DRAM based refresh-free ternary CAM

Details DRAM based refresh-free ternary CAM DRAM based refresh-free ternary CAM

2000-06-09

2001-12-18

Le, Vu A. (Department: 2824)

Static information storage and retrieval

Read/write circuit

Data refresh

C365S049130, C365S149000

Reexamination Certificate

active

06331961

ABSTRACT:

FIELD OF INVENTION
The invention relates to content addressable memory (CAM), and more particularly to CAM based on dynamic random access memory (DRAM).
BACKGROUND OF INVENTION
For implementing CAM, DRAM offers compelling advantages over static random access memory (SRAM). DRAM is smaller in size than SRAM. Thus, on a given area, DRAM offers inherently a denser memory than what SRAM can offer. As such, more information can be stored using DRAM. Also, because the more integration on a chip the better the performance, DRAM improves performance. Moreover, DRAM inherently consumes less power than SRAM, making DRAM well suited for high density CAM, wherein power consumption is a critical issue. Further still, DRAM requires fewer transistors per cell when compared to SRAM. Thus, DRAM lowers manufacturing cost. In summary, compared to SRAM, DRAM offers higher density, higher performance, lower power consumption, and lower manufacturing cost.
However, DRAM necessitates refresh overhead that slows the speed performance of CAM. This speed penalty defeats the purpose of using CAM, which is to provide high-speed memory access. In fact, CAM is required precisely in time critical applications. Nevertheless, because of the very nature of DRAM, refresh cannot be avoided. The voltage leakage of the capacitors in DRAM cells demands that refresh be performed to preserve stored data.
Thus an impasse has been reached, wherein on the one hand DRAM-based CAM offers many distinct advantages compelling benefits over SRAM-based CAM, and wherein on the other hand speed penalty of DRAM-based CAM defeats the purpose of using a CAM for time critical applications. In view of this impasse, a need exists for taking advantages of DRAM in implementing CAM while not paying the speed penalty caused by DRAM refresh.
SUMMARY OF INVENTION
The invention is a system and a method for a content addressable memory (CAM) based on dynamic random access memory (DRAM). The invention does not use static random access memory (SRAM) to implement CAM. Rather, the invention is drawn to a DRAM-based CAM that offers the advantages of DRAM (over SRAM) without the speed penalty associated with DRAM refresh. In the invention, a refresh mechanism at the system level of the CAM makes the refreshing process transparent to other processes of the CAM system. The invention devotes an entire port to refresh; this is the port additional to the port to typical data and control functions. In so doing, the invention answers the need to take advantage of DRAM in a CAM while preserving the high speed performance demanded of any CAM.
Preferably, a CAM system includes a ternary state CAM cell constituted by two DRAM cells. A set of wire lines are devoted for refreshing the DRAM cells. In particular, a refresh word line (RWL) is coupled to both DRAM cells. This RWL's function is to refresh the DRAM cells making up the CAM cell. Two refresh bit lines (RBL's) are also implemented for refreshing these two DRAM cells. One of the RBL is coupled to one of the DRAM cell, while the other RBL is coupled to the other DRAM cell. Advantageously, the invention offers the benefits of DRAM for the CAM system while not slowing the speed performance of the CAM system.
In addition to a refresh mechanism, a swing line (SL) is implemented to regulate voltage swing in a local match line (LML) of the CAM system. Specifically, the SL is coupled to the both DRAM cells and the LML. Moreover, voltage level of the SL can be adjusted. The SL has an adjustable voltage level for changing voltage swing in the LML to regulate trade-off between power consumption and speed of the CAM cell. Advantageously, the SL enables a user to regulate and fine-tune power consumption of the CAM system.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.


REFERENCES:
patent: 4672580 (1987-06-01), Yau et al.

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