A 7Gb/s/link non-contact memory module for multi-drop bus system using energy-equipartitioned coupled transmission line

Abstract

As computing power and speed increases, the demand for higher memory bandwidth increases as well. Recently, the memory interface has been improved up to 20Gb/s/link [1]. Considering PCB routing area, a multi-drop bus architecture is still preferable for large memory capacity to the point-to-point connection. However, the multi-drop approach suffers from performance degradations due to reflections at each stub. To mitigate this problem, reference [2] proposes an impedance-matched bidirectional multi-drop DQ bus architecture that is difficult to realize due to smaller series resistor if more than 4 modules are used. To avoid multi-reflections from each stub, other approaches have used coupled transmission lines (CTL) [3, 4]. While a horizontal directional coupler buried in the PCB was used [3], coupled traces on the bent loop of flex fixed to a module were used for signal delivery in vertical direction [4]. In [3], a long coupler with long main bus line was used so that the signal integrity degrades at the far-end coupler. In [4], the coupling traces on the motherboard and the flex have zigzag geometries for better misalignment tolerance, which result in large area of routing due to the minimum required pitch between traces. In [5], the CTL needs to be placed close to a Tx/Rx chip, as there are no extended transmission lines for signal lead. Therefore, it cannot be used for memory modules. DRAM multi-drop bus interface technology mapping is described in Fig. 2.8.1.