Simultaneous process self-calibration method using TDC for 3D DDR4 DRAM

Abstract

Three-dimensional (3D) dynamic random-access memory (DRAM) with TSVs has been proposed due to continuous demands for low-power and high-density memory without IO loading limitation. However, the process difference among the stacked dies causes the timing mismatch of internal signals. To remove signal confliction and reduce signal skews among the stacked dies, the simultaneous process self-calibration scheme is proposed. The stacked dies using the proposed scheme detect the slowest signal among the stacked dies and internal signals are aligned with the slowest signal at the same time. The time for aligned operation is within one read loop and the scheme is turned off after calibration to reduce additional standby current. The 3D double-data rate 4 (DDR4) DRAM using the proposed scheme is operated over 2133 Mbit/s at 1.2 V.