Hello John,

Your analysis is partly correct.

The slew-rate of the output is indeed rather slow when used in open drain/pu mode. A lower value for the pu resistance will increase the slew rate (it's dependant on 1/RC). This explains the effects you observe on your LCD.

As for the I/O pins in push-pull mode: This is output only! If you want to use a pin as input, you have to configure it as open drain and, depending on the speed required, use the internal or an external pu. For driving your LCD, however, push-pull mode would have done the trick (without the pu resistors!).

Best regards,
Rob Klein.