Time Digitization firmware for the new Drift Tubes electronics for HL-LHC

The electronics of the CMS Drift Tube (DT) chambers will need to be replaced in order to cope with the increased occupancy and rates at the high luminosity phase in the HL LHC (High-Luminosity Large Hadron Collider). A new electronic system is being designed, that will allow forwarding signals from all DT chambers at the maximum resolution to the backend system, where improved trigger primitives generation will take place. The on-detector boards, which will be attached to the DT chambers inside the CMS wheels are called OBDT (On Board electronics for Drift Tubes) and are built around a Microsemi Polarfire FPGA that performs the time digitization of the chamber signals at $\sim$~1~ns resolution, and forwards them to a high-speed link, optimizing bandwidth and latency. Thirteen prototypes of this board have been installed in parallel to the present system in the CMS detector, fully instrumenting one out of sixty sectors, and are taking data routinely integrated in the CMS DAQ chain. The firmware implementation of this FPGA performs the time digitization of up to 240 input channels, the high-speed link implementation and several slow control functionalities, including a very thorough clock and reset signals distribution that allows ensuring that the time measurements are stable across power cycles and reconfigurations of all the CMS clock chain. The description and implementation details, together with the results of the performance of this firmware in the data taking campaigns at CMS are presented in this contribution.