Commissioning the ATLAS Level-1 Central Trigger System

The ATLAS Level-1 Central Trigger (L1CT) electronics is a central part of ATLAS data-taking. It receives the 40 MHz bunch clock from the LHC machine and distributes it to all sub-detectors. It initiates the detector read-out by forming the Level-1 Accept decision, which is based on information from the calorimeter and muon trigger processors, plus a variety of additional trigger inputs from detectors in the forward regions. The L1CT also provides trigger-summary information to the data acquisition and the Level-2 trigger systems for use in higher levels of the selection process, in offline analysis, and for monitoring. In this paper we present the completion of the installed L1CT system, its performance during cosmic-ray data taking with the full ATLAS detector, and the experience with first beam in the LHC. Particular emphasis is given to results from timing-in of the trigger signals, synchronization of the detector read-out systems, synchronization of the trigger with beam, and coherent configuration, operation and monitoring of the system.