For the Phase-I luminosity upgrade of the LHC, a higher granularity trigger readout of the ATLAS LAr Calorimeters is foreseen in order to enhance the trigger feature extraction and background rejection. The new readout system digitizes the detector signals, grouped into 34000 so-called Super Cells, with 12 bit precision at 40 MHz and transfers the data on optical links to the digital processing system, which extracts the Super Cell energies. A demonstrator version of the complete system has now been installed and operated on the ATLAS detector. Results from the commissioning and performance measurements will be reported.