAL_PWREx_EnableGPIOPullDown(PWR_GPIO_C, PWR_GPIO_BIT_4);

  // Apply the pull-up bit with APC bit 10
  HAL_PWREx_EnablePullUpPullDownConfig();

  /* The Following Wakeup sequence is highly recommended prior to Standby mode entry
     - Enable wakeup
     - Clear wake up pin flag depending in edge detection & pin level.
     - Enter the Standby mode.
  */

  /* Enable WakeUp Pin PWR_WAKEUP_PIN2 connected to PC.13 */
  HAL_PWR_EnableWakeUpPin(PWR_WAKEUP_PIN2_HIGH);

  /* Clear all related wakeup flags*/
  __HAL_PWR_CLEAR_FLAG(PWR_FLAG_WUF2);

  /* Enter the Standby mode */
  HAL_PWR_EnterSTANDBYMode();

  /* This code will never be reached! */