The CERN Proton Synchrotron (PS) is equipped with high frequency RF systems at 20 MHz, 40 MHz and 80 MHz. These are used to perform longitudinal beam manipulations in order to provide short bunches with 25 ns spacing to the Super Proton Synchrotron (SPS). As part of the "LHC Injectors Upgrade" (LIU) project, a new digital low level RF (LLRF) firmware has been developed for the high frequency cavities. The firmware combines an automatic voltage control (AVC) loop and a multi-harmonic feedback (MHFB) into an integrated cavity controller. The AVC loop and internal RF source provide pulse voltage regulation and phase control, while the MHFB reduces the cavity impedance throughout the acceleration cycle. The controller is implemented on the pre-existing PS one turn delay feedback board, built around the Altera Stratix II FPGA. Due to the modest size of this FPGA, a multiplexed CORDIC and complex multi-harmonic source are used to optimise the amount of FPGA resources required. Beam tests of the AVC loop and commissioning results of the fully integrated controller are presented.