The new readout system for the Alice Zero Degree Calorimeters in LHC Run 3.

The Alice Zero Degree Calorimeters (ZDC) provide information on centrality, time of the collision, vertex position, event plane and luminosity in heavy-ion operation. The operating conditions for the ZDC in LHC Run 3 will be very challenging in heavy-ion collisions, due to the presence of a physical background from electromagnetic dissociation processes with a resulting event rate that could reach $5 M event/s.$ The new readout system, that was redesigned in order to operate in continuous mode without dead time, is based on a commercial 12-bit digitizer with a sampling rate of about $1 {GSps}$, assembled on an FPGA Mezzanine Card. The signals produced by the 26 ZDC channels are digitized and the samples are processed through an FPGA where an auto trigger algorithm is implemented, allowing the online selection of the interesting physical events and data reduction directly at the front end level. Information as timing, baseline average estimation and luminosity measurements are also extracted. The architecture of the new readout system, the operation of the digitizer, the auto trigger strategy, the firmware organization and performance during the commissioning are presented.