floresta:
Where have you been able to find information about the internal workings of the controller? I have been looking for this.
I confess - I had to use a magnifying glass (actually, head-mounted magnifier). Otherwise simple inspection.
My memory serves that some (ten?) years ago I studied the HD44780 datasheet which explains the six-level multiplexing voltages of the LCD display. These voltages are set by a resistor chain on the board comprising R1 to R5.