Abstract: To meet the control requirement of the amplitude, phase, startup logic, interlock and online parameter modification of cyclotron, a general DSP firmware based on finite state machine was proposed. The firmware is divided into 5 states, and they are blockading RF signal, searching cavity resonance, enhancing power, phase match-ing, and phase closed-loop control. The firmware communicates with the local and remote control machines through the serial port and modifies the parameter of the RF system online. It adopts asynchronous event drive method to achieve fast response to spark and reflection. The firmware is successfully applied in the CYCIAE-100 cyclotron low level RF system. In the CYCIAE-100 cyclotron beam commission process, the firmware solves the problem of starting up and ensures the safety of the equipment. The firmware is stable and reliable and meets the requirement of accelerator RF system.