23.3 A 3-bit/2UI 27Gb/s PAM-3 Single-Ended Transceiver Using One-Tap DFE for Next-Generation Memory Interface

Bandwidths of memory interfaces have been increased tremendously to enable high-data throughput while maintaining single-ended signaling and the supply voltage of I/O has been scaled down. Due to the increasing interface bandwidth the required area and power consumption has increased as well, resulting in higher I/O circuit design costs [3]. A high-loss channel causes ISI, which in turn limits the maximum data rate. Therefore, complex equalizers are needed for compensation, resulting in additional power dissipation and area overhead. As the data sampling rate increases, the deterministic and random noises degrade the data sampling margin and further limit the bandwidth. To lessen the negative impact of high channel loss and to reduce the forwarded clock frequency, multi-level signaling, such as PAM-4, can be used, as shown in Fig. 23.3.1 [2]. While the voltage sense margin for PAM-4 is theoretically $\frac{1}{3}$ of NRZ, in practice it is smaller due to simultaneous switching noise (SSN), crosstalk, and random noise in single-ended signaling. Eventually, the reduced voltage sense margin degrades the SNR, which causes a reduction in the BER. On the other hand, PAM-3’s voltage sense margin is ${\textstyle \frac {1}{2}}$ of NRZ’s. Duo-binary signaling is commonly used for PAM-3 signaling [1]. However, the pin efficiency and the forwarded clock frequency for duo-binary signaling is the same as for NRZ. In this paper, a 3b/2UI PAM-3 single-ended memory interface is proposed, with a pin efficiency of 150% and a reduced clock frequency, compared to NRZ signaling. To address PAM-3 equalizer inefficiencies a tri-level decision feedback equalizer (DFE) is implemented in the receiver (RX).