Abstract:
An integrated circuit including a programmable logic array with a plurality of logic cells and programmable interconnections to receive input signals and to perform logical functions to transmit output signals. The integrated circuit may also include megacells comprising a plurality of functional blocks receiving inputs and transmitting outputs. The integrated circuit may also include a programmable interconnections subsystem to cascade the megacells. The megacells are coupled to the programmable logic array.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/401,055 filed Mar. 10, 2009, U.S. Pat. No. 7,830,173, which is a continuation of U.S. application Ser. No. 12/148,071 filed Apr. 15, 2008 now abandoned, which is a continuation of U.S. application Ser. No. 11/219,597 filed Sep. 1, 2005, U.S. Pat. No. 7,382,156, which is a continuation of U.S. application Ser. No. 10/811,422 filed Mar. 25, 2004, U.S. Pat. No. 6,975,138, which is a continuation of U.S. application Ser. No. 10/412,975 filed Apr. 11, 2003, U.S. Pat. No. 6,781,410, which is a continuation of U.S. application Ser. No. 10/231,320 filed Aug. 28, 2002, U.S. Pat. No. 6,624,658, which is a continuation of U.S. application Ser. No. 09/960,916 filed Sep. 24, 2001, U.S. Pat. No. 6,504,399, which is a continuation of U.S. application Ser. No. 09/243,998 filed Feb. 4, 1999, U.S. Pat. No. 6,329,839, which is a continuation of U.S. application Ser. No. 08/708,403 filed Sep. 4, 1996, U.S. Pat. No. 6,034,547, all of which are herein incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention is directed to a programmable, configurable bus system of lines to interconnect electrical components for an electrical/electronics system. 
     BACKGROUND 
     Megacells are described as block components such as static random access memory (SRAM), microcontrollers, microprocessors and buffers. Sometimes it is desirable to interconnect a plurality of megacells together to provide a larger functional entity. One way to interconnect multiple megacells and logic circuits is through a hardwired bus system. Examples are illustrated in  FIGS. 1   a ,  1   b  and  1   c .  FIG. 1   a  illustrates a bus interface to a dual port SRAM megacell. Bus lines include DATA  0 -DATA 15 , READA 0 -READA 9 , WRITEA 0 -WRITEA 9 . To couple multiple megacells, the data lines are shared among the coupled cells. However, separate read and write lines would be required for each megacell. To the contrary, if the megacells were coupled to generate a deeper combined megacell, the data lines would be separate for each megacell and the read and write lines would be shared among the megacells. Control signals are then be used to select a particular megacell for a particular operation. This is illustrated in  FIGS. 1   b  and  1   c.    
     Such configurations are hardwired and cannot easily be changed to accommodate different configurations. Furthermore, if errors occur in the mask generated, repairs are not easily made, as configurability is minimal. In addition to providing a bus system to interconnect multiple megacells, tristatable input ports are sometimes used to enable multiple inputs to be input to a particular bus line thus allowing a system level communication between logic to megacells or megacells to megacells. However, a single tristate can directly couple to only one line. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, features and advantages of the present invention will be apparent to one skilled in the art from the following detailed description in which: 
         FIGS. 1   a ,  1   b  and  1   c  illustrate interconnections of prior art megacells. 
         FIGS. 2A and 2B  illustrate one embodiment in which logic is programmably coupled to the megacell. 
         FIG. 3  is a block diagram illustration of exemplary programmable logic utilized to implement one embodiment of the configurable bus system of the present invention. 
         FIGS. 4A and 4B  illustrate the organization of the programmable logic of  FIG. 3 . 
         FIGS. 5A and 5B  provide further illustration of the organization of the programmable logic of  FIG. 3 . 
         FIG. 6  illustrates the programmability of connections to bussed signal lines to multiple megacells in accordance with the teachings of the present invention. 
         FIG. 7   a  is a block diagram illustration of one embodiment of a megacell connected to the bus system and I/O. 
         FIG. 7   b  illustrates one embodiment of a dual-port static random access memory (SRAM) megacell with a field programmable gate array (FPGA). 
         FIG. 8   a  is a block diagram illustration of an alternate embodiment and  FIG. 8   b  illustrates the embodiment incorporated into a dual port SRAM with a FPGA. 
