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@Gordon
The voltage swing is good, but as far as I can tell from that trace (I'm not 100% sure where 0v is on the scope) the 'resting' voltage is 0v, so it's oscillating between -0.7 and 0.7v...
That sort of implies that the pullup on the pin isn't working for some reason? Ideally it should sit at around 1.6v, so would go between 0.9v and 2.3v depending on the signal.