The Electromagnetic Calorimeter (ECAL) of the Compact Muon Solenoid (CMS) experimentwill be upgraded for the High-Luminosity phase of the LHC (HL-LHC) to cope with the new,harsher conditions, in terms of luminosity and pileup. The current lead tungstate crystals andavalanche photodiode detectors in the barrel region of the ECAL will remain, while the front-endand off-detector read-out electronics of the calorimeter will be upgraded. The new electronicswill have to fulfill the requirements of the upgraded Level 1 hardware trigger system, in termsof increased latency and data bandwidth, in order to preserve detector performance despite theincreased instantaneous luminosity. The upgrade will provide single crystal granularity for thehardware trigger and will enable a full read out of the detector. A crucial characteristic of the newdesign will be the ability to measure the time of electrons and photons with high precision, of theorder of 30 ps for energies above 100 GeV. This excellent time resolution will improve the overallCMS physics performance under the expected pile-up conditions. The status of the ongoing R&D activities is presented, together with the latest beam test results with prototypes.