Phase-1 ATLAS Level-1 Trigger in Run-3

The ATLAS Level-1 Trigger, crucial for the selection of LHC events at CERN, has been upgraded for Run-3 with the state-of-the-art processors and electronic devices, extensively using optical links to enhance performance. The Calorimeter Trigger system, utilizing 10-fold higher granularity, enhances object identification alongside a redesigned Topological Processor for detailed angular analysis. Notably, the Muon Trigger system, with input from the newly installed muon sub-detector (New Small Wheel), improves Level-1 Trigger background rejection. Trigger Decision and Clock distribution were enhanced with new single-board modules. The Central Trigger system includes a System-on-Chip for the new Muon Interface, running control and monitoring applications directly on the hardware. This overview highlights the impact on the performance of the upgraded system, particularly under Run-3 high-intensity conditions.