The CMS muon system is planning an upgrade, which is necessary to maintain the high level of performance in the high luminosity phase of the LHC. One of the improvements is the installation of an additional set of muon detectors - Gas Electron Multiplier (GEM) - in the first endcap muon station of the CMS. Recently a novel and complex chip - VFAT3 - for the readout of GEM detectors was designed within the CERN Microelectronics section. In this seminar I will present the architecture of the CMS-GEM readout system with a focus on the VFAT3 chip, including the main challenges, which include the low-power and low-noise front-end required to tolerate high range of detector capacitances, a wide dynamic range and reduction of the discriminator time-walk to less than five nanosecond. The digital back end of the chip includes a novel radiation-hard communication port and data formatting techniques resulting in a lossless compression of the data-stream, amongst other functionality. The entire design has been made radiation hard, for both, TID and SEE.