Commissioning of the ATLAS level1 endcap muon trigger system

The ATLAS level1 endcap muon trigger system at the LHC selects interesting events containing muons with transverse momenta greater than 6 GeV/c from 40 MHz proton-proton collisions. This system consists of 3588 Thin Gap Chambers (TGC) and the total number of readout channels is 3.18x10^5. The trigger logic is based on the coincidence between 7 layers of TGC. All processes are performed on fast electronics within 2.5 μs. To be ready for the first beam collisions scheduled in 2009, we have succeeded in sending trigger signals of cosmic ray muons with the synchronous operation at 40 MHz and fine signal timing alignments. During the single proton beam circulation in the LHC in 2008, we have also confirmed that the timing of the endcap muon trigger for tracks of beam halo muons coming from the beam pipe was well aligned for beams. We report on status of the commissioning and results from runs with cosmic ray muons and single beams.