Integrated circuit

Abstract: A configurable logic array, includes a plurality of configurable logic cells which include a tristate output buffer (806,906,1006,1106), having an input receiving a logic signal (TY1) from within the configurable logic cell, an output (Y1) connected to the configurable interconnect structure and an output enable input (OE1). A plurality of selectors (1201-1204), controlled by the configuration memory, supply output enable signals for controlling corresponding tristate output buffers (806,906,1006,1106). The inputs to the plurality of selectors include a "common output enable signal," and at least a second logic signal, such as a constant high or constant low logic level. A circuit responsive to program data in the configuration memory and input signals from the interconnect structure generates the common output enable signal. One input of the selector is provided by an invertor (1210,1211,1212,1213) connected from the input of the tristate output buffer to the selector for connecting an output signal to a long line in a wired-AND configuration.

Description: The present invention relates to an integrated circuit which includes an array of configurable logic cells, a plurality of input/output cells, and a configurable interconnect structure. More particularly, the present invention relates to programmable gate arrays with logic cells having individually configurable output enable circuits. Description of Related Art The programmable gate array is a high performance, user programmable device containing three types of configurable elements that are customized to a user's system design. The three elements are (1) an array of configurable logic blocks (CLBs), (2) with input/output blocks (IOBs) around a perimeter, all linked by (3) a flexible programmable interconnect structure. The system design desired by a user is implemented in the device by configuring programmable RAM cells. These RAM cells control the logic functionality performed by the CLBs, IOBs and the interconnect. The configuration is implemented using PGA design software tools. It is generally accepted that the programmable gate array was first, successfully, commercially introduced by Xilinx of San Jose, California. Xilinx originally introduced the XC2000 series of logic cell arrays and has more recently introduced a second generation XC3000 family of integrated circuit programmable gate arrays. A description of the 2000 series, as well as related programmable logic device art, can be found in THE PROGRAMMABLE GATE ARRAY DESIGN HANDBOOK, First Edition, published by Xilinx, pages 1-1 through 1-31. The architecture for the XC3000 family is provided in a technical data handbook published by Xilinx entitled XC3000 LOGIC CELL ARRAY FAMILY, pages 1-31. (See, in particular, Figure 15b re three-state buffers in the interconnect structure.) The prior art in programmable gate arrays is further exemplified by U.S. Patent Nos. 4,642,487; 4,706,216; 4,713,557; and 4,758,985; each of which is assigned to Xilinx, Inc. As mentioned above, the programmable gate array consists of a configurable interconnect, a ring of configurable input/output blocks, and an array of configurable logic blocks. It is the combination of these three major features that provides flexibility and data processing power for programmable gate arrays. However, the programmable gate arrays of the prior art suffer certain limitations in each of the interconnect structure, the input/output block structures, and the configurable logic block structures. One of the limitations resides in the flexibility of the connections from outputs of the configurable logic cells to the interconnect structure. For instance, it is often desirable to have a tristatable output with an output enable logic, in certain parts of a network. Prior art systems have distributed tristate buffers in the interconnect structure to serve this purpose. However, utilization of the interconnect structure to reach the tristate buffers, and the limited number of tristate buffers, have increased the complexity of programming the programmable gate arrays implemented in this manner. Accordingly, it is desirable to implement the logic cells in the programmable gate array with flexible, programmable output structures for connection to the configurable interconnect. Document EP-A-0 415 542 published on 06.03.1991 and having a date of priority of 15.08.1989, thus falling within the terms of Article 54(3) EPC, discloses an integrated circuit comprising: an array of programmably configurable logic elements ; a programmably configurable interconnect structure including a plurality of conductive longlines, said plurality of configurable logic elements being coupled to the interconnect structure, each member of at least a subset of plurality of configurable logic elements comprising: input means for receiving a set of input signals from the interconnect structure; programmably configurable logic means coupled to the input means for generating a set of output signals in response to a first subset of input signals; and output means coupled to the logic means for outputting one of said output signals to a corresponding conductive longline, each output means including a tri-state output buffer for outputting, when enabled, a selective one of said output signals to a corresponding conductive longline, said output means being provided in a wired-logic configuration. The present invention increases the flexibility of connection between the configurable logic cells and the interconnect structure. According to the invention there is described an integrated circuit comprising: an array of programmably configurable logic elements; a programmably configurable