Problem in using Nios2

I'm using a DE2 board.I have read image data in sdram in order to display on screen.However the vga contoller works on 27.715MHz,it read data on that frequency.But nios2 system operated on 50MHz,so the sdram output data on 50MHz.That's the question:How can vga controller cooperate with nios2,that is to control the reading data rate to 27.715? Thanks!