The digital algorithm processors for the ATLAS Level-1 Calorimeter Trigger

The ATLAS Level-1 Calorimeter Trigger identifies high-E_T jets, electrons/photons and hadrons, and measures total and missing transverse energy in proton-proton collisions at the Large Hadron Collider. Two subsystems-the Jet/Energy-sum Processor (JEP) and the Cluster Processor (CP)-process data from every crossing, and report feature multiplicities and energy sums to the ATLAS Central Trigger Processor, which produces a Level-1 Accept decision. Locations and types of identified features are read out to the Level-2 Trigger as regions-of-interest, and quality-monitoring information is read out to the ATLAS data acquisition system. The JEP and CP subsystems share a great deal of common infrastructure, including a custom backplane, several common hardware modules, and readout hardware. Some of the common modules use FPGAs with selectable firmware configurations based on their location in the system. This approach saved substantial development effort and provided a uniform model for firmware and software development. We present an in-depth description of the two subsystems as manufactured and installed. We compare and contrast the JEP and CP systems, and discuss experiences during production, installation and commissioning. We also briefly present results of recent tests that suggest an interesting upgrade path for higher luminosity running at the LHC.