Keypad header file for atmega

There's a header file at the bottom of this page: https://people.ece.cornell.edu/land/courses/ece4760/keypadRW88/keypad.html

 

It is common for the R/W pin to be tied to GND when not in use (to save MCU pins), which means you can only write data to the LCD.

If you wish to read and write data to the LCD, you can connect the R/W to a MCU pin... Now you can set LOW to write data, HIGH to read data.