Content addressable memory system

Abstract: A system includes a plurality of content addressable memory (CAM) arrays which are cascaded. The CAM arrays are connected to a commonly shared bus. Each of the arrays provides search results (hit, match address and multiple match). A hit signal is propagated from array to array. The hit result of the system in search operation is given from the furthest down stream array. The match address result of the system is given from the commonly shared bus. Self-timed signal generated by logic local to each array prevents the occurrence of driving contentions onto the commonly shared bus.

Description: Technical Field The present invention relates to a content addressable memory (CAM) system in which a plurality of CAM arrays are cascaded. Background Information Content addressable memories (CAMs) are known. In CAMs, data is selected based on contents, rather than physical location. This function is useful for many applications, especially when performing a look-up for the purposes of mapping from a long identification word to a shorter word. This operation is required in many telecom functions, including Asynchronous Transfer Mode (ATM) address translation. Often, system storage requirements exceed the number of entries stored on a single CAM array. Multiple CAM arrays, possibly on multiple chips, are then required, and it is necessary to cascade the multiple CAM arrays such that they may be searched as a single entity. An appropriate "user-friendly" cascading capability enables the same CAM array to be used in a range of systems with different capacity requirements, and allows for easy expandability and scalability, as well. United States Patent No. 5,568,416 granted to K. Kawana et al on October 22,1996 discloses an associative memory in which multiple CAM chips are cascaded by propagating result address and status through all chips in the cascade. Each chip contains a status register for itself, and another for all upstream chips. It also discloses means of identifying the last device in the cascade, and separate storage areas for common and unique data entries. A 288-KB FULLY PARALLEL CONTENT ADDRESSABLE MEMORY USING A STACKED -CAPACITOR CELL STRUCTURE' IEEE JOURNAL OF SOLID-STATE CIRCUITS, vol. 27; no. 12, 1 December 1992, pages 1927-1933, XP000329045 from TADATO YAMAGATA ET AL discloses an architecture for high-density CAM's comprising cascading means for propagating the search results received from the CAM arrays. Summary Of The Invention It is an object of the present invention to provide an improved content addressable memory system. According to one aspect of the present invention, there is provided A system comprising: a plurality of content addressable memory (CAM) arrays, each CAM array comprising an array of w words x b bits of core cells, each cell comprising data storage means, each CAM array being able to provide a hit signal and a match address signal resulting from a search operation; and cascading means for propagating the hit signals and match address signals received from the CAM arrays and for providing a system hit signal and a system match address signal; wherein the cascading means comprises: a commonly shared bus for receiving the match address signals from the CAM arrays; and a plurality of driving means for together preventing more than one match address signal from the CAM arrays from propagating to the bus simultaneously, each driving means being associated with a respective CAM array for preventing the match address signal received from the CAM array from propagating to the bus, characterised in that each driving means is disabled by a signal generated locally to the CAM array and enabled by a signal received from an upstream driving means. According to the present invention, it is possible to implement a plurality of CAM arrays that has the same kind of search result outputs as a single CAM (e.g., hit, match address). It is thus possible that n CAM arrays, each with a capacity of w entries (or words), are integrated into a single multi-chip CAM system with n x w words. The cascading means may comprise a plurality of driving means and propagating means, each being associated with a respective CAM array. In the system, a hit signal is propagated downstream by the propagating means. Brief Description Of The Drawings An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a block diagram of a system including a plurality of CAM arrays which are cascaded, according to an embodiment of the present invention; Figure 2 is a circuit diagram of an example of a CAM array shown in Figure 1; Figure 3 is a circuit diagram of an example of a logic circuit shown in Figure 1; Figure 4 is a timing chart showing self-timed signal and search results in one CAM array; Figure 5A is a timing chart showing relative timing of search signals responsive to the transition from 0 to 0 of a propagation-in hit signal; Figure 5B is a timing chart showing relative timing of search signals responsive to the transition from 1 to 0 of the propagation-in hit signal; Figure 5C is a timing chart showing relative timing of search signals responsive to the transition from 0 to 1 of the propagation-in hit signal; Figure 5D is a timing chart showing relative timing of search signals responsive to the transition from 1 to 1 of a propagation-in hit signal; Detailed Description Referring to Figure 1 which shows a system according to an embodiment of the present invention, the system includes n CAM arrays 110 which are cascaded through n logic circuits 120. Each logic circuit 120 is associated with the respective CAM array 110 to receive hit, match