Like the 6800 and 6502, the 6809 uses a two-phase clock to gate the latches. This two-phase clock cycle is used as a full machine cycle in these processors. Simple instructions can execute in as little as two or three such cycles. The 6809 has an internal two-phase clock generator (needing only an external crystal) whereas the 6809E needs an external clock generator. There are variants such as the 68A09(E) and 68B09(E); the internal letter indicates the processor's rated clock speed.