Implementation of the ALICE Trigger System

The ALICE trigger system consists of the Central Trigger Processor (CTP) and 24 Local Trigger Units (LTU) that act as a uniform interface to sub-detector front-end electronics. The CTP generates three levels of hierarchical hardware triggers - L0, L1 and L2. At any time, the 24 sub-detectors of the ALICE experiment are dynamically partitioned into up to 6 independent clusters. Trigger selection includes the past-future protection a fully programmable hardware mechanism of controlling the event pile-up. The system contains a number of options that enhance the testability: the SnapShot memories both emulate logic inputs and monitor the logic operation - they enable on-line and in situ detection of system malfunction; more than 1200 signal counters, with simple on-line access, monitor the system performance and check the consistency; the ScopeProbe option enables a direct oscilloscope access to the system inputs, outputs and relevant internal signals. Some of the "common" CTP functions - down-scaling of trigger classes, synchronization of trigger inputs, etc. - have been realized in a way that is economical on logic resources and offers performance benefits. The implementation of those options using the FPGAs is the main topic of the presentation.