Programmable logic device having complex logic blocks with improved logic cell functionality

[0001] 1. Field of the Invention

[0002] The present invention generally relates to Complex Logic Block (CLB) based Programmable Logic Devices (PLDs), and more particularly, CLBs including a plurality of slices, each having one or more logic cells with improved logic, register, arithmetic, logic packing and timing functions and capabilities.

[0003] 2. Description of Related Art

[0004] A Programmable Logic Device (PLD) is a semiconductor integrated circuit that contains fixed logic circuitry that can be programmed to perform a host of logic functions. In the semiconductor industry, PLDs are becoming increasingly popular for a number of reasons. Due to the advances of chip manufacturing technology, application specific integrated circuits (ASICs) designs have become incredibly complex. This complexity not only adds to design costs, but also the duration of time needed to develop an application specific design. To compound this problem, product life cycles are shrinking rapidly. As a result, it is often not feasible for original equipment manufacturers (OEMs) to design and use ASICs. OEMs are therefore relying more and more on PLDs. The same advances in fabrication technology have also resulted in PLDs with improved density and speed performance. Sophisticated programming software enables complex logic functions to be rapidly developed for PLDs. Furthermore, logic designs generally can also be easily migrated from one generation of PLDs to the next, further reducing product development times. The closing of the price-performance gap with ASICs and reduced product development times makes the use of PLDs compelling for many OEMs.

[0005] The architecture of most PLDs defines a two-dimensional array of logic blocks. Row and column inter-logic block lines, typically of varying length and speed, provide signal and clock interconnects between the blocks of logic in the array. In one type of commercially available PLD, the blocks of logic are referred to as Complex Logic Blocks or CLBs. Each CLB includes a pair of slices. The two slices in each CLB do not have direct connections with one another and are organized in different columns. The slices in each column, however, are connected to the slices of the above and below CLBs, creating independent carry chains. For example, the slices in the first and second columns each receive a carry in (Cin) signal from the previous CLB and generate a carry out signal provided to the next CLB respectively.

[0006] Each slice typically includes four logic cells (hereafter simply referred to as “cells”). Each cell includes a 6-input logic function generator, typically referred to as a Look Up Table (LUT), arithmetic circuitry for receiving and generating Cin and Cout carry signals, an output register for generating a registered output, a non-registered output, and storage elements for performing ROM functions. The LUT can be configured as either a single 6-input LUT, or two five input LUTs, both receiving the same set of inputs (e.g., A1-A5).

[0007] For more details on CLB based PLDs having slices, see for example the “Virtex-5 User Guide”, UG190 (v3.0), Feb. 2, 2007, published by Xilinx Corporation, San Jose, Calif., pages 155-181, incorporated by reference herein for all purposes.

[0008] The problem with the aforementioned cells is its lack of flexibility and limitations in performing logic, various register, arithmetic functions and a lack of flexibility for efficiently packing logic functions into the cell.

[0009] A PLD with CLBs defining slices having one or more logic cells with improved logic, register, arithmetic, logic packing and timing functions and capabilities is therefore needed.

[0010] A CLB-based PLD with logic cells having improved logic, register, arithmetic, logic packing and timing functions and capabilities is disclosed. The CLBs of the PLD are arranged in rows and columns of an array and are interconnect by a plurality of interconnect lines. Each of the plurality of CLBs has a first slice of logic cells and a second slice of logic cells arranged in a first column and a second column respectively. First and second carry chains are provided between each of the logic cells of each column of slices. At least one of the logic cells includes one or more Look Up Tables for implanting logic functions on a set of inputs provided to the one logic cell and an arithmetic logic circuit configured to receive a carry-in signal and to generate a carry-out signal forming part of the first carry chain. In one embodiment, the logic cell further includes a first output register and a second output register and the set of outputs generated by the logic cell are partitioned among the first output register and the second output register. In another embodiment, an output of one of the registers is provided as an input to one of the Look Up Tables of the cell through a register feedback connection. In yet another embodiment, the set of inputs provided to a first and a second of the Look Up Tables are different, enabling a higher degree of logic efficiency or “packing” by enabling each cell to perform logic functions on two different sets of inputs as opposed to only the same set of inputs. Finally, in another embodiment, the arithmetic logic circuit is capable of generating two SUM output signals.

[0020] It should be noted that like reference numbers refer to like elements in the figures.

[0021] The present invention will now be described in detail with reference to a various embodiments thereof as illustrated in the accompanying drawings. In the following description, specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without using some of the implementation details set forth herein. It should also be understood that well known operations have not been described in detail in order to not unnecessarily obscure the present invention. Further, it should be noted that the techniques of the present invention could be applied to a variety of systems or electronic devices such as programmable devices and application-specific integrated circuit (ASIC) devices.

