LCD 16 x 2 1602A Interfacing
According to the table shown here the display controller is ST7066U or compatible.

According to ST7066U


  The ST7066U has pin function compatibility with the HD44780, KS0066 and SED1278 that allows the user to easily replace it with an ST7066U


So you can either refer to the appropriate datasheet or search for HD44780 LCD usage info or AVR libraries. As an example you can check http://homepage.hispeed.ch/peterfleury/avr-lcd44780.html which provided interface info and includes a link to a free HD44780 LCD library.