Autonomous Dual-Mode (PAM2/4) Serial Link Transceiver with Adaptive Equalization and Data Recovery

Abstract—This paper describes an adaptively equalized, dualmode (PAM2 one-tap DFE/PAM4) 0.13 m CMOS transceiver chip, and the techniques used to continuously adapt the link. Interestingly, with only minor modification the same hardware needed to implement a PAM4 system can be used to implement a PAM2 loop-unrolled single-tap decision-feedback equalization (DFE) receiver. Adaptive equalization using data-based update filtering allows continuous updates while minimizing the required sampler front-end hardware and significantly reduces the cost of implementation in multi-level signaling schemes. To allow the transmitter to adapt to the channel, the link uses common-mode signaling to create a back-channel communication path over the existing pair of wires. The design uses a three-level return-to-null signaling scheme which allows the receiver to simultaneously extract voltage and timing references and minimize the required receiver hardware. The measured results indicate that this back-channel achieves reliable communication without noticeable impact on the forward link for back-channel data rates of up to 16 Mb/s and swings of 20–100 mV. Index Terms—Adaptive equalization, back-channel, commonmode, data recovery, decision-feedback equalization (DFE), link.