The CMS experiment requires various technical upgrades to sustain its performance whilst
coping with a harsher radiation environment and having to resolve a greater number of
proton-proton interactions per beam cross-over, both brought by the increased luminosity
projected for the high-luminosity LHC. One key upgrade is replacing the existing endcap
calorimeter with a novel device, the High Granularity Calorimeter (HGCAL). This thesis
will focus on two mutually related aspects considering the design and implementation of
its electronics system.

The first aspect is about a test system that prototypes a full vertical slice (start-to-end) of
the detector electronics chain. As the final system, this includes both on-detector (front-
end) and off-detector (back-end) components, as well as trigger and DAQ sub-systems.
Key operational aspects including fast control, timing distribution, slow control, trigger
and DAQ were activated and tested. This allowed operating the system in a beam test,
measuring a data transfer bit error rate upper limit of O(1E−16), and evaluating the
digital agreement between the different devices in the chain. These results mark the
first-ever implementation of an HGCAL readout system prototype of such complexity.

The second aspect involves the design and FPGA implementation of two back-end al-
gorithms, including simulations to estimate their physics performance. The first algorithm
emulates front-end readout chip buffers and occasionally throttles CMS Level-1 trigger
decisions to prevent data loss due to buffering limitations. An analysis showing this
algorithm results in approximately 1.5E−8 deadtime will be presented. The second
algorithm calculates 3D cluster properties (energy, position, shape) from the front-end
energy deposits for use in the CMS Level-1 trigger. It will be shown that this algorithm
has excellent and very good ability to identify photons and Vector Boson Fusion jets,
respectively. Both blocks were implemented in software and firmware, and successfully
tested in hardware.