interconnect structure including a plurality of conductive longlines, said plurality of configurable logic elements being coupled to the interconnect structure, each member of at least a subset of plurality of configurable logic elements comprising: input means for receiving a set of input signals from the interconnect structure; programmably configurable logic means coupled to the input means for generating a set of output signals in response to a first subset of input signals; and output means, coupled to the logic means for outputting one of said output signals to a corresponding conductive longline, each output means including a tri-state output buffer for outputting, when enabled, a selective one of said output signals to a corresponding conductive longline, said output means being provided in a wired-logic configuration; wherein said interconnect structure includes pull-up resistors connected to a subset of the conductive longlines to provide a normally logic high state on said subset of the conductive longlines. Other aspects and advantages of the present invention can be seen upon review of the drawings, the detailed description and the claims that follow. Brief Description of the Figures Figure 1 is a schematic diagram of a programmable gate array structure. Figure 2 is a schematic diagram of the configuration memory. Figure 3 is a diagram illustrating nomenclature for the vertical bus, and the use of pull up resistors on long lines of the vertical bus according to the invention. Figure 4 is a diagram illustrating nomenclature for the horizontal buses and illustrating the pull up resistors used on long lines of the horizontal buses according to the invention. Figure 5 is a schematic diagram of a configurable logic cell. Figure 6 illustrates connections of the tristatable outputs of the configurable logic cells to the long lines and bidirectional general interconnect of the interconnect structure. Figure 7 illustrates connection of the tristatable outputs to the uncommitted long lines. Figure 8 is a schematic diagram of a first output macrocell in a configurable logic cell. Figure 9 is a schematic diagram of a second output macrocell in a configurable logic cell. Figure 10 is a schematic diagram of a third output macrocell in a configurable logic cell. Figure 11 is a schematic diagram of a fourth output macrocell in a configurable logic cell. Figure 12 is a schematic diagram illustrating a circuit for generation of the output enable signals for the respective output macrocells. Detailed Description I. Layout and Programming Structure Fig. 1 illustrates the layout of a configurable logic array integrated circuit. Also provided in Fig. 1 is a notation which is utilized to describe the configurable logic array in this application. Accordingly, the configurable logic array shown in Fig. 1 consists of an array of configurable logic blocks illustrated by the square symbol with bold lines shown at the upper left hand corner of the figure. Each configurable logic block in the array is labeled with a row and column number, i.e. in the upper left hand corner of the array, the configurable logic blocks are labeled R1C1, R1C2, and so on until the lower right hand corner of the array where the configurable logic block is labeled R8C8. Around the periphery of the array are 110 pads for connection to external pins. Pads 2-13, 16-27, 29-40, 43-54, 57-68, 71-82, 85-96 and 99-110 are coupled to configurable input/output blocks represented by the symbol shown in the upper left hand corner of the figure. Pads 1, 14, 15, 28, 41, 42, 55, 56, 69, 70, 83, 84, 79 and 98 are utilized for functions other than configurable input/output blocks, such as power, ground, global clock and reset signal inputs, and programming mode control signals. The connection of these miscellaneous pads is similar to that done in prior art programmable gate arrays and is not further described here. The interconnect structure consists of nine horizontal buses labeled HBUS1 through HBUS9 with nine intersecting vertical buses VBUS1 through VBUS9. The intersections of vertical bus 1 and vertical bus 9 with the horizontal buses 2-8 are characterized by having segment boxes which provide programmable interconnection between the respective horizontal buses and vertical buses as described in detail below. Likewise, the intersections of horizontal bus 1 and horizontal bus 9 with vertical buses 2-8 are characterized by segment boxes providing the programmable interconnection between the horizontal and vertical buses. The intersections of the vertical buses 2-8 with the horizontal buses 2-8 are characterized by switching matrices providing for interconnection between the respective horizontal and vertical buses. The placement of the segment boxes and switching matrices is schematically illustrated in Fig. 1 using the symbols illustrated in the lower left hand corner of the figure. The detailed structure of the switching matrices and segment boxes is described below. The configurable logic array according to the present invention contains three types of configurable elements that are customized to a user system design which is specified in a configuration memory. The three configurable elements are the array of configurable logic blocks (CLBs), the configurable input/output blocks (IOBs) around the perimeter, and the programmable interconnect network. The system design of a user is implemented in the programmable gate array by configuring programmable RAM cells known as a configuration memory. These RAM cells control the logic functionality performed by the CLBs, IOBs, and the interconnect. The loading of the configuration memory is implemented using a set of design software tools as