Commissioning of the ALICE-PHOS trigger

The Photon Spectrometer (PHOS) of the ALICE experiment at LHC is a high-resolution electromagnetic calorimeter dedicated to the precise measurement of direct photon and neutral meson yields in p+p and Pb+Pb collisions at mid-rapidity in a pT range up to 100 GeV/c. A multi-level trigger system selects events of interest and reduces the overall data flow. PHOS generates two triggers: i) the Level 0 (L0) trigger for high luminosity p+p runs acting mainly as a spectrometer trigger and for selecting high pT photons in Pb+Pb and in p+p collisions; ii) the Level1 (L1) trigger corresponds to a more sophisticated shower analysis which refines the pT selection. The algorithms perform a pulse-shape analysis of the oversampled Avalanche Photo Diode (APD) signal followed by a sliding-window cluster finder. Both triggers are implemented in the firmware which, since SRAM-based FPGAs are used, can be changed on-line between runs. The trigger system was commissioned by cosmic rays and particles from interactions. The status of the trigger system and first results will be presented.