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Switching GPIO cause ripple on VDDA. Very strange problem.

Question asked by polyakov.mikhail on Jun 28, 2012
Latest reply on Jun 29, 2012 by malund.erik
Good day everybody !


I have two strange problems with GPIO  in STM32L151CBU6


1)  This code I compile  in Keil. Using an oscilloscope I see the time  ports switching.
    Why PB7 switches slower than  PA5 PA11 ?
   
     HSI_ON;
    SYSTEM_CLOCK_HSI;


  RCC->AHBENR |= RCC_AHBENR_GPIOBEN | RCC_AHBENR_GPIOAEN; 
  
    GPIOA->MODER &= ~GPIO_MODER_MODER5;
    GPIOA->MODER |= GPIO_MODER_MODER5_0;  //PA5 is output
    GPIOA->OSPEEDR|= GPIO_OSPEEDER_OSPEEDR5;  // Maximum speed  PA5


    GPIOA->MODER &= ~GPIO_MODER_MODER11;
    GPIOA->MODER |= GPIO_MODER_MODER11_0;  //PA11 is output
    GPIOA->OSPEEDR|= GPIO_OSPEEDER_OSPEEDR11;  // Maximum speed  PA11


    GPIOB->MODER &= ~GPIO_MODER_MODER7;
    GPIOB->MODER |= GPIO_MODER_MODER7_0;  //PB7 is output
    GPIOB->OSPEEDR|= GPIO_OSPEEDER_OSPEEDR7; // Maximum speed  PB7


    while(1)
    {        
        GPIOA->BSRRL=GPIO_BSRR_BS_5;  //Hi level on  PA5
        GPIOA->BSRRH=GPIO_BSRR_BS_5;  //Low level  PA5


        GPIOA->BSRRL=GPIO_BSRR_BS_11;  //Hi level on PA11
        GPIOA->BSRRH=GPIO_BSRR_BS_11;  //Low level PA11


        GPIOB->BSRRL=GPIO_BSRR_BS_7;   //Hi level on PB7
        GPIOB->BSRRH=GPIO_BSRR_BS_7;   //Low level PB7
  }
 
2)But  problem number 1  is  very  important. I do not need fast switch on PB7.
   Real trouble is that switching  PA5 causes current consumption pulsation on VDDA !
   This pulsation causes changes in the ADC readings. Сapacitors on VDDA do not solve the problem.
   In attachment  there is measurement scheme  and  pulsations with amplitude 100mV  when transmitting  one byte through SPI1  , SCK of which  is on PA5.
 When switching PA11 or PB7 , there is no pulsations on VDDA.


 Why  PA5  is associated with VDDA  in so strange way ?
 How to get the correct results from ADC when switching PA5 ?


Thank you in advance
Mikhail Polyakov,
Moskow.A5.


  

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