well known in the art. The perimeter of configurable IOBs provide a programmable interface between the internal logic array and device package pins. The array of CLBs performs user specified logic functions. The interconnection consists of direct connections between specific CLBs or IOBs, and a general connect that is programmed to form networks carrying logic signals among the blocks. The logic functions performed by the CLBs are determined by programmed lookup tables in the configuration memory. Functional options are performed by program controlled multiplexers. Interconnecting networks between blocks are composed of metal segments joined by programmable interconnect points (PIPs). The logic functions, functional options, and interconnect networks are activated by a program data which is loaded into an internal distributed array of configuration memory cells. The configuration bit stream is loaded in to the device at power up and can be reloaded on command. Fig. 2 is a schematic diagram of the configurable logic array as seen by the program data. The array includes a plurality of distributed memory cells referred to as the configuration memory 200. Program data on line 201 is loaded into shift register 202 in response to a clock signal on line 203. The detect logic 204 determines when the shift register is full by reading a preamble from data on 201. When the shift register is full, the detect logic 204 signals across line 205 a frame pointer logic 206 which generates frame pointer signals across lines 207. Control logic 208 is responsive to the mode inputs to the device on line 209 to control the detect logic 204 across line 210 and the frame pointer during loading of the configuration memory 200. The configuration memory 200 is organized into a plurality of frames F1-FN. As program data is loaded into the shift register, the frame pointer F1 is activated to load the first frame in the configuration memory. When the shift register is loaded with the second frame of data, the frame pointer for F2 is activated, loading the second frame F2, and so on until the entire configuration memory is loaded. Control logic 208 generates a program done signal on line 210. II. General Interconnect Structure Fig. 3 illustrates the notation used for the vertical buses. Each vertical bus has 25 lines. Lines 1-4 and 15-17 are long lines which run across the entire array. According to the present invention lines 1-4 may have pull up resistors (shown schematically at R) connected to a logic high voltage VCC to establish a normally high state on the long lines. Lines 5-14 consist of bidirectional general interconnect (BGI) segments which are coupled through switching matrices and segment boxes as described below. Lines 18-25 are uncommitted long lines which run the entire length of the array. The long lines 15-17, the uncommitted long lines 18-25, and the BGI segments 5-14 could also have pull up resistors establishing a normally high state, if desired. Fig. 4 illustrates the notation used for the horizontal buses. Each horizontal bus is a 23 line bus in which lines 1-4 and 15 are long lines. Lines 1-4 may be coupled to pull up resistors R according to the present invention. Lines 5-14 are BGI segments, and lines 16-23 are uncommitted long lines. As with the horizontal buses, lines or segments other than long lines 1-4 could be coupled to pull up resistors, if desired. Long lines are characterized by extending across the array, and being coupled through programmable interconnect points to specific inputs and outputs of the configurable logic cells and input/output cells and to lines on intersecting buses. The BGI segments are characterized by having terminations in segment boxes or switching matrices, rather than extending across the entire array, as well as by being coupled through programmable interconnect points to specific inputs and outputs of the configurable logic cells and input/output cells. The uncommitted long lines are characterized by extending across the array and by being coupled through programmable interconnect points to specific outputs of configurable logic cells and input/output cells, to BGI segments, and to long lines; and no connections committing the lines to specific inputs of configurable logic cells or input/output cells. In order to construct networks through a device, the horizontal and vertical buses require means of interconnection. This occurs at the intersections of the horizontal buses and the vertical buses. The interconnections between the lines at the intersection are made through programmable interconnect points, switch matrices, and segment boxes. III. Configurable Logic Block and Tristate Output Connections The configurable logic block 500 shown in Fig. 5 consists of a combinational function and control generator 501 which receives inputs from four sides, schematically illustrated by buses 502-1, 502-2, 502-3, and 502-4. The combinational function and control generator 501 communicates with four independently configurable output ports (macrocells) 503-1, 503-2, 503-3, and 503-4. The output ports receive signals and supply feedback signals to and from the combinational function and control generator 501 across respective buses 504-1, 504-2, 504-3, and 504-4. Each output port supplies a plurality of output signals, schematically illustrated by the respective output buses 505-1, 505-2, 505-3, and 505-4 to the configurable interconnect structure. The block diagram of Fig. 5 illustrates at a high level the symmetry of the configurable logic block 500. Input signals can be received from all four sides of the block, likewise, output signals can be supplied to any of the four sides of the block. Furthermore, as seen