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Maximum Output Speed

Question asked by jenkins.james on Jan 7, 2012
Latest reply on Jan 11, 2012 by schaffer.tim
Hi,
I am curious as to what the '50MHz' output speed of the GPIO ports refers to (with correct configuration bits set). Is it the maximum frequency of a square wave output, or does it have something to do with the rise/fall times?

The camera module I am using works with an STM32-driven 5MHz square wave, but not a 12MHz one. The 5MHz square wave shows to be exactly 1.65V (3.3V/2), however, the 12MHz waveform shows 0.3V-0.5V, which suggests that the signal is never reaching 3.3V - the rise time is too long. Unfortunately I do not have an oscilloscope so can't see what's going on.

It seems strange because other people seem to have managed something akin what I am trying to do.
The following are quotes from http://forums.leaflabs.com/topic.php?id=774:
- "That 50MHz refers to the speed that the external electrical signal driven by a GPIO pin changes from low to high, or vice versa; the rise-time. This configures the rate that the drive transistors work at."
- "The best I have achieved is 18MHz to toggle a pin repeatedly with some odd-looking code, but 12MHz was okay."
- "If all that is needed is a square wave at a fixed frequency on a pin, it would be almost as easy to program one of the timers ... When I loaded data too, it came down to about 12MHz."

If anybody has any insight to offer, I'm all ears!

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