address and multiple match signals ht, sa and mt therefrom; the timing of these three signals is know but not necessarily controllable. The logic circuit 120 has input terminals HTI and MTI for receiving hit and multiple match signals, respectively, from the upstream logic circuit 120. Both input terminals MTI and HTI of the furthest upstream logic circuit 120 are connected to logic 0 terminals. Also, each logic circuit 120 has output terminals HT and MT for providing hit and multiple match signals to the downstream logic circuit 120. An address output terminal SA of each logic circuit 120 is connected to a commonly shared bus 122. The system includes n AND gates 124 which have inverting and non-inverting input terminals. The hit input terminal HTI and hit output terminal HT of each logic circuit 120 are connected to the inverting and non-inverting input terminals of the respective AND gate 124. A clock generator 126 provides clock signals ck to the CAM arrays 110 and the logic circuits 120. The bus 122 and the hit and multiple match output terminals HT and MT of the furthest downstream logic circuit 120 are connected to a search result observing circuit 128. The output terminals of the AND gates 124 are connected to an n-to-log 2 n encoder 130. Referring to Figure 2 which shows an example of the CAM array 110 implemented with an array of w (=4) words (rows) x b (=4) bits (columns), the CAM array 110 includes core cells 230 of w rows x b columns. Each of the core cells 230 includes data storage means (not shown) and is at the intersection of a match line 232 and a pair of bit lines 234. A pair of bit lines 234 carry differential data representing a single bit, rather than two bits of data. Each core cell 230 acts to store a single bit of data and is capable of performing a single-bit comparison (logical exclusive NOR (XNOR)) operation, in addition to its bit storage capability. The bit lines 234 for differential data are connected to reference word storage and bit line drivers 236 which receive input data D for loading the contents of the CAM array and for the search reference word. The CAM array 110 includes an encoder 238 which is connected to the match lines 234. The structure of the CAM array is known. See a paper by K.J. Schultz et al. entitled "Architectures for Large-Capacity CAMs", INTEGRATION: the VLSI Journal, Vol. 18, pp. 151-171, 1995, which is incorporated herein by reference. The CAM array is not limited to one shown in Figure 2. There are many variations. For example, the data comparison function of a CAM array is not performed by the core cells and is performed by separate comparators placed adjacent to the core cells. Such a CAM array is described in United States Patent Application No. 08/748,928 entitled "Large-Capacity Content Addressable Memory", filed on November 14, 1996 by K.J. Schultz et al. Also, a core cell array of a CAM array may be chained as described in United States Provisional Application No. 60/037,834 entitled "Content Addressable Memory", filed on February 6,1997 by K.J. Schultz et al. Referring to Figures 1 and 2, in response to the clock signal ck, when differential data is asserted on a pair of bit lines 234 in a search operation, the core cell 230 compares its stored data bit with this differential data (also known as reference data, or a single bit of the comparand). When the stored data is not equal to the reference data, the core cell 230 pulls the match line 232 (which is precharged to a logical high state) down to a low state. When the stored data is equal to the reference data, the cell 230 has no effect on the match line 232 to which it is connected. Because all b core cells 230 in a given word are connected to the match line 232 in the same way, the match line 232 will be pulled low if any bit in its word is unequal to (or mismatches) the corresponding reference bit. The match line 232 remains in a logical high state only if all bits in its word match their corresponding reference bits. Each CAM array 110 is able to provide search results (i.e., hit, match address and multiple match signals ht, sa and mt), via the encoder 238, which are fed to the respective logic circuit 120. Each logic circuit 120 propagates the hit and multiple match results by array-to-array and transfers the match address result to the commonly shared bus 122. The hit and multiple match results SHT and SMT of the system are available at the far right side (the furthest downstream). An additional useful piece of status information is the ordinal location of the array that has driven its result onto the bus 122 (i.e., the highest-priority array with a match); this information is generated by the AND gate 124 and the encoder 130. With reference to a single array 110, the binary address of a matching word is encoded onto the "sa" output. In the event that a plurality of words have matched the reference data, the multiple match signal mt is asserted to a logical high state. In this event, the match address output of the encoder 238 may produce (a) an invalid result, (b) an address representing the location of a single one of the multiple matches, or (c) a sequence of outputs, representing the locations of each of the matched words. Note that some applications may not require the multiple match result, and all references to the multiple match function may be eliminated from this disclosure, without loss of utility. Figure 3 is a circuit diagram of an example of the logic circuit 120. Because the CAM array 110 of the system shown in Figure 1 provides the hit and multiple match search results, the logic circuit 120 propagates them in a similar way. One logic circuit 