Till today I haven't asked anyone to write actual code on whiteboard. Recently I had a chance to interview a Stanford grad who had previously worked on embedded sys in some start-up. I asked him to draw system level architecture of his product in OSI layer format like where HAL/ firmware, RTOS, insys mem, db, application, UI are placed and how they are communicating with each other ...and this guy was staring at me like I am asking him to jump down from 10th floor. Then I thought did I do something wrong... I mean if you have really worked on/experienced something then you should know that thing thoroughly and then you don't need any special prep for interviews