below, input signals from the input bus 502 can be used to generate output signals across bus 505-1, 505-2, 505-3, or 505-4. Similar flexibility is provided from all of the other input buses in the configurable logic block. Fig. 6 shows the programmable connections of the outputs Y1 through Y4 to the long lines and BGI. The outputs Y1 through Y4 are also connected to the uncommitted long lines as shown in Fig. 7. Also, the outputs are coupled differently to the vertical bus 1 and horizontal bus 1, vertical bus 9 and horizontal bus 9 as it relates to the long lines 1-4 in the respective buses. Fig. 6 shows that the output Y1 is coupled to PIPs associated with long lines 3, 4, and 15, and BGIs 5, 9, 13, and 14 in HBUS i. The output Y2 of CLB CiRi is coupled to VBUS i+1 long lines 1, 2 and 15, and BGIs 5, 7, 11, and 14. Output Y3 of CiRi is coupled to HBUS i+1 long lines 1, 2, and 15, and to BGI lines 5, 8, 12, and 14. The output Y4 of CiRi is coupled to VBUS i long lines 3, 4, and 15, and to BGI 5, 6, 10, and 14. The connection of the configurable logic blocks to the uncommitted long lines is shown in Fig. 7. Each CLB, such as CLB R3C4, has outputs Y1 through Y4 coupled to one uncommitted long line each. The connections will not be recited because they are shown in Fig. 7. In Fig. 7, only lines 18-25 of the vertical buses, and lines 16-23 of the horizontal buses are shown, because these are the only uncommitted long lines. In order to provide an example for reading Fig. 7, the CLB R3C4 output Y1 is coupled to uncommitted long line 21 of HBUS 3. The output Y2 of R3C4 is coupled to uncommitted long line 23 of VBUS 5. The output Y3 is coupled to uncommitted long line 21 of HBUS 4. The output Y4 is coupled to uncommitted long line 23 of VBUS 4. Note that the uncommitted long lines do not have programmable connections to inputs of CLBs. The selection of the connections of the outputs of the uncommitted long lines has been carried out to achieve a distributed uniform pattern that facilitates programming of nets through the array. IV. CLB Output Macrocells The output macrocells for the configurable logic block are shown in Figs. 8-11. The input signals to these macrocells are generated within the configurable logic cell in response to inputs to the cell from the configurable interconnect and the program data in the configuration memory. The macrocell in Fig. 8 supplies signals to outputs X1 and Y1 of the configurable logic block. Inputs to the macrocell include FC1, FE1, H and FD1 generated within the CLB. The inputs FC1, FE1 and H are coupled to multiplexer 800. The output DQ1 of multiplexer 800 is supplied as a D input to register 801. The output Q1 of register 801 is coupled as an input to multiplexer 802. Two additional inputs to multiplexer 802 include FC1 and FE1. The output of multiplexer 802 is coupled to line 803. Line 803 supplies the signal QF1 as feedback to the combinational logic. Likewise, it is coupled directly to an output buffer 804 for driving the output signal X1 for the direct connect. Signal 803 is also coupled to multiplexer 805. The second input to multiplexer 805 is a signal FD1. The output TY1 of multiplexer 805 is coupled to a tristate output buffer 806. The output of buffer 806 is the Y1 signal for connection to the interconnect structure as shown in Figs 6 and 7. The tristate buffer 806 is controlled by the control signal OE1 generated within the configurable logic block as described below with reference to Figure 12. Fig. 9 shows the macrocell supplying signals to the outputs X2 and Y2. The inputs to macrocell 2 in Fig. 9 include FC2, FE2, H, and FD2 generated within the CLB. FC2, FE2, and H are supplied through multiplexer 900 to generate the signal DQ2. DQ2 is supplied to register 901. The output Q2 of register 901 is supplied as an input to multiplexer 902. Other inputs to multiplexer 902 include FC2 and FE2. The output QF2 of multiplexer 902 is supplied on line 903 as feedback and directly to output buffer 904 supplying the signal X2 to the direct connect. The signal on line 903 is also supplied to multiplexer 905. The second input to multiplexer 905 is the signal FD2. The output TY2 of multiplexer 905 is supplied as an input to tristate output buffer 906, which drives the signal Y2. Tristate buffer 906 is controlled by control signal OE2. The output macrocell of Fig. 10 drives signals to outputs X3 and Y3. Its inputs include the signals FC3, FE1, D1, H and FD3 generated within the CLB. The inputs FC3, FE1, and D1 are coupled through multiplexer 1000 to supply the signal DQ3. Signal DQ3 is coupled to register 1001. The output Q3 of register 1001 is supplied as an input to multiplexer 1002. Two other inputs to multiplexer 1002 include FC3 and H. The output QF3 of multiplexer 1002 is supplied on line 1003 as feedback and directly to the buffer 1004 which drives the signal X3. Also, the signal on line 1003 is supplied to multiplexer 1005. The second input to multiplexer 1005 is signal FD3. The output TY3 of multiplexer 1005 is supplied to the tristate buffer 1006 driving the signal Y3. The tristate buffer 1006 is controlled by the signal OE3. The output macrocell for the driving signals to outputs X4 and Y4 is shown in Fig. 11. It is similar to the macrocell of Fig. 10. The input signals include FC4, FE2, D2, H, and FD4 generated within the CLB. The signals FC4, FE2 and D2 are supplied through multiplexer 1100 to supply the signal DQDQ4. Signal DQ4 is supplied through register 901 to generate the output signal Q4. The output signal Q4 is supplied to multiplexer 902. Other inputs to multiplexer 902 include FC4 and H. The output of multiplexer 902 is the signal QF4 on line 1103 which is supplied as feedback and is coupled