A 25.6-Gb/s Interface Employing PAM-4-Based Four-Channel Multiplexing and Cascaded Clock and Data Recovery Circuits in Ring Topology for High-Bandwidth and Large-Capacity Storage Systems Takashi Toi

Abstract

This article presents a pulse-amplitude modulation (PAM)-4-based 25.6-Gb/s serial interface for high-bandwidth (BW) and large-capacity storage systems consisting of NAND flash memory. A conventional interface with multi-drop bus topology between the NAND flash memories and their controller has an inevitable tradeoff between BW and capacity if we assume a reasonable PCB design in which the numbers of pins and wires near the NAND controller is limited. Although a daisy-chain-based interface can resolve this tradeoff, it requires the additional over- heads of bridge chips and procedures for distinguishing between bridge chips. In order to address these challenges, this article presents three key techniques: 1) ring topology; 2) PAM-4-based four-channel multiplexing; and 3) cascaded clock and data recovery (CDR) circuits with phase-error-dependent bang-bang phase detector (PED-BBPD). The fabricated transceiver for the proposed interface using a 28-nm CMOS process achieves energy efficiency of 3.69 pJ/b at 25.6-Gb/s PRBS31 with a bit error r a t e( B E R )o fl e s st h a n1 0 −15 through a short channel with 1.84-dB loss. The proposed interface mitigates the overhead of the bridge chips with higher data rate than previous works, and it can achieve a state-of-the-art figure of merit of 1.80 PKG Gb/s/mW, defined as “No. of NAND packages (PKGs) × data rate/power consumption,” with a controller and four bridge chips.