536
W. H. Smith
measurement. The efficiency of the trigger is evaluated with respect to benchmark
physics processes derived from the physics goals of the experiment. The criteria are
a sharp turn-on curve of the efficiency at its threshold and an asymptote as close to
100% as possible. The L1T thresholds should be somewhat smaller than the offline
physics analysis cuts. The reason for such a requirement is that the efficiency turnon curves for the L1T will be somewhat softer than can be achieved with a full
analysis including the best resolutions and calibration corrections.
Much of the logic in contemporary L1T systems is contained in custom
Application Specific Integrated Circuits (ASICs), semi-custom or gate-array ASICs,
Field Programmable Gate Arrays (FPGAs), Programmable Logic Devices (PLD), or
discrete logic such as Random-Access Memories (RAM) that are used for memory
Look-Up Tables (LUT). Given the remarkable progress in FPGA technology, both
in speed and number of gates, the technology of many trigger systems has mostly
moved towards full implementation in FPGAs.
The key to a good trigger system is flexibility. Not only should all thresholds be
programmable, but also as mentioned above, algorithms are either implemented in
FPGAs or LUTs. Reprogramming the FPGAs or downloading new LUT contents
allows for revisions of the trigger algorithms. The only fairly fixed aspect of the
trigger system is which data is brought to which point for processing. However,
this is determined by the detector elements, size of showers and curvature of tracks,
which are well known and basic features of the detectors and physics signals. There
are new technologies being developed that are expected to provide flexibility in
data routing, including backplanes and cards that use programmable cross-point
switches.
The L1T system sustains a large dataflow. This is either carried on optical fibres,
copper cables, or on backplanes within crates. At the LHC, the data carried by
these means may be sent in parallel at either 40 MHz, or a higher multiple of
this frequency, or converted from parallel to serial and transmitted at a higher rate
on a single lines or pair of lines. Serial data transmission has the advantage of
transmitting more data per cable wire or backplane pin but the disadvantage of
extra latency for the parallel to serial and serial to parallel operations plus the risk
of data errors involved with the encoding, high frequency transmission and link
synchronization. In many cases this requires the overhead of monitoring and error
detection bits. Copper cables in general avoid the necessity for optical drivers with
their cost, size and power requirements, but have limited length capability, take up
more volume and use more material.
12.1.2 L1T Requirements
The L1T has to be inclusive, local, measurably efficient, and fill the DAQ bandwidth
with a high purity stream. The local philosophy of the trigger implies an initial trigger selection of electrons, photons, muons and jets that relies on local information
tied directly to their distinctive signatures, rather than on global topologies. For
W. H. Smith
measurement. The efficiency of the trigger is evaluated with respect to benchmark
physics processes derived from the physics goals of the experiment. The criteria are
a sharp turn-on curve of the efficiency at its threshold and an asymptote as close to
100% as possible. The L1T thresholds should be somewhat smaller than the offline
physics analysis cuts. The reason for such a requirement is that the efficiency turnon curves for the L1T will be somewhat softer than can be achieved with a full
analysis including the best resolutions and calibration corrections.
Much of the logic in contemporary L1T systems is contained in custom
Application Specific Integrated Circuits (ASICs), semi-custom or gate-array ASICs,
Field Programmable Gate Arrays (FPGAs), Programmable Logic Devices (PLD), or
discrete logic such as Random-Access Memories (RAM) that are used for memory
Look-Up Tables (LUT). Given the remarkable progress in FPGA technology, both
in speed and number of gates, the technology of many trigger systems has mostly
moved towards full implementation in FPGAs.
The key to a good trigger system is flexibility. Not only should all thresholds be
programmable, but also as mentioned above, algorithms are either implemented in
FPGAs or LUTs. Reprogramming the FPGAs or downloading new LUT contents
allows for revisions of the trigger algorithms. The only fairly fixed aspect of the
trigger system is which data is brought to which point for processing. However,
this is determined by the detector elements, size of showers and curvature of tracks,
which are well known and basic features of the detectors and physics signals. There
are new technologies being developed that are expected to provide flexibility in
data routing, including backplanes and cards that use programmable cross-point
switches.
The L1T system sustains a large dataflow. This is either carried on optical fibres,
copper cables, or on backplanes within crates. At the LHC, the data carried by
these means may be sent in parallel at either 40 MHz, or a higher multiple of
this frequency, or converted from parallel to serial and transmitted at a higher rate
on a single lines or pair of lines. Serial data transmission has the advantage of
transmitting more data per cable wire or backplane pin but the disadvantage of
extra latency for the parallel to serial and serial to parallel operations plus the risk
of data errors involved with the encoding, high frequency transmission and link
synchronization. In many cases this requires the overhead of monitoring and error
detection bits. Copper cables in general avoid the necessity for optical drivers with
their cost, size and power requirements, but have limited length capability, take up
more volume and use more material.
12.1.2 L1T Requirements
The L1T has to be inclusive, local, measurably efficient, and fill the DAQ bandwidth
with a high purity stream. The local philosophy of the trigger implies an initial trigger selection of electrons, photons, muons and jets that relies on local information
tied directly to their distinctive signatures, rather than on global topologies. For