to buffer 1104 to drive the signal X4. The signal on line 1103 is also supplied to multiplexer 1105. A second input to multiplexer 1105 is the signal FD4. Multiplexer 1105 generates a signal TY4 which is coupled to the tristate buffer 1106. Tristate buffer 1106 is controlled by the signal OE4 and drives the output Y4 of the configurable cell. Note that the macrocells of Figs. 10 and 11 provide for utilization of the registers 1001 and 1101 even if they are not used for driving the output of the combinational logic. This is provided by allowing the inputs D1 and D2 of the CLB to be directly coupled to the registers in the output macrocells. Although not shown in Figs. 8-11, each register includes a clock, clock enable and reset control. Furthermore, each of the multiplexers shown in the figures, unless a dynamic control signal is explicitly shown, is controlled by memory cells in the configuration program. Thus, the configuration of the macrocells is set during programming of the device. Note also that the macrocell allows the output X1 and the output Y1 to be driven from different sources at the same time. This gives the configurable logic block the ability to produce up to eight outputs at a time. The Y1 through Y4 signals are each applied to drive eight bus lines in the interconnect through PIPs, one of which is an uncommitted long line. The outputs X1 through X4 provide a high speed signal path to adjacent and next adjacent configurable logic cells or input/output cells in the array. V. Output Enable Generator Figure 12 illustrates the circuit for generating the output enable signals 0E1 through 0E4. In Figure 12, the tristatable output buffers 806, 906, 1006 and 1106, generating the outputs Y1 through Y4, respectively, of the macrocells shown in Figures 8-11, respectively, are illustrated. The circuit generating the output enable signals is based on a plurality of selectors, 1201, 1202, 1203 and 1204, which supply, at their respective outputs, the signals 0E1, 0E2, 0E3 and 0E4 to the tristatable output buffers. Each of the selectors, 1201, 1202, 1203, 1204, includes a plurality of inputs and supplies a signal from one of the plurality of inputs to its output in response to program data in the configuration memory. (not shown). The inputs to the selectors include a first signal on line 1205 supplying a logic low level and a second signal on line 1206 supplying a logic high level. Also, each of the selectors 1201-1204 is connected to a common 0E control line 1207. The signal on the common 0E control line is supplied at the output of selector 1208 which is controlled in response to control signal CT5 and CT8 which are generated in the configurable logic cell in response to inputs from the configurable interconnect and to the program data in the configuration memory. The inputs to the selector 1208 are 4 bits of RAM 1209 of the configuration memory. The fourth input to each of the plurality of selectors 1201-1204 is supplied to enable the tristatable buffer to operate as a connection to a wired-AND line. This fourth input for selector 1201 is supplied at the output of invertor 1210. The input of invertor 1210 is connected to receive the signal TY1, which is also supplied as input to the tristatable output buffer 806. The fourth input to selector 1202 is supplied at the output of invertor 1211, which receives as input the signal TY2, which is also connected as input to the tristatable buffer 906. The fourth input of the selector 1203 is supplied at the output of invertor 1212 which receives as input the signal TY3 which is also connected as input to the tristatable buffer 1006. The fourth input to selector 1204 is supplied at the output of invertor 1213 which receives as input the signal TY4 which is also connected as input to the tristatable buffer 1106. The circuit supplying the output enable signals, shown in Figure 12, provides the ability to individually configure the output macrocells in each configurable logic cell in the array. Each of the cells, in response to the program data controlling the respective selectors, can operate in one of four states. In the first state, the tristate buffer is permanently enabled. In the second state, the tristate buffer is permanently tristated. In the third state, the tristate buffer is dynamically controlled in response to the common 0E control signal on line 1207. In the fourth state, the tristate buffer operates for coupling the signal TY1 to the configurable interconnect in a wired-AND configuration. The wired-AND configuration operates by connecting the output, such as Y1, to one of the long lines having a normally high state due to pull up resistors. The wired-AND configuration is accomplished by coupling a plurality of logic cells to a single line having normally high state with the tristatable output buffer coupled in the wired-AND configuration. In this case, if the input TY1 is high, the inverse of TY1 is selected as the output enable signal 0E1 and tristates the output buffer. Because the line is in a normally high state, the tristated buffer has the same effect on the line as supplying a logic 1. If a logic 0 is supplied at the signal TY1, the inverse is supplied as signal 0E1, enabling the output buffer 806. This presents the logic 0 on the output Y1 and pulls down the line. Thus, the wired-AND configuration accomplishes the AND logic function for a potentially large number of outputs from the configurable logic cells, without the use of an additional gate. Other wired-logic functions, such as wired-NAND or wired-OR, could be implemented as well using other techniques. VI. Conclusion The foregoing description of preferred embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims.

