The High Luminosity upgrade of the LHC at CERN will provide unprecedented instantaneous and integrated luminosities of around 5 × 10$^{34}$ cm$^{−2}$ s$^{−1}$ and 3000 fb$^{−1}$ , respectively, from 2025 to 2035. During this operational period, an average of 140 to 200 collisions per bunch-crossing (pile-up) is expected. In order to cope with these extreme pile-up conditions, increased data rates, and harsh environment, the Compact Muon Solenoid detector is undergoing a radical Phase II upgrade program. In the barrel region of the CMS electromagnetic calorimeter (ECAL) the entire readout and trigger electronics will be replaced. A dual gain trans-impedance amplifier and an ASIC providing two 160 MHz ADC channels, gain selection, and lossless data compression will be installed. The trigger decision will be moved off-detector and performed by powerful FPGAs, allowing for more sophisticated trigger algorithms to be applied. The upgraded ECAL will be capable of high-precision timing measurements, of the order of 30 ps, for photons and electrons above 50 GeV. The time resolution improvement will enhance the overall CMS physics performance by mitigating the high pile-up effects. The design of the full upgraded ECAL barrel readout chain and the status of the components R&D will be presented.