STM32 I²C LCD
LCD Display modules are readily available and they are dirt cheap (< $2 typically). They come in different "resolutions" typically expressed as number of characters and number of lines. Common ones are 1602 (16 characters in 2 lines) and 2004 (20 characters in 4 lines).
The modules are interfaces by an 8 bit parallel data bus (can be operated in 4 bit mode) and some control signals:
These can of course be "driven" by an STM32 by interfacing these pins directly, but at a minimum that would tie up 8 GPIO pins for one display.
To cut down on the number of pins needed a GPIO extender can be used and because this approach is quite a lot easier a lot of the LCD modules come with an I²C GPIO extender already attached.
The I²C extender is connected in the following way:
In other words, the 8 bits of the GPIO extender is mapped as follows:
|P7||D7||MSB of data|