The Phase-II upgrades will prepare the ATLAS experiment for the High Luminosity LHC (HL-LHC), planned to begin operation in 2026. The HL-LHC is expected to deliver more than ten times the integrated luminosity of LHC Runs 1-3 combined. To achieve this in a reasonable amount of time, an increase in instantaneous luminosity corresponding to up to 200 simultaneous $pp$ interactions per bunch crossing is required. This large luminosisty increase presents significant challenges to the detector, trigger, and data acquisition systems in the form of increased trigger rates and detector occupancy. The results from the tests with beam performed at CERN, as well as the latest results on the development of the on- and off-detector electronics, firmware, data processing, and simulation components of the Tile Calorimeter Demonstrator readout system are presented.