This paper presents a BIST architecture for Finite State Machines that exploits Cellular Automata (CA) as pattern generators and signature analyzers. The main advantage of the proposed approach, called C2 BIST (Circular Cellular BIST) is that the same CA is used for generation and compaction, thus lowering substantially the area requirements. The configuration of the CA rules is performed through a Genetic Algorithm, that is shown to provide good results both in terms of fault coverage and number of reconfigurations. In many cases, no reconfiguration is necessary, and the corresponding area occupation is competitive with current BIST approaches.