The digital LLRF controller based on embedded microcontroller for the booster of the Dubna superconducting accelerator complex NICA is presented. It allows setting the frequency in accordance with the magnetic field value in the 0.5-5 MHz range with an inaccuracy +/- 110 Hz. A novel method of frequency calculation based on the derivative is proposed. The specialized tester module allows tuning up and checking the RF system during comissioning and operation.