Structure and method of an encoded ternary content addressable memory (CAM) cell for low-power compare operation

[0002] 1. Field of the Invention

[0003] The present invention relates to the field of integrated circuits, and particularly to a content addressable memory (CAM).

[0004] 2. Description of Related Art

[0005] Networking companies are scrambling in a race to design and develop high performance network processing products for the terabit router market while reducing the cost to implement 10 giga-bits per second/OC192 and above optical carrier network interfaces. Terabit routers demand a fatter throughput of data packets for examining an incoming packet, retrieves a next hop location, and transfers the packet to destination. Among various designs and memory architectures, a ternary CAM is a popular choice due to its fast processing speed for parallel address matching and rule-based classification.

[0006] A ternary CAM cell is capable of storing information in three different logic states, a binary “0” state, a binary “1” state, and an “X” or “don't care” state. One characteristic of a CAM is that the entire chip is being accessed every cycle for a look-up, which consumes a large amount of power. Conversely, most other types of memories access only a specific address, which in turn consumes a relative small amount of power. But in a CAM, the entire chip has to be checked in parallel. Electrical power in a CAM is generally consumed from two sources, a set of compare lines and a set of match lines. Compare and matching lines are the ones that are actually switching in a CAM.

[0007] FIG. 1 illustrates a conventional ternary CAM cell 5 that employs a pair of data storage and comparator groups 11 and 12 and a conventional ternary CAM cell 10 that employs a pair of data storage and comparator groups 13 and 14. If data is stored in data storage 15, a compliment data is stored in data storage 19. Transistors {overscore (A)} 16, A 20, B 17, and {overscore (B)} 21 form an exclusive OR (XOR) function of {overscore (A)} B and A{overscore (B)}. In combination, the first pair of data storage and comparator groups 11 and 12 is used to code one bit. When data is to be stored, the data is sent to data storage 15, and a complimentary data is being stored in data storage 19.

[0008] During a compare operation, data storage and comparator group 11 is used for comparison with true comparand dataline C018, and data storage and comparator group 12 is used for comparison with the compliment value of the true comparand dataline {overscore (C0)} 22. During a non-compare operation, comparand datalines C018 and {overscore (C0)} 22 are both grounded to zero. For operation in a NMOS compare circuit, one of the comparand datalines C018 or {overscore (C0)} 22 must be in a high-voltage state, while the other comparand dataline is in a low-voltage ,tate. The high-voltage dataline is the one that is activated for performing a compare operation. As a result, one-half of the datalines in the CAM cell region is toggled for each compare operation. Given the large amount of parallel compare operations in a CAM cell, the total electrical current consumed by comparand datalines represent a significant portion in a CAM.

[0009] Similar operations apply to data storage and comparator group 13 including a comparand dataline C126 that couples to a transistor B 25, a transistor {overscore (A)}, and a data storage 23, and data storage and comparator group 14 including a comparand dataline {overscore (C1)} 30 that couples to a transistor {overscore (B)} 29, a transistor A 28, and a data storage 27. The raw data is stored and the raw data is compared. A miss is detected if a match line 31 is pulled low through one of the four CAM cells.

[0010] FIG. 2 is a time diagram depicting conventional ternary CAM cell 10 with two datalines C018 and C126, with corresponding compliments of comparand datalines {overscore (C0)} 22 and {overscore (C1)} 30. For every cycle, both the comparand datalines C0 or {overscore (C0)}, and C1 or {overscore (C1)}, may switch up for evaluation and reset for precharge. As a result, there are four switching operations in two cycles, which consumes a significant amount of power from aCAM.

[0011] Accordingly, it is desirable to have a CAM circuit and method that reduces the number of switchings on a compare line, and thus effectively reducing the amount of power consumed.

[0012] The invention discloses a low-power ternary CAM by utilizing four encoded comparand datalines, C0, C1, C2, and C3 in a twin ternary cell. The twin ternary cell is a composite of two ternary CAM bits. The two binary CAM bits are coded so that only one of the four comparand datalines is toggled during a compare operation. The encoded data is stored and used for comparison. In one embodiment, the four possible states for the 2-bit comparands are coded as 0001, 0010, 0100, and 1000.

[0013] Advantageously, the invention reduces power consumptions in a CAM by avoiding the storing and comparing of raw data. Moreover, the total number of switchings during a compare operation is significantly reduced, thereby resulting in a 50% or more power saving.

[0014] FIG. 1 is prior art circuit diagram illustrating a conventional ternary CAM cell that employs a pair of data storage and comparator groups and a conventional ternary CAM cell that employs a pair of data storage and comparator groups.

[0015] FIG. 2 is a prior art timing diagram illustrating the assertions of comparand datalines in the conventional ternary CAM configuration.

[0016] FIG. 3 is a simplified block diagram illustrating a low-power ternary CAM configuration in accordance with the present invention.

[0017] FIG. 4 is a circuit diagram illustrating a low-power ternary CAM configuration with encoded comparand datalines in accordance with the present invention.

[0018] FIG. 5 is a flow diagram illustrating the low-power ternary CAM configuration with encoded comparand datalines in accordance with the present invention.

[0019] FIG. 6 is a flow diagram illustrating the low-power ternary CAM configuration with encoded comparand datalines in accordance with the present invention.