     
    
    
     DETAILED DESCRIPTION 
     The system of the present invention provides a flexible programmable bus structure system of lines to couple one or more circuits for input and output as well as to each other. In the following description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that these specific details are not required in order to practice the present invention. In other instances, well known electrical structures and circuits are shown in block diagram form in order not to obscure the present invention unnecessarily. 
     One embodiment of the programmable bus system is illustrated in  FIGS. 2A and 2B . The system is illustrated using a megacell circuit; however, it is readily apparent that the system can be utilized with a variety of types of circuits and/or components. The type of megacell component used in the following discussion is a 256×8 dual port static random access memory (SRAM). However, the bus system described herein is not limited to SRAM components. A variety of components, such as microcontrollers, buffers, digital signal processors (DSPs) can be coupled to the bus system described herein. 
       FIGS. 2A and 2B  illustrate one embodiment of the configurable bus system of the present invention. Referring to  FIGS. 2A and 2B , the configurable bus system of lines includes groups of lines  210 , lines  215 , and lines  220 ,  225 . Each data input/output port of the megacell  205  is connected to one line of lines  210 . For example, DI[ 0 ] is connected to Data[ 0 ], DI[ 1 ] is connected to Data[ 1 ], etc. In addition, each read or write address port of the megacell  205  is connected to one of the group of lines  215 . Furthermore, lines  225  are connected to the control ports of the megacell  205 . It is recognized that the exemplary system described herein has been programmed to convey address, data and control information across certain of the lines which form the bus system of lines. It is readily apparent that in other applications the system may only convey other combinations of information such as data and control. In addition, one skilled in the art recognizes that the lines are programmable and can be configured for a variety of types of information in addition to the types of information described herein. 
     In the present embodiment, data is preferably input to the megacell  205  and output from the megacell through interface logic  230 . As will be described below, the interface logic is embodied in a programmable logic device, such as a field programmable gate array (FPGA); however, other types of logic can be used. A first set of programmable connections programmably couple the interface logic  230  to the data input/output ports of the megacell  205  (e.g., elements  235 ,  240 ,  245 ,  250 ). For example, programmable elements  235 ,  240  selectively connect a first line  255  from the interface logic  230  to lines Data[ 0 ]  211  and Data[ 8 ]  212 . In addition, in the present embodiment, the programmable elements of the first set of programmable elements programmably couple the interface logic  230  to line  215 . For example, programmable elements  237 ,  247  selectively connect a first line  256  from the input/output logic  230  to bussed lines READA[ 0 ]  216  and WRITEA[ 0 ]  217 . Furthermore, the location of the programmable elements and the lines that each programmable element selectively connect to can be varied according to application.  FIGS. 2A and 2B  illustrate one arrangement of programmable elements of the first set of programmable elements that provides flexibility in configuring the bus system of lines. 
     The control signals to the megacell  205  can be similarly transmitted over the configurable bus system described herein. A second set of programmable connections are used to selectively connect control signals received from the interface logic  230  to the lines  225  and megacell  205 . For example, programmable elements  261 ,  262  selectively connect a global clock input to lines  226 ,  227 . In addition, in the present embodiment, lines READA[ 8 ], READA[ 9 ], WRITEA[ 8 ], WRITEA[ 9 ] ( 220  collectively) are used to provide the higher order address bits as control inputs to select other coupled megacells. This illustrates the capability of this innovative bus system to provide system level integration. 
     Preferably, a third set of programmable connections are used to selectively extend the number of megacells coupled to the configurable bus system. The bus system is configurable using elements of the third set of programmable connections to selectively connect on or more megacells to the bus system of lines. The third set of programmable connections selectively limit the load on the lines for better performance by extending the lines (and therefore increasing the load) only when needed. In the present embodiment, for example, programmable elements  270 ,  271  selectively extend the lines  210  and lines  215 . 
