TD-Link: A Daisy-Chain Optical Architecture for Integrated Data Readout and Deterministic Timing Distribution in Large-Scale Detector Systems

Abstract

TD-Link is an optical communication architecture that combines high-throughput data readout and sub-nanosecond timing synchronization over a single fiber for large-scale detector systems. It adopts a multidrop daisy-chain ring topology connecting a Data Concentrator to up to sixteen FERS front-end boards per link, with up to eight links per concentrator. Operating at 3.125 Gb/s, TD-Link carries data, synchronization, and control traffic in the same serial stream through a token-based streaming protocol that minimizes per-hop latency and supports on-the-fly payload fragmentation. Deterministic latency is enforced at two levels. On the concentrator, transmitter lanes are aligned by using the half-full flag of the multi-gigabit transceiver (MGT) elastic buffer as a one-bit phase detector: a firmware state machine steps the transmit phase interpolator (PI) until the buffer pointer difference crosses half-depth, locking every lane to the same phase condition. On each front-end board, where the synchronization pulse must also cross from the recovered-clock to the transmit domain, the residual three-to-five-cycle ambiguity of the synchronizer is removed by a calibration-free multiphase oversampling time-to-digital converter (TDC) with 0.8 ns bins, which time-stamps the incoming edge on the transmit time base so that a scheduler regenerates it at a fixed, programmable latency. A Digital Dual Mixer Time Difference (DDMTD) circuit measures the inter-concentrator phase. Tests with CERN PicoTDC-equipped FERS boards show a board-to-board synchronization $\sigma$ of 24 ps on the same ring, 27 ps across transceiver quads, and 28 ps across two cascaded concentrators, against an intrinsic front-end floor of 7 ps. Over more than one thousand cold boots without re-calibration, the boot-to-boot spread of the board-to-board skew stayed below 20 ps, consistent with the 18.6 ps phase-interpolator step.

re-submitted to IEEE Transactions on Nuclear Science (IEEE Real Time Conference 2026) after the first round of peer review