The ATLAS Level-1 trigger timing setup

The ATLAS detector at CERN's LHC will be exposed to proton-proton collisions at a bunch-crossing rate of 40 MHz. In order to reduce the data rate, a three-level trigger system selects potentially interesting physics processes. The first trigger level is implemented in electronics and firmware. It aims at reducing the output rate to less than 100 kHz. The central trigger processor (CTP) combines information from the calorimeter and muon trigger processors and makes the final Level-1-Accept (L1A) decision. It is a central element in the timing setup of the experiment. Three aspects are considered in this article: the timing setup with respect to the Level-1 trigger, with respect to the experiment, and with respect to the world. Trigger signals from the muon and calorimeter trigger processors have to be synchronized in phase with respect to the local clock, and aligned in terms of the bunch crossing they originate from. The Level-1 latency is defined as the time between the collision and the arrival of the L1A at the sub-detectors. It is fixed and less than 2.5 musec. During this time, the data from all sub-detectors are stored in front-end pipeline buffers. In order to guarantee read-out of the same collision, the pipeline lengths must be carefully tuned in order to match the Level-1 latency using several strategies with and without particle beam. The CTP further calculates a UTC time stamp derived from a GPS-based time-stamping system with a stability of 5 nsec and high absolute time precision. The time stamp will allow us to correlate ATLAS events with those in other particle-physics or astronomic detectors at CERN or elsewhere