This article summarizes the current implementation of the time-of-flight (TOF) energy analysis for plasma potential measurements by the heavy ion beam diagnostic of the tokamak ISTTOK. The technique beam modulation, TOF energy analyzer (TOFEA), and data acquisition system, the calibration procedures, the method used to obtain the plasma potential, and the resolution of the measurements are described in detail. The main TOFEA advantage is the possibility to realize simultaneous multichannel detection with a multiple cell array detector. The first measurements of the plasma potential profile obtained with a four-channel TOFEA are presented.