120 receives hit, match address and multiple match signals ht, sa and mt of a search result from the respective CAM array 110. The hit signal ht is fed to AND gates 332 and 342 and an OR gate 336. The match address signal sa is fed to the input terminal of a transfer gate 334, the output terminal of which is connected to the match address output terminal SA of the logic circuit 120. The multiple match signal mt is fed to an OR gate 344. The propagation-in hit signal hti is fed to the AND gate 342, the OR gate 336 and the inverting input terminal of the AND gate 332, the output (a match address enable signal sae) of which is fed to the transfer gate 334. The multiple match input terminal MTI is connected to the OR gate 344. The output of the AND gate 342 is fed to the OR gate 344. The output of the OR gate 344 is fed to a buffer 346, the output terminal of which is connected to the multiple match output terminal MT of the logic circuit 120. A self-timed signal st is generated by a self-timed signal generator. There are many possible embodiments of self-timed signal generators. It is the intended scope of this invention to subsume any such embodiment, provided the resulting self-timed signal st is employed as described above to enable contention-free result bus sharing. Referring to Figure 3, in one possible embodiment of a self-signal generator, the clock signal ck is fed to the reset input terminal R of the flip-flop 352. Also, the clock signal ck is fed to the set input terminal S of the flip-flop 352 through the buffers 354. The self-timed signal st is provided from the Q output terminal of the flip-flop 352 to the AND gate 332 and the inverting input terminal of the OR gate 336. The falling edge of the self-timed signal st is generated by the rising edge of the clock signal ck, while the rising edge of the self-timed signal st is generated by a delayed version of the rising edge of the clock signal ck. Timing both edges of the self-timed signal st from the rising edge of the clock signal ck results in duty cycle independence. The delay of the delay chain can be set equivalent to the delay between the rising edges of the clock signal ck and the hit signal ht. Alternatively, if the duty cycle of the clock signal ck is known and well controlled, timing of the rising edge of the self-timed signal st may be controlled by the falling edge of the clock signal ck. Note that hit timing should be predictable, in order to achieve maximum operating speed using the embodiment; in embodiments where the hit timing is not predictable, the width of the st low pulse must be sufficient to be longer than any possible ht delay. Figure 4 is a timing chart showing self-timed signal and search results. An address signal sa from the CAM array 110 is fed to the transfer gate 334 which prevents the address signal from passing through the gate when the enable signal sae is low. An output signal from the transfer gate 334 is provided to the address output terminal SA. A propagation-out hit signal hto from the OR gate 336 is fed to the buffer 338. During the time interval when st=0 on all logic circuits 120 in the system, no arrays are enabled to drive the bus 122. During the same interval, all hit signals HT in the cascade are at logic 1, due to st=0 (see Figure 5A-D), doubly disabling SA output drivers through the propagation-in hit signal hti. This partial redundancy may be removed by re-timing the signals and decreasing the number of inputs to the gates, without departing from the scope of this invention. Note that such an approach would lead to a less robust design. Waveforms of all of the relevant signals on a single array are shown in Figures 5A - 5D, for the four different cases of the propagation-in hit signal hti (transition from 0 to 0, 1 to 0, 0 to 1, and 1 to 1). As can be seen, correct operation is independent of (a) speed differences between arrays and (b) routing delay, because de-selection occurs based on logic local to the circuit 120, and only selection is gated by upstream signals. This feature also supports expandability, as additional arrays added to a system may be subject to different processing conditions, or even a completely different fabrication technology. When worst-case timing is characterized, the slowest path to SA driving will be from the propagation-in hit signal HTI input. The downward transition on the propagation-in hit signal HTI may further propagate to the propagation-out hit signal HT (assuming ht=0), such that the worst-case system performance is equal to that of a single array standing alone, plus (n-2) times the propagation-in hit signal HTI -to-HT delay plus the propagation-in HTI signal hti -to-SA delay. System performance can be characterized by the following expressions: tCH-SAV = tCH-HTV + (n-2) x tHTIL-HTL + tHTIL-SAV tCH-SAV = tCH-HITV + (n-2) x tHITIL-HITL + tHITIL-SAV tCH-SHTV = tCH-HTV + (n-1) x tHTIL-tHTL tCH-HITV = tCH-HITV + (n-1) x tHITIL-HITL Note that, without the self-timed signal st, disabling and enabling SA drive would be dependent on HTI timing. Bus contention would be difficult to prevent, and disable timing would depend on an array's position in the cascade. In another alternative embodiment, or simply an additional function, search address results may be stored in registers (not shown). The output of the encoder 130 may be used to determine which array's result register is read. It is understood that there are many possible variations in embodiment detail that are logically subsumed by this invention disclosure, including different signal polarities, equivalent Boolean gate-level implementations, small timing variations, and so on.

