Level-1 Triggering on High-Granularity Calorimeter information at the HL-LHC

The CMS collaboration is building the high granularity calorimeter (HGCAL) for the endcap regions as part of its planned upgrade for the High-Luminosity LHC. The calorimeter data will form part of the Level-1 trigger (hardware) of the CMS experiment, reducing the event rate from the nominal 40 MHz to up to 750 kHz with a decision time (latency) of 10 microseconds. Together with basic tracking information, which will also be available at Level-1 trigger, this facilitates particle-flow techniques to be used as part of this trigger. Around 1 million "trigger channels" are read at 40 MHz from the HGCAL, presenting a significant challenge in terms of data manipulation and processing for the trigger system the trigger data volumes will be an order of magnitude above those currently handled at CMS. In addition, the high luminosity will result in an average of 140 (or more) interactions per bunch crossing that give a huge background rate in the forward region and these will need to be efficiently rejected by the trigger algorithms. Furthermore, reconstruction of the particle clusters to be used for particle flow in events with high hit-rates is also a complex computational problem for the trigger. We present the status of the trigger architecture and design, as well as the concepts for the algorithms needed in order to tackle these major issues.