     In addition, it is preferred that the interface logic  230  is programmable and provides bidirectional access to the bus. In addition, it is preferably that the interface logic provides three-statable control to the bus. In particular, control bits and associated logic is used to provide bidirectional, three state control and selective input/output of a plurality of external connections to the lines of the bus system. Referring to  FIG. 2 , the input/output logic  230  includes a plurality of elements, e.g.,  231 ,  232 ,  233 ,  234 . Each element is coupled to two external connections  280 ,  281 . Each element is further coupled to enable control signals, e 0   282 , e 1   283 . The enable control signals e 0 , e 1  and control bits  284 ,  285  function to provide the three state bus functionality that selects one of two external connections for input to or output from the bus. Control bit  284  controls the connection as input to the megacell  205  and control bit  285  controls the connection as output from the megacell  205 . If the control bit  284  is set to a first state, e.g., zero, the three-state connection is disabled. If the control bit  284  is set to a second state, e.g., 1, the state of the connection is controlled by enable control signals e 0 , e 1 . Although the present embodiment incorporates the bidirectional, three state access to the bus system of lines, it is contemplated that bidirectional three state access mechanism is implemented separate form the interface logic. 
     The programming of the bus system of lines can be achieved a variety of ways. One method is to manually program the different programmable connections associated with particular lines of the bus system of lines. Other automated methods are also contemplated. Obviously, once programmed, the programmable connections can remain in the programmed state. Alternately, a dynamic programmable system can be provided wherein control circuitry coupled to the bus system and the programmable connections can determine, prior to a data transfer, those connections to program in order to configure the bus system of lines to transfer the data. This control circuitry could reside in a circuit coupled to the bus system for the transfer of data or in a circuit external to the bus system and connected circuits. For example, the bus system may couple a processor or arithmetic logic unit and memory. The processor or ALU can contain the control circuitry to configure the bus for each data transfer or plurality of transfers. 
     Furthermore, it is contemplated that the connections to be programmed can be determined a variety of ways in order to configure the bus system for a general transfer or specific transfers of data. For example, the control circuitry could examine the content of the data to be transferred and the control signals issued prior to or contemporaneous with a request to transfer or a signal indicating data is to be transferred (e.g., read or write signals or commands) to determine the programmable connections to be programmed. 
     The bus system described can be used to connect components, logic circuits and the like which span across one or more elements. In the present example, as noted above, the bus system is used to connect memory (SRAM) to the logic of a programmable logic device (PLD) such as a field programmable gate array (FPGA). More particularly, in the present embodiment, the bus system is used to integrate the memory into the same component as the FPGA. The FPGA, embodied as the interface logic in the present embodiment, preferably functions as control logic for accessing the SRAM or as interface logic between the SRAM and other devices. Preferably, a programmable logic device such as those described in U.S. Pat. No. 5,457,410 and U.S. patent application Ser. No. 08/534,500, filed Sep. 27, 1995 is used. 
       FIG. 3  is a block diagram of an exemplary FPGA  300 . The I/O logic blocks  302 ,  303 ,  311 , and  312  provide an interface between external package pins of the FPGA  300  and the internal user logic either directly or through the I/O to Core interface  304 ,  305 ,  313 ,  314 . The external package pins are coupled to the lines of bus system ( 210 ,  215 ,  FIG. 2 ), the signals that are processed through the input/output logic ( 230   FIG. 2 ), and the ports of the megacell ( 205 ,  FIG. 2 ). Four interface blocks  304 ,  305 ,  313  and  314  provide decoupling between core  306  and logic  302 ,  303 ,  311  and  312 . 
     The Core  306  includes configurable logic and an interconnect hierarchy. In the present embodiment, the logic is organized in a number of clusters  307  of logic which are intraconnected by an I-Matrix  301  and interconnected by MLA routing network  308 . The core also includes control/programming logic  309  to control the bits for programming the intraconnection and interconnection lines. In the embodiment described herein, SRAM technology is utilized. However, fuse or antifuse, EEPROM/ferroelectric or similar technology may be used. In order to minimize skewing, a separate clock/reset logic  310  is used to provide clock and rest lines on a group basis. 
     The present embodiment provides logic in groups called clusters.  FIG. 4   a  is an example of a logic cluster. It is contemplated that the logic cluster illustrated by  FIG. 4   a  is illustrative and logic cluster can be formed of other elements such as logic gates and flip-flops. Referring to  FIG. 4   a , the logic cluster  400  is formed of four logic elements. These elements include one 2 input combinational logic or configurable function generator (CFG)  402 , two three input CFGs  404 ,  406  and D flip-flop  408 . CFG  402  can also be a three input CFG. The CFGs  402 ,  404 ,  406  are programmable combinatorial logic that provide a predetermined output based using two input values (for CFG  402 ) or three input values (for CFGs  404 ,  406 ). The CFGs are programmed with values to provide output representative of a desired logic function. The D flip flop  408  functions as a temporary storage element such as a register. 
