Posted on November 03, 2015 at 16:04 Hi Werner,
That's an interesting question. I am using an 8MHz SD clock and if I view the waveform on the scope the rising and falling edges are
curved rather than straight up and down. But there's no ringing. The scope says the rise and fall times are around 31 nS. If I change
the output speed to medium which (according to the datasheet) raises the max output frequency from 2 to 25 MHz, the clock
waveform looks almost identical with just a very small reduction in rise/fall times to around 29 nS. What is more, it works quite reliably.
I have a 12pF cap to ground on that clock signal to stop some ringing that was occuring.
One last thing, the datasheet for the STM32405 that I have, specifies the max GPIO freqs with 50pF of loading and doesn't actually
say what the max freqs are for lower loadings.
Cheers,
Mark
