Ensuring clock phase repeatability by preventing loss of the 40.078 MHz clock in time-critical detectors: a non-disruptive clock switching approach

Abstract For the High Luminosity phase of the LHC (HL-LHC), the CMS detector electronics require precise and accurate timing to discriminate pileup events. The upgraded Back-End electronics of the Barrel Electromagnetic Calorimeter supply the required high-precision clock (Time Interval Error σ < 5 ps) to the Front-End. However, after a reset is applied on the timing distribution system, the resulting phase of the clock is not accurately repetitive but it gives an up to 10 ps offset for each FPGA node of the timing distribution chain. That is resulting to the uncertainty on the total clock skew with the maximum being N x 10 ps between neighbor channels of the system (where N is the number of FPGA-nodes of the subdetector branch). We have studied a solution minimizing the need to reset the timing distribution system by switching on-the-fly and seamlessly the source clock at the top of the tree to a local LHC-frequency clock. This approach prevents the loss of the links towards the on-detector electronics ensuring clock phase repeatability and offers stability to the on-detectors electronics.