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PROBLEM: External pull-up of FT pin not reaching 5V

Question asked by red on Aug 13, 2015
Latest reply on Sep 10, 2015 by red
Hi,

I am having trouble using an external pull up resistor to 5V. The voltage at the GPIOs is clamping to ~3.65V.

Notes:
       
  • Processor: STM32L151VBT6 (100-LQFP)
       
  • Test Board: Proprietary (nothing else connected to these pins, not even protection diodes)
  •    
  • Pins tested: PD8 and PD9
  •    
  • MCU Supply voltage: 3.0V
  •    
  • Pull-up voltage: 5.0V
  •    
  • Pull-up Resistor: 1.5 kOhm
  •    
  • A fix not requiring a board change would be best
Test code:

int main( void )
{
    GPIO_InitTypeDef GPIO_InitStructure;
     
    RCC_AHBPeriphClockCmd( RCC_AHBPeriph_GPIOD, ENABLE );
     
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_8;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_40MHz;
    GPIO_InitStructure.GPIO_OType = GPIO_OType_OD;
    GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
    GPIO_Init(GPIOD, &GPIO_InitStructure);
     
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_9;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_40MHz;
    GPIO_InitStructure.GPIO_OType = GPIO_OType_OD;
    GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
    GPIO_Init(GPIOD, &GPIO_InitStructure);
 
    while(1);
}

What I've done so far:
       
  • Tested two GPIOs PD8/PD9
  •    
  • Tried configuring the pins as inputs/outputs/AF(USART) (all worked as desired except the pull up was clamped)
  •    
  • Tried different pull ups (1.5k and 5.6k)
  •    
  • Searched the forums, couldn't find anything except that pins with analog features are clamped to 3.3V, but these pins don't have any analog functionality.

Any help would be greatly appreciated.

Thanks!

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