The High Luminosity Large Hadron Collider (HL-LHC) will deliver five times the LHC nominal instantaneous luminosity, after a series of upgrades that will take place during the shutdown of 2024 –2026. The ATLAS Hadronic Calorimeter (TileCal) will require the complete replacement of the readout electronics in order to accommodate its acquisition system to the increased radiation levels, trigger rates, and high pile-up conditions during the HL-LHC era.The upgraded readout electronics will digitize the PMT signals from every TileCal cell for every bunch crossing and will transmit them directly to the off-detector electronics. In the counting rooms, the off-detector electronics will store the calorimeter signals in pipelined buffers while transmitting reconstructed trigger objects to the first level of trigger at 40 MHz.The TileCal upgrade project has undergone an extensive R&D program and several test beam campaigns. A Demonstrator module has been assembled using the upgraded on-detector electronics and tested in the ATLAS experiment. The Demonstrator module is operated and read out using a prototype of Tile PreProcessor (TilePPr) which also permits integrating the Demonstrator module into the present ATLAS TDAQ system. This contribution presents the status and performance of the Demonstrator module in the ATLAS experiment.