     This combination of one two input, one output CFG, two three input one output CFGs and a D flip flop enable a variety of logic and arithmetic functions to be performed. For example, the elements can be programmed to perform such functions as comparator functions or accumulator functions. In the present embodiment, it is used to selectively couple bus signal lines to input/outputs of a megacell and to input/output logic. It should be noted that this combination of elements provides a fine granularity without the addition of redundant elements which add to the die size and speed of processing. Furthermore, the combination of elements also maximizes usage of elements thereby maximizing usage of die size space. The fine granularity characteristic resulting in more output points that can be tapped is a desirable characteristic as often an intermediate signal generated by a particular combination of elements is needed. 
     In addition, the local interconnect within the cluster is structured to enable signals to be processed within minimum delays. The cluster elements,  402 ,  404 ,  406 ,  408 , are connected through interconnection lines I-M 0  through I-M 5  (referred to herein collectively as I-Matrix lines) which are oriented horizontally and vertically through the logic cluster. These intraconnections of a cluster are programmable through switches, for example switches  420 - 444 . Intraconnections lines I-M 0  to I-M 5  and switches  420 - 444  form what is referred to herein as the I-Matrix. The I-Matrix provides connectability among the elements  402 ,  404 ,  406 ,  408  to at least one other element of the cluster. For example, the output of the CFG  202  can be connected to the input of CFG  404  by enabling switches  424  and  428 . 
     To ensure minimum signal delays during processing, separate, direct connections are provided between the D flip flop  408  and the three input CFGs  404 ,  406 . Continuing reference to  FIG. 4   a , switches  450 - 455  and connected lines provide such connections. It has been determined that the input and output of the three input CFGs  404 ,  406  often perform programmed functions in conjunction with the register  408 . For example the three input CFGs can be utilized with the register to provide a one bit multiplexing function. 
     The bi-directional switches  450 - 455  can be programmed a variety of ways to route the signal to achieve a specific function. For example, a signal output by CFG  404  can drive D flip-flop  408  by enabling switch  451 . Alternately, the signal may be driven onto the I-Matrix by enabling switch  450 . Similarly, the output of CFG  406  can drive the input of the D flip-flop  408  by enabling switch  455 . Other routing paths by selectively enabling switches are also possible. Furthermore, the output of the CFG  402  can drive the D flip-flop  408  by an indirect connection through the I-Matrix. thus, extreme flexibility is achieved. 
     The routing of the output signal of the D flip-flop is also programmable through switches  452  and  453 . By selectively enabling switches  452  or  453  and selective switches of the I-Matrix, the output signal can be routed to any one of the elements of the cluster or of other clusters. The signal output is selectively routed through the switches  433 - 435  adjacent to the CFG  204  or to switches  441 ,  442  and  443  adjacent to CFG  406 . Die savings are achieved without decreasing the level of usage of elements in the device. 
     Each logic cluster is connectable to the other logic clusters inside the logic block through switches extending the I-matrix between neighboring clusters.  FIG. 4   b  illustrates I-matrix interconnection lines I-M 0  to I-M 5  of a first logic cluster  460  selectively connected to the I-Matrix lines of adjacent logic clusters  461  and  463 , respectively through switches  464 ,  465 ,  466 ,  467 ,  475  and  476 . 
     The flexibility herein described is partially achieved through the numerous bi-directional switches used. It was also noted previously that the switches can be implemented a variety of ways. For example, the switches can be implemented as fusible links which are programmed by blowing the fuse to open or short the switch. Alternately, the switch can be a passgate controlled by a bit in an SRAM array. The state of the bits in the array dictate whether a corresponding passgates are open or closed. 
