U3 Counter Speed
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Posted 18 April 2013 - 12:35 PM
It has to do with how fast the signal rises and falls internally, due to the series impedance and stray capacitance. With the 8 MHz spec for a 3.3V square wave, we are saying you need a rise time of at least 63ns and a fall time of at least ns, for the voltage to swing above and below the logic thresholds. I suspect if you did your own test, the point where it starts to miss counts is around 16 MHz. EIO/CIO might be faster as they have lower series impedance, and a 5V square wave might work faster than a 3.3V square wave.
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