Claims: An integrated circuit comprising: an array of programmably configurable logic elements (500); a programmably configurable interconnect structure (HBUS1....HBUS9, VBUS1..... VBUS9) including a plurality of conductive longlines (Y1, Y2, Y3, Y4), said plurality of configurable logic elements being coupled to the interconnect structure, each member of at least a subset of plurality of configurable logic elements comprising: input means (502/1-4) for receiving a set of input signals from the interconnect structure; programmably configurable logic means (501) coupled to the input means for generating a set of output signals (504/1-4) in response to a first subset of input signals; and output means (503/1-4), coupled to the logic means for outputting one of said output signals to a corresponding conductive longline, each output means including a tri-state output buffer (806) for outputting, when enabled, a selective one of said output signals to a corresponding conductive longline, said output means being provided in a wired-logic configuration; wherein said interconnect structure includes pull-up resistors (R) connected to a subset of the conductive longlines to provide a normally logic high state on said subset of the conductive longlines. An integrated circuit as claimed in claim 1 wherein each given output means includes output-enabling means (1201, 1210) for selectively enabling and disabling the corresponding tri-state output buffer (806) such that a wired-logic function may be carried out on the corresponding, pulled-up longline, the wired logic function being that of the output signal (Y1) output by the given output means and that of an output signal (Y1') output by another of the given output means. An integrated circuit as claimed in claims 1 or 2 in which said interconnect structure comprises a plurality of buses each extending across said array and each including a conductive longline. An integrated circuit as claimed in claim 3, wherein at least two of the said buses extend in different directions (H,V) and said pull-up means connects to the longlines in said at least two buses that extend in different directions. An integrated circuit as claimed in claim 1, wherein said output enabling means includes: a first multiplexer (1201), having a plurality of inputs (1205-1207) and an output (OE1); an inverter (1210) having an input coupled to an output (TY1) of the tri-state output buffer and further having an output coupled to the first input of the first multiplexer. An integrated circuit as claimed in claim 5, wherein the output means of each member of the subset of configurable logic elements comprises: a second multiplexer (1208) having an output coupled to a second input (1207) of the first multiplexer, said second multiplexer having a plurality of inputs each respectively receiving one of a set of configuration bits provided by a configuration defining means (1209), said second multiplexer generating a common output enable signal (1207) in response to a supplied set of control signals (CT5,CT6) and program data in the configuration memory. An integrated circuit as claimed in claim 6, wherein a third (1205,1206) of said plurality of inputs of the first multiplexer (1201) connects to a first constant logic level signal (VCC,VSS). An integrated circuit as claimed in claim 7, wherein a fourth (1205,1206) of said plurality of inputs of the first multiplexer connects to a second constant logic level signal that is opposite said first constant logic level signal. An integrated circuit as claimed in claim 1, wherein the output means includes a plurality of inverting means each connected to the input of a respective tri-state output buffer for inverting signals on the input of the respective tri-state buffer to generate a signal for the output-enabling means.

Inventors: AGRAWAL OM PRAKASH, WRIGHT MICHAEL JAMES, AGRAWAL, OM PRAKASH, WRIGHT, MICHAEL JAMES
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 1991
Main classification: H03K19
IPC codes: H01L21, H03K19, G06F13