  The CMS Electromagnetic Calorimeter (ECAL) is a high resolution crystal
calorimeter operating at the CERN LHC. It is responsible for the identification
and precise reconstruction of electrons and photons in CMS, which were crucial
in the discovery and subsequent characterization of the Higgs boson. It also
contributes to the reconstruction of tau leptons, jets, and calorimeter energy
sums, which are vital components of many CMS physics analyses. The ECAL trigger
system employs fast digital signal processing algorithms to precisely measure
the energy and timing information of ECAL energy deposits recorded during LHC
collisions. These trigger primitives are transmitted to the Level-1 trigger
system at the LHC collisions rate of 40 MHz. These energy deposits are then
combined with information from other CMS sub-detectors to determine whether the
event should trigger the readout of the data from CMS to permanent storage.
This presentation will summarize the ECAL trigger performance achieved during
LHC Run 2 (2015-2018). It will describe the methods that are used to provide
frequent calibrations of the ECAL trigger primitives during LHC operation.
These are needed to account for radiation-induced changes in crystal and
photodetector response, and to minimize the spurious triggering on direct
signals in the photodetectors used in the barrel region ($\eta<$1.48). Both of
these effects are increased relative to LHC Run 1 (2009-2012), due to the
higher luminosities experienced in Run 2. Further improvements in the energy
and time reconstruction of the CMS ECAL trigger primitives are being explored
for LHC Run 3 (2021-23). The main features of these improved algorithms will be
described and preliminary estimates of the potential performance gains will be
given.