Claims: A system comprising: a plurality of content addressable memory (CAM) arrays (110), each CAM array (110) comprising an array of w words x b bits of core cells (230), each cell (230) comprising data storage means, each CAM array (110) being able to provide a hit signal and a match address signal resulting from a search operation; and cascading means (120, 122) for propagating the hit signals and match address signals received from the CAM arrays (110) and for providing a system hit signal and a system match address signal; wherein the cascading means (120, 122) comprises: a commonly shared bus (122) for receiving the match address signals from the CAM arrays (110); and a plurality of driving means (120) for together preventing more than one match address signal from the CAM arrays (110) from propagating to the bus (122) simultaneously, each driving means (120) being associated with a respective CAM array (110) for preventing the match address signal received from the CAM array (110) from propagating to the bus (122), characterised in that each driving means (120) is disabled by a signal generated locally to the CAM array (110) and enabled by a signal received from an upstream driving means (120). The system of claim 1, wherein the cascading means (120, 122) further comprises a plurality of propagating means (120), each propagating means (120) being associated with a respective CAM array (110) for propagating a hit signal received from a CAM array (110) to a downstream propagating means (120). The system of claim 2, wherein the cascading means (120, 122) further comprises observing means (128) for observing a hit signal received from the furthest downstream propagating means (120) and for observing the match address signal on the bus (122), thereby providing the system hit signal and system match address signal. The system of claim 3, wherein the observing means (128) comprises means for determining a priority of each CAM array (110) according to its position, and wherein the further upstream a CAM array (110) is, the higher its priority. The system of claim 1, wherein the cascading means (120, 122) further comprises means for providing a plurality of match address signals from the CAM arrays (110) to the bus (122) in a sequence of subsequent cycles. The system of claim 1, wherein the cascading means (120, 122) further comprises means for encoding the ordinal location of a highest priority CAM array (110) having a match address signal. The system of claim 1, wherein the cascading means (120, 122) further comprises means for driving the match address signals from the CAM arrays (110), the encoded ordinal location of a highest priority CAM array (110) having a match address signal and associated data onto the bus (122) without contention. The system of claim 3, further comprising means (126) for providing clock signal to the CAM arrays (110) and the cascading means (120, 122), so that the search operations in the CAM arrays (110) are conducted in response to the clock signal and the hit signals and match address signals form the CAM arrays (110) are propagated in response to the clock signal. The system of claim 8, wherein the cascading means (120, 122) further comprises signal timing means for generating a self-timed signal, and wherein each propagating means (120) operates in response to the self-timed signal. The system of claim 9, wherein the signal timing means comprises means for making a first transition of the self-timed signal in response to the clock signal. The system of claim 9, wherein the signal timing means comprises means for making a second transition of the self-timed signal after the hit signals and match address signals from the CAM arrays (110) are guaranteed to have been generated. The system of claim 9, wherein the signal timing means comprises a simple delay chain, and wherein a first edge of the self-timed signal is initiated by the clock signal and a second edge of the self-timed signal is initiated by a delayed clock signal. The system of claim 9, wherein each propagating means (120) comprises means for determining a hit signal of the propagating means (120), the hit signal being logically determined as the OR of: the inverted self-timed signal; the hit signal from an upstream propagating means (120); and the hit signal from the associated CAM array (110). The system of claim 13, wherein the self-timed signal is decomposed into two inputs of a four-input OR gate, a first of the two inputs being a delayed self-timed signal, thereby widening a self-timed signal pulse to allow for a delay due to a combination of search results from the CAM arrays (110). The system of claim 1, wherein each CAM array (110) is able to provide a multiple match signal, the multiple match signals of the CAM arrays (110) being propagated by the plurality of propagating means (120). The system of claim 15, wherein the cascading means (120, 122) further comprises observing means (128) for observing a multiple match signal received from the furthest downstream propagating means (120).

Inventors: GIBSON GARNET FREDERICK RANDAL, SCHULTZ KENNETH JAMES, SHAFAI FARHAD, GIBSON, GARNET FREDERICK RANDALL, SCHULTZ, KENNETH JAMES, SHAFAI, FARHAD
Applicants: NORTEL NETWORKS LTD, NORTEL NETWORKS LIMITED
Application year: 1998
Main classification: G11C15
IPC codes: G11C15