Usually most of the companies which hires for the post of embedded openings. They will just check whether you are strong enough in C programming. Usually for interviews they'll ask some C concepts based on your experience. They'll give you some logic's which you need to implement using C programming. If you are core to this field, they will ask you about micro-controller architectures and its features like interrupts, timers, PWM, UART and some serial protocols, . etc. Anyway it depends on there requirement also.
Best of luck
Sijo