Abstract
The ALICE experiment at the LHC is equipped with an electromagnetic calorimeter (EMCal) designed to enhance its capabilities for jet measurement. In addition, the EMCal enables triggering on high energy jets. Based on the previous development made for the Photon Spectrometer (PHOS) level-0 trigger, a specific electronic upgrade was designed in order to allow fast triggering on high energy jets (level-1). This development was made possible by using the latest generation of FPGAs which can deal with the instantaneous incoming data rate of 26 Gbit/s and process it in less than 4 μs.
Highlights
After digitization, each fastOR is digitally integrated over a time sliding window of 4 samples
For computing two kinds of L1 triggers: the L1gamma trigger which uses the same patch size as L0, but without the inefficiencies displayed by the L0 (i.e. 2 × 2 patches across several TRU regions can be computed), and the L1-jet trigger, which is built by summing energy over a sliding window of 2 × 2 subregions (1 subregion = 4x4 fastOR = 8x8 towers)
Pair usage is as follows: one pair is dedicated for the LHC reference clock transfer to the TRU, another is used by the TRUs to forward their local L0 candidates and the 2 remaining are used for synchronous serial data transfer without any encoding
Summary
Each fastOR is digitally integrated over a time sliding window of 4 samples. The results of these operations are constantly fed to 2 × 2 spatial sum processors that compute the energy deposit in patches of 4 × 4 towers (or 2 × 2 fastOR), see fig. 2. Each patch energy is constantly compared to a minimum bias threshold; whenever it is crossed and the maximum of the peak has been found, a local L0 trigger is fired. In preparation for the L1 algorithm, the time integrated sums are stored in a circular buffer for later retrieval
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