Speaker
Description
Summary
The ATLAS first-level calorimeter trigger (L1Calo) is a
hardware-based system with a high degree of adaptability
provided by widespread use of FPGAs. The real-time path of the
trigger is subdivided into a preprocessor (PPr) which
takes analogue signals from the calorimeters and digitises them,
followed by two digital processor systems working in parallel:
the jet/energy-sum processor (JEP) and the cluster processor
(CP). This contribution discusses the digital processor systems
and their commissioning: the PPr is discussed in a separate
contribution.
The input to both CP and JEP systems are 8-bit digitised
transverse energy deposits covering either 0.1x0.1 (CP) or
0.2x0.2 (JEP) in eta-phi space. These signals are transmitted
to the CP and JEP systems over high-speed serial LVDS cables.
The CP and JEP systems are each comprised of two parts: a
parallel set of high-density processor modules - the CPMs and
JEMs respectively - and a common merger module (CMM) which
collates results and sends them to the ATLAS central trigger
processor (CTP), which in turn makes the overall first-level
trigger decision. Both CP and JEP systems take events at the
bunch-crossing rate of 40 MHz, and are heavily pipelined.
The CP and JEP processors are allocated about 400ns processing
time (in parallel), as their share of the overall trigger
latency. The CP occupies four 9U VME crates, the JEP two.
One of the principal challenges of the processor systems is
the connectivity over the large eta-phi array of trigger towers,
required in order to avoid double-counting of energy deposits.
The main component that ensures this is a dense custom backplane
with approximately 22 000 pins per crate.
The output from the CP system passed to the CTP consists of a
set of multiplicities of electromagnetic (EM) objects passing
flexible Et thresholds on shower energy as well as limits on
activity in surrounding cells, and in the hadronic samplings
behind the EM cells. A simple alteration of how hadronic
samplings are used also allows triggering on isolated
hadrons from tau decays. Co-ordinates and classifications
of these candidates are also made available to the second-level
trigger system via regions-of-interest (RoIs).
The output from the JEP system includes information about
hadronic jets passing Et thresholds, in a similar scheme to
that of the CP system. In addition, the total Et deposited
in the calorimetry is found, as well as a vector sum of
Et deposits; these are compared to thresholds and the
results are passed to the CTP.
Production of the modules for the full-scale calorimeter
trigger system has taken place during 2006-7, and installation
and commissioning of the full system is taking place during the
spring and summer of 2007. The installation of the processor
systems into the ATLAS counting rooms is outlined, together with
results of commissioning tests, and lessons learnt. Integration
with the CTP, ATLAS DAQ systems and higher-level trigger
systems is also in progress and will be discussed.