     To allow an efficient implementation of a carry chain as well as other applications, staggered or barrel connections between clusters is used to increased connectivity.  FIG. 4   b  illustrates the extensions of the I-Matrix within a logic cluster to neighboring clusters. For example, switch  475  connects I-M 5  of cluster  460  to I-M 0  of cluster  461  and switch  476  connects I-M 1  of cluster  460  to I-M 2  of cluster  461 . 
     A plurality of interconnected logic clusters form a logic block. In the present embodiment each logic block consists of four logic clusters organized in a 2×2 array as generally illustrated by  FIG. 5   a . Each logic block has a set of bi-directional routing lines to which all CFGs inside the logic clusters are programmably connected. The bi-directional routing line provide the path for signals to travel into and out of the logic block to the routing lines of a hierarchical routing architecture having multiple lengths of interconnections at different levels of the hierarchy. It can also be seen that the block connectors can also provide connections among the CFGs of the logic clusters of the same block and adjacent blocks. Although the input and output of each element of each logic cluster of the logic block can be selectively connected to each block connector, to control the expansion on die size it is preferred that each input and output is selectively connected to a subset of block connectors. An example of such an embodiment is shown in  FIG. 5   b.    
     Referring to  FIG. 5   b , a symbolic representation of one embodiment of the connections to block connectors within a block  300  is shown. Each element of each cluster  500 , e.g., CFG 1 , CFG 2  and CFG 3  is connected to two identified block connectors (BC) at the inputs. Two block connectors are identified as coupled to the output of the two input CFG 1  and three block connectors are coupled to the output of the three input CFGs (CFG 2 , CFG 3 ). The specific block connectors coupled to each elements are distributed among the elements of the block to maximize connectivity. 
     The block connectors provide the input and output mechanism for interconnecting to higher levels of connections of the routing hierarchy referred to as the multiple level architecture (MLA) routing network. The network consists of multiple levels of routing lines (e.g., MLA- 1 , MLA- 2 , MLA- 3 , MLA- 4 , etc.) organized in a hierarchy wherein the higher level routing lines are a multiple longer than the lower level routing lines. For example, MLA- 2  routing lines are twice as long as MLA- 1  routing lines and MLA- 3  routing lines are twice as long as MLA- 2  routing lines and MLA- 4  routing lines are twice as long as MLA- 3  routing lines. 
     Using the logic and interconnect hierarchy described, the user can program the PLD and the bus to access the memory in a variety of configurations without requiring significant space on the component. 
     The flexibility and utility of the configurable bus system of the present invention is illustrated with reference to  FIG. 6 .  FIG. 6  shows the bus system configured to couple to 4 SRAM megacells arranged in a 2×2 configuration. The programmable elements are configured as passgates controlled by a bit in one of the SRAMs or other coupled memory. As is illustrated, no extra logic or interconnect is required for the bus system configuration. By enablement of the proper links which control the interconnect, the bus system is easily configured for the particular arrangement of megacells. 
     In the present example, the bus system is programmed to be coupled to the interconnect of the PLD (e.g., block connectors (bc), I-matrix lines (IM) and MLA lines (MLA- 1 )) to enable the logic of the PLD to provide the necessary interface logic to interface the SRAM to components or devices external to the system. For example, the PLD provides logic to assert the necessary control signals to transmit the address information and receive and transmit data. In the example shown in  FIG. 6 , data and address information is communicated through the bi-directional block connectors. Control information, including control signals to control the state of the enable signals (e 0 , e 1 ) are communicated via the I-matrix and MLA- 1  lines. 
       FIG. 7   a  is a block diagram illustration of one embodiment of megacell  701 ,  702 , coupled to the bus system of the present invention. A program controlled interface  703 ,  704 , to the bus system of lines  705  and megacells  701 ,  702  are provided. The interface from the core bus  705  to the I/O  706 ,  707  can be achieved using hardwired or program controlled connections  708 ,  709 . Preferably, these connections are achieved using a programmable, peripheral bus system of lines  710 ,  711  to provide further flexibility. The peripheral bus system is preferably programmable in the same manner as described above with respect to  FIG. 2 . In the present embodiment, the interface logic ( 230   FIG. 2 ) provides the program controlled interface  703 ,  704  to the bus system  705  which is also programmed controlled. 
