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GPIO speed

Question asked by grossi.marco on Oct 6, 2015
Latest reply on Oct 6, 2015 by Clive One
I am working on a STM32 NUCLEO 152RE.
When initializing a I/O pin of a digital port, one of the options is to set its speed. The available options for L152 are:

GPIO_Speed_400KHz = 0x00, /*!< Very Low Speed */
  GPIO_Speed_2MHz   = 0x01, /*!< Low Speed */
  GPIO_Speed_10MHz  = 0x02, /*!< Medium Speed */
  GPIO_Speed_40MHz  = 0x03  /*!< High Speed */

However the GPIO frequency cannot be higher than HCLK.
If, for example, I set the SystemClock to 12MHz (HSI with PLLMul 3 and PLLDiv 4) with no AHB prescaler, HCLK is 12MHz.
Then, what happens if I set GPIO_Speed_40MHz for a digital I/O? Is the pin speed scaled down to the highest frequency possible (12MHz) or what else?