GPIO Speed setting Fast/High (50/100MHz) - IO Compensation cell

[flyer31]

Posted on January 27, 2012 at 10:28

Hi,

in the reference manual RM0090 for STMF4, in chapter 7.1 it is recommended to activate the ''IO compensation cell'' (Register SYSCFG_CMPCR), if outputs are used with speed settings of 50 or 100 MHz (GPIOx_OSPEEDR-Register). If I search for ''SYSCFG_CMPCR'' in the folder STM32F2xx_StdPeriph_Lib_V1.0.0\Libraries, then I get a hit in the file stm32f2xx_syscfg.c, there is a function named ''SYSCFG_CompensationCellCmd'', and in the release notes, they specify that this command was added in April 2011 only. But if I search the source folder for the usage of this command, it is never used. Anyone who has experience with the influence of this ''IO compensation cell''? Is it really a good a advise to use it? Further: In the reference manual 6.4.3, description of register GPIOx_SPEEDR, it says for hi-speed setting: 100 MHz High Speed on 30pF (80 MHz Output max speed on 15pF). Is this real or maybe some misprint? (for higher capacitive pin load I definitely would expect some lower speed ...). (??)

[shannon2]

Posted on January 28, 2012 at 22:27

Perhaps if you described the problem you are having we could help you.  Have you looked at the signals on an o'scope?

[flyer31]

Posted on January 28, 2012 at 23:17

Yes, I looked at the signals with a 100MHz oscilloscope (50MHz Timer outputs).

After I included some master-slave relationships between the timers (to synchronize them), I recognized that there is a phase shift between the timers, if I switch on this compensation cell. So I anyway think, I cannot use this. I was just curious, whether I should see some other influence due to this compensation cell setting, because I did not see any influence on the pulse form. (But today I recognized, that it might be necessary, to switch on the SYSCFG clock source first - which I did not do yesterday on my first testing). No problems - just curiosity.