The ATLAS Level-1 Calorimeter Trigger is one of the main elements of the first stage of event selection for the ATLAS experiment at the LHC. It consists of over 300 custom built VME modules, of several different types. The full installation of this system has been in operation since the start of 2008, during which time it has undergone intensive testing, both in standalone mode, and in conjunction with the ATLAS detector in combined running. During this time, the performance of the system has been investigated, problems understood and fixed, and configuration parameters tuned to continuously improve the quality of the trigger decisions made.
An overview of the calorimeter trigger hardware will be presented, along with some analysis of its performance during this first year of running, mostly with cosmic rays, but also, briefly, with beam in LHC.