       FIG. 7   b  depicts an overview of an exemplary component configured with dual port SRAM megacells and a FPGA. The FPGA, including its interconnect structure, is represented by elements  712 ,  715 ,  720 ,  725 . Each element  712 ,  715 ,  720 ,  725  comprises a plurality of logical blocks organized in 16×16 array with a corresponding hierarchical interconnect structure as discussed in U.S. Pat. No. 5,457,410 and U.S. patent application Ser. No. 08/534,500. The FPGA elements  712 ,  715 ,  720 ,  725  are connected by the interconnect, e.g., block connectors, I-matrix lines and MLA lines (see  FIG. 6 ), through the configurable bus system of lines (e.g., as represented by elements  730 ,  735 ,  740 ) to an SRAM (e.g.,  745 ,  750 ,  755 ,  760 ). SRAM  745 ,  750 ,  755 ,  760  and elements  730 ,  735  and  740  correspond to the structure illustrated by  FIG. 6 . It should be noted that the bus system preferably spans the entire component to the adjacent array of SRAMs  775 ,  780 ,  785 ,  790  through programmable elements (not shown). The bus system is further coupled to I/O ports or pads (e.g.,  791 ,  792 ) for input/output to/from the system to external components or devices. Although the bus system can be coupled through hardwired connections, it is preferred that the connection be made via programmable elements, e.g.,  765 ,  770  and bus system of lines  775 . 
       FIG. 8   a  is a block diagram illustration of an alternate embodiment in which gateway interface logic  801  is used to interface the core bus system  802  to the I/O  803 . In addition, this diagram illustrates alternative programmable connections that can be implemented to provide further programmability and flexibility to the system. 
     The gateway interface logic  800  is composed of hardwired logic, metal programmable logic, or programmable logic such as a plurality of logic clusters and is directly or indirectly coupled (i.e., direct hardwired connections or indirect program controlled connections) to the megacell  804 .  FIG. 8   a  shows the gateway interface logic  800  is coupled to the megacell  804  via peripheral bus  805  which preferably includes bi-directional, three-statable connections (e.g.,  808 ). The gateway interface logic  800  provides an additional level of logic to the interface between the megacell and the I/O pads or ports to external components or devices. The gateway interface logic can enable faster transfer of information. For example, the gateway interface logic can be structured to provide the specific bus protocols or handshaking required to interface to external devices. The gateway interface logic can also provide address decode functionality (e.g., wide decode) to expedite processing of information. 
     In the present embodiment, the gateway interface logic  800  is implemented as a logic cluster  801 , consistent with the logic clusters referred to herein and in U.S. Pat. No. 5,457,410 and U.S. patent application Ser. No. 08/534,500. I-Matrix lines are used to connect the gateway logic to the peripheral bus  805 . It should be recognized that the gateway interface logic is not limited to the specific implementation described herein and a variety of logic implementations can be used. 
       FIG. 8   b  illustrates dual port SRAMs with FPGA and the configurable bus system. In this embodiment, further programmability is provided at the I/O ports of the system using gateway interface logic. In particular, the programmable gateway logic (e.g,  830 ) is located between the core bus system of lines (e.g., elements  810 ,  815 ,  820 ) and the I/O (e.g.,  825 ). In the present embodiment a logic cluster as illustrated in  FIG. 4   a  is used; however, as noted above, it is contemplated that other forms of logic can be utilized. In addition, this embodiment includes a peripheral bus system of lines  840 , which functions is a manner similar to the core bus system of lines, providing a programmable bus system for transferring information. Preferably, each of the programmable connections of the bus system (e.g.,  846 ,  847 ) are bi-directional, three-statable connections. 
     Further enhancements and interconnect flexibility is achieved by providing programmable connections from the core bus (e.g.,  820 ) direct to the peripheral bus  840  and from the megacell (e.g.,  845 ) direct to the peripheral bus  840 . For example, programmable connection  822  selectively enables the bus element  820  to be connected to peripheral bus  840 . Similarly, programmable element  824  selectively connects megacell  845  directly to peripheral bus  840 . Such flexibility is advantageous when speed is a consideration. For example, it may be desirable to directly connect externally received control input data to the megacell. 
     The invention has been described in conjunction with the preferred embodiment. It is evident that numerous alternatives, modifications, variations and uses will be apparent to those skilled in the art in light of the foregoing description.