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GPIOx_OSPEEDR and GPIO Compensation Cells

Question asked by Godshall.Robert on Feb 19, 2014
Latest reply on Feb 20, 2014 by Godshall.Robert
Hi All

I've been working with STM32F405/07 (STM32FDiscovery & my own designs) for a while now... though I'm certainly no expert:)

This is a request for any discussion or useful information generally related to high speed GPIO. While some aspects of this seem pretty obvious I really hate to make assumptions... "behind the baddest of bugs are even worse assumptions!" I've searched the forums and available literature and am not really finding much; maybe I've missed something? Some of my questions are: 
  • First and foremost, is there a document (other than the main reference manual and data sheet) that covers GPIO speed and/or the compensation cells in detail?
  • Is there any good reading on the mechanics of AHB1 with respect to the GPIO ports? I think this could help my understanding of the speed aspect. 
  • What sorts of effects do GPIOx_OSPEEDR values have on pins when configured as:    
    • Output?
    • Input, if any? 
    • Alternate function, if any? ie, if I'm expecting 10MHz SPI and my MOSI line is setup with GPIOx_OSPEEDR = 2MHz, should I expect trouble? 
  • What exactly do the compensation cells do?
  • What are the pros and cons of using the compensation cells? And what are the possible consequences of not using them when toggling an input or output at >= 50MHz?
Thanks in advance for any and all feedback!