  The ATLAS detector at CERN's Large Hadron Collider will be exposed to
proton-proton collisions from beams crossing at 40 MHz. A three-level trigger
system was designed to select potentially interesting events and reduce the
incoming rate to 100-200 Hz. The first trigger level (LVL1) is implemented in
custom-built electronics, the second and third trigger levels are realized in
software. Based on calorimeter information and hits in dedicated muon-trigger
detectors, the LVL1 decision is made by the central-trigger processor yielding
an output rate of less than 100 kHz. The allowed latency for the trigger
decision at this stage is less than 2.5 microseconds. Installation of the final
LVL1 trigger system at the ATLAS site is in full swing, to be completed later
this year. We present a status report of the main components of the first-level
trigger and the in-situ commissioning of the full trigger chain with cosmic-ray
muons.