[0022] Referring to FIG. 1, a block diagram of a programmable logic device (PLD) 10 of the present invention is shown. The PLD includes a plurality of Complex Logic Blocks (CLBs) 12 arranged in an array of rows and columns. Each CLB 12 includes a first slice 14A and a second slice 14B. The slices 14A of the CLBs 12 in a given column are interconnected by a first carry chain 16A, as designated by the carry-in (Cin) and carry-our (Cout) lines received and outputted by the slices 14A. Similarly, the slices 14B are interconnected by a second carry chain 16B, as designated by the carry-in (Cin) and carry-our (Cout) lines received and outputted by the slices 14B. Although not illustrated, the individual CLBs 12 are interconnected by a plurality of inter-CLB lines, as is well known in the art. For more details on CLB based PLDs having slices, see for example the “Virtex-5 User Guide”, UG190 (v3.0), Feb. 2, 2007, published by Xilinx Corporation, San Jose, Calif., pages 155-181, incorporated by reference herein for all purposes.

[0023] Each slice 14 includes one or more logic cells. In one specific embodiment, each slice 14 includes four logic cells. In other embodiments, either more or fewer logic cells may be used per slice 14. In alternative embodiments for example, one to three, or five through sixteen or more logic cells may be included per CLB 14. In general, the number of specific logic cells in a CLB is arbitrary, and should not be construed as limiting the present invention.

[0024] Referring to FIG. 2, a block diagram of a logic cell 20 according to the prior art is shown. The logic cell 20 includes a six-input Look Up Table 22, an arithmetic logic circuit 24, two output multiplexers 26A and 26B, and an output register 28.

[0025] The six input Look Up Table 22 includes two five-input Look Up Tables 30A and 30B, each configured to receive the same set of inputs D1 through D5, and a Look Up Table output multiplexer 32, which is configured to receive a sixth input D6. With this configuration, the Look Up Table 22 can be configured as either a single six-input Look Up Table capable of generating a single logic function “O6”, or two five-input logic functions designated F1(D1, D2, D3, D4, D5) and F2(D1, D2, D3, D4, D5) respectively.

[0026] The arithmetic circuit 22 is configured to implement arithmetic logic functions, such as counters and shifter registers. The arithmetic circuit 22 is configured to receive a seventh input (DX) and a carry-in (Cin) from the logic cell below. In response, the arithmetic circuit generates a SUM signal and a carry out signal (Cout), which is provided to the cell above.

[0027] The output multiplexer 26A generates a non-registered output (DMUX) selected from inputs (O6, O5, SUM and Cout). The output multiplexer 26B selects from inputs (O6, O5, SUM, COUNT, and DX). The register 28 generates a registered output (DQ) from the input selected by multiplexer 26B. A third output (D) is derived from the Look Up Table output multiplexer 34 (O6).

[0028] FIG. 3 is a logic cell with enhanced register functionality according to one embodiment of the present invention. The logic cell 36 is similar to the cell 20, except it additionally includes a second output register 38, output multiplexer 40, and a scan-chain 42. In the embodiment shown, the output multiplexer 26B is configured to receive (scan-in, O6 and Cout), while the multiplexer 40 is configured to receive (DQ, O5, SUM, and DX). The second output register is configured to generate output (DQ1), the value of which is determined by the selection of multiplexer 40.

[0029] The scan-chain 42 in FIG. 3 allows the output of one register (i.e., 28) to be provided as an input to the next register (i.e., 38). For example in FIG. 3, the register 28 receives the “scan-in” signal from the above cell, whereas the register 38 provides the “scan-out” signal to the next cell.

[0030] The enhanced register functionality of logic cells 36 provides a number of advantages over conventional cells, such as that illustrated in FIG. 2. The scan-chain 42 through the registers 28 and 38 allows greater functionality, such as pipelining and/or the creation of shift registers, between cells. This feature was not possible in the prior cell. The partitioning of the outputs among the two registers 28 and 38 also provides greater flexibility in register packing, which is the ability to efficiently place or “pack” more logic into a given cell.

[0031] It should be noted that this specific partitioning of the output signals generated by the logic cell 36 in FIG. 3 are merely exemplary. The output signals may be partitioned in any arbitrary way among the output registers. Furthermore, the output signals do not have to be limited to those specifically identified above. Rather, the output signals can be selected from generic signals, such outputs of the Look Up Tables of the cell, a sum, carry-out signal, or any other signal generated by the arithmetic circuit 24, or any of the inputs to the logic cell.

[0032] Referring to FIG. 4, a logic cell 56 with register feedback according to another embodiment of the present invention is shown. The logic cell 56 is similar to that of FIG. 2, except it includes a multiplexer 58 and a feedback path 60 between the (DQ) output of the register 28 and the multiplexer 58. In the embodiment shown, the multiplexer 58 receives two inputs, D1 and DQ. Accordingly, the registered output of the cell 56 can be selectively provided as an input to both five-input Look Up Tables 30A and 30B. Although the multiplexer 58 is shown coupled to the input D1, it should be noted that the multiplexer 58 can be used to “swap” the registered output (DQ) for any of the logic cell inputs D1 through D6.

