The CMS ECAL Upgrade for Precision Crystal Calorimetry at the HL-LHC

The Compact Muon Solenoid Experiment (CMS) is operating at the Large Hadron Collider (LHC) with proton-proton collisions at 13 TeV center-of-mass energy and at a bunch spacing of 25 ns. New further challenging running conditions for CMS are expected after the High-Luminosity upgrade of the LHC (HL--LHC). The CMS electromagnetic calorimeter (ECAL) will need to be upgraded to substain the hardned environment.The design and R\ and D studies for the ECAL upgrade are presented together with first test beam studies. Particular challenges at HL--LHC are the harsh radiation environment, the increasing data rates and the extreme level of pile-up events, with up to 200 simultaneous proton-proton collisions. Precision timing can be exploited to reduce the effect of the pile-up. Time resolution measurementscarried out during test-beams are shown. Plans are also shown for R\ and D for the new readout and trigger electronics, which must be upgraded due to the increased trigger and latency requirements at the HL--LHC