Posted on January 07, 2012 at 14:59It controls the slew rate of the pin, how hard it drives high/low. The amount of external loading will impact how effectively this occurs. If you are trying to drive a high load, consider using an external buffer.
Using code to drive GPIOs at high frequencies is a fools errand. Use a timer to generate fast/accurate frequencies, or peripherals like SPI, synchronous USART for clock/data type applications. Think creatively, and avoid bit level processing in software, try to do 8 or 16-bit at a time to achieve some decimation.
With high timer rates you definitely don't want them generating interrupts as they will assuredly saturate the processor, you could experiment, I have in the past, but off the top of my head I'd say below 100 KHz.
Also think creatively about the frequency you run the processor at. You can use different crystals and PLL settings that hit magic frequencies, and maybe run below the 72 MHz ceiling, but will mesh better with the rates the external hardware works optimally with. The STM32 is not good with fractional settings. For instance if you need a clean 256 KHz, you'll need to run at 32 or 64 MHz.
Consider using the MCO pin for higher frequencies, should be capable of 36 MHz, perhaps 48-50 MHz if you under clock. Timer pins, perhaps 18 MHz, I'm sure they'd go higher, but will grind and be generally inflexible. At these rates I'd be using different hardware, with finer granularity. I'd need to stick a scope on the board to give you a high confidence number.
Also consider F2 or F4 series parts, or external clocks.