[0020] FIG. 3 is a simplified block diagram illustrating a low-power ternary CAM configuration. An encoder/decoder 51 is coupled to a twin ternary CAM cell 50 for encoding raw data from a writing operation 52. Encoder/decoder 51 executes a write operation 53 or a compare operation 54 to twin ternary CAM cell 50. When reading from twin ternary CAM cell 52, encoder/decoder 51 decodes the encoded data to generate an output of raw data 56.

[0021] FIG. 4 is a circuit diagram illustrating twin ternary CAM cell 50 with encoded comparand datalines, including a first CAM bit 60, a second CAM bit 61, a third CAM bit 62, and a fourth CAM bit 63, interconnected by a match line 80. Each CAM bit is associated with a comparand dataline. Four comparand datalines, C067, C171, C275, and C379 are used in twin ternary CAM cell 50.

[0022] Twin ternary CAM cell 50 includes four CAM bits, one cell as the combination of CAM bits 60 and 61, and the other cell as the combination of CAM bits 62 and 63. It is apparent to one of ordinary skill in the art that the number of cells in twin ternary CAM cell 50 can be expanded to any number of cells, provided there are no limitations to physical lines.

[0023] A state table defines nine ternary states for a two-bit combination, as shown in Table 1. The two ternary bits provide a total of nine binary states: 00, 01, 10, 11, X0, X1, 0X, 1X, and XX. The corresponding stored data states [D3:0] in twin ternary CAM cell 50 are stored as 1110, 1101, 1011, 0111, 1010, 0101, 1100, 0011, and 0000. The four stored data states 1110, 1101, 1011, 0111, are encoded in comparand dataline states C[3:0] as 0001, 0010, 0100, and 1000 respectively. The encoded bits of C[3:0] are storage respectively, in data D064, D168, D272, and D376.

[0024] Comparand datalines C067, C171, C275, and C377 represent four physical lines that only one of the four comparand datalines switches. Switching in a comparand dataline C067, C171, C275, and C379 requires two switches, asserted high and asserted low. Power consumed in each twin ternary CAM cell 50 is thus significanty reduced. For example, a conventional CAM 10 with two complimentary compaerand datalines {overscore (C0)} 18, C022, C126, and {overscore (C1)} 30 require C018 or {overscore (C0)} 22 to switch and {overscore (C1)} 26 or C130 to switch, for a total of four switchings. Therefore, twin ternary CAM cell 50 in this embodiment conserves 50% of power. One of ordinary skill in the art should recognize that the number of comparand datalines can be increased without departing from the spirits in the present invention.

[0025] Data in data storage D064, D168, D272, and D376 are stored as encoded data. Encoded data are used during compare operations. Therefore, the number of switching is compressed. When the binary value in twin ternary CAM cell 50 corresponds to 00, comparand dataline C067 is asserted. If the binary value in twin ternary CAM cell 50 corresponds to 01, comparand dataline C171 is asserted. When the binary value in twin ternary CAM cell 50 corresponds to 10, comparand dataline C075 is asserted. If the binary value in twin ternary CAM cell 50 corresponds to 11, comparand dataline C179 is asserted.

[0026] FIG. 5 is a flow diagram illustrating the low-power ternary CAM configuration with encoded comparand data lines. Twin ternary CAM cell 50 starts 86 with a command decode, and performs one of the three operations: store, read, or compare. For a store operation, encoder/decoder 51 encodes 87 content of received raw data 52 and updates 88 cells in first, second, third, and fourth CAM bits 60, 61, 62, and 63.

[0027] For a compare operation, encoder/decoder 51 encodes 92 comparands and performs 93 an XOR comparison. Encoder/decoder 51 then priority encodes 94 the result, and generates 95 match or mismatch output. During a read operation, twin ternary CAM cell 50 reads 89 the encode data, decodes 90 the encoded data, and generates 91 a raw data output.

[0028] FIG. 6 is a flow diagram illustrating the low-power ternary CAM configuration with encoded comparand datalines. During encode and compare operations, data in D065, D169, and D273 are asserted high, while data in D377 remains unasserted. In this illustration, the stored data state D3 is asserted as “011” for D[3:0], which in turn asserts the comparand dataline C380 in binary value of “1000”.

[0029] The patent disclosure includes copyrightable material. The copyright owner gives permission for facsimile reproduction of material in Patent Office files, but reserves all other copyright rights whatsoever.

[0030] The above embodiments are only illustrative of the principles of this invention and are not intended to limit the invention to the particular embodiments described. Although the term “CAM bit” is used in the invention, other similar or equivalent terms may be practiced without departing from the spirit in the present invention, such as CAM unit, partial CAM cell, incomplete CAM cell, and sub-CAM cell. For example, the encoding scheme in Table 1 serves as one embodiment. Other variations of encoding schemes can be used in implementing the present invention. Moreover, although the present invention is described in the context of a ternary CAM, one of ordinary skill in the art should recognize that the encoding scheme in comparand datalines are applicable to a binary CAM. The present invention can be implemented in an embedded CAM, or a standalone CAM for sorting out conditions quickly. The first, second, third, and fourth CAM bits as used in the present invention include memory cells in a DRAM (Dynamic Random Access Memory), SRAM (Static Random Access Memory), E²PROM (Electrical Erasable Progrommable Read-Only Memory), EPROM (Erasable Programmable Read-Only Memory), PROM (Programmable Read-Only Memory), ROM (Read-Only Memory), or other types of storage elements, either electrically implemented or through an etched technique. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the appended claims.