Dirka:
maybe it is something else?
Dirk,
I'm assuming that you are wanting to free up the I2C pins?
What you did is ok.
However, the issue you are bumping into is that
A6 and A6 are only analog pins. They cannot be used as digital pins
as the AVR internally didn't hook them up to a register for digital i/o.
The Arduino core code doesn't do any sanity checks for this and will
end up indexing off the end of a table so there is no telling what actually
happens.
You should be able to digital pin 13, but use it for something else other than EN.
Pin 13 is the LED which probably blinks on reset and that may send some
bad data to the LCD when it blinks during reset.
To do this will require changing two pins, EN and some other pin to use 13.
Maybe hook r/w to pin 13. (the LED will be on for reads and off for writes).
Alternatively, you could pick a different pin for EN other than A6, A7 or 13,
perhaps pin 2 or 3?
--- bill