A 32Gb/s Digital-Intensive Single-Ended PAM-4 Transceiver for High-Speed Memory Interfaces Featuring a 2-Tap Time-Based Decision Feedback Equalizer and an In-Situ Channel-Loss Monitor

Single-ended transceivers that can deliver high-data rates at reduced supply voltages are required to meet the ever-growing demands of future memory interfaces. The performance of conventional non-return-to-zero (NRZ) links is usually limited by inter-symbol-interference (ISI) noise caused by high channel losses. Alternative schemes such as duobinary [1], three or four level pulse amplitude modulation (PAM-3, PAM-4) [2], and multi-band signaling [3] were proposed to increase bandwidth efficiency. In particular, PAM-4 signaling utilizes four signal levels to send 2b per unit interval, at the expense of complex TX and RX circuits resulting in higher power consumption and larger chip area. While this approach has been gaining popularity for ultra-high speed (>50Gb/s) links, a more compact implementation is needed for memory interface applications. In this paper, we propose a digital-intensive PAM-4 receiver targeted at memory interfaces; time-based circuits are used for the decision feedback equalization (DFE). Unlike traditional current-mode logic, time-based circuits can be realized using inverters and programmable loads, making them ideally-suited for low-voltage energy-efficient memory interfaces.