The Compact Muon Solenoid detector was originally designed to operate for about ten years, for LHC instantaneous luminosity up to $1 \cdot 10^{34}$ $cm^{-2}$ $s^{-1}$ and integrated luminosity of 500 fb$^{-1}$. The High Luminosity LHC will increase the instantaneous luminosity by about a factor of 5 from current levels and CMS will accumulate an integrated luminosity of 3000 fb$^{-1}$ by about 2035. With such high luminosity the electromagnetic calorimeter of CMS will have to cope with a challenging increase in the number of interactions per bunch crossing and in radiation levels. The front-end readout electronics will be completely redesigned, with the goals of providing precision timing, low noise and added flexibility in the trigger system. It will use a faster pre-amplifier, increase the sampling frequency from 40 MS/s to 160 MS/s and implement a trigger system that resides entirely off-detector.