[0033] Pipelining is common in circuit design technique. With pipelining, a complex circuit is broken down or partitioned into a plurality of clocked pipelined stages. At each stage, the circuit performs a specific operation. With each clock cycle, the results of each stage are transitioned to the next stage. The speed (i.e., frequency) or clock cycle of the pipelined circuit is dictated by the longest propagation path between any two stages in the pipeline. For example, if a number of stages have a propagation delay of 5 nanoseconds, but one stage has a delay of 10 nanoseconds, then the entire circuit must operate at the 10 nanosecond rate. Higher speed performance can therefore be achieved if a better balance of the delays at the various stages of the circuit can be achieved.

[0034] Referring to FIG. 5A, a block diagram 65 illustrating how register feedback aids in balancing delays between pipelined stages of a logic circuit according to the present invention is shown. In the diagram, three stages A, B and C are provided. The first stage A has a circuit 68 with a 5 nanosecond delay followed by register 28a. The second stage B has a long wire 69 with a 5 nanosecond delay, a first circuit 68b with a 5 nanosecond delay and a second circuit 68b with another 5 nanosecond delay, followed by a register 28b. The third stage C has two circuits 68c with a combined 10 nanosecond delay and a register 28c. By using the register feedback 60 as illustrated in FIG. 4, the delay of the long wire 69 can be made to occur before (i.e., to the left of) the register 28a, rather than after (i.e., or to the right of the register). This shifting of the delay associated with wire 69 is logically illustrated in FIG. 5B. As a result, the delay at each of the three stages has been balanced with each being 10 nanoseconds. Consequently, as illustrated in this example, the register feedback 60 can be used to improve the clock speed or frequency at which the logic circuit operates, thereby improving performance.

[0035] FIGS. 6A-6E show various embodiments of the logic cell of the present invention having improved logic functionality capabilities. As noted above, the two Look Up Tables in the cell 20 of FIG. 2 have limited flexibility, as each is limited in receiving the same inputs (D1, D2, D3, D4, D5). FIGS. 6A-6E in contrast show various embodiments where different combinations of inputs can be provided to the two five-input Look Up Tables 30A and 30B respectively. This flexibility aid in improving the density or “packing” as more logic can be squeezed into two Look Up Tables 30A and 30B capable of operating on different inputs, as opposed to operating only on the same set of inputs.

[0036] In FIG. 6A, the Look Up Table 30A receives inputs (D1A, D2, D3, D4, D5) and generates function F1 (D1A, D2, D3, D4, D5). Look Up Table 30B receives (D1B, D2, D3, D4, D5) and generates function F2 (D1B, D2, D3, D4, D5). The D1 input to the two Look Up Tables 30A and 30B is therefore different. This arrangement improves flexibility when the two Look Up Tables 30A and 30B are used in a fractured mode. For example, since 30A and 30B are sharing only four inputs, they can be configured as two independent three-input Look Up Tables, or two four-input Look Up Tables, with only two common inputs. It should be noted that the use of different inputs for D1 is only exemplary. Any of the inputs D1 through D5 can be different in accordance with this embodiment.

[0037] In FIG. 6B, the Look Up Table 30A receives inputs (D1A, D2A, D3, D4, D5) and generates function F1 (D1A, D2A, D3, D4, D5). Look Up Table 30B receives (D1B, D2B, D3, D4, D5) and generates function F2 (D1B, D2B, D3, D4, D5). The D1 and D2 inputs to the two Look Up Tables 30A and 30B is therefore different. This arrangement also improves flexibility when the two Look Up Tables 30A and 30B are used in a fractured mode. For example, since 30A and 30B can each implement four-input Look Up Tables with two common inputs. It should be noted that the use of different inputs for D1 and D2 is only exemplary. Any two of the inputs D1 through D5 can be different in accordance with this embodiment.

[0038] FIG. 6C is similar to FIG. 6B with the Look Up Table 30A receiving inputs (D1A, D2A, D3, D4, D5) and Look Up Table 30B receiving inputs (D1B, D2B, D3, D4, D5). In addition, a swap multiplexer 62, AND gate 64 and a programming bit “R” are provided to control the multiplexer 62 and gate 64. The swap multiplexer 62 allows either the input D5 or D6 to be provided to the fifth input of Look Up Table 30A. When the value of the programming bit R is set so that the input D6 is selected by the multiplexer 62, the output of the AND gate will 64 force the multiplexer 32 to select the function F1 from the Look Up Table 30A. Thus, if D6 is used as an input, the Look Up Table 30A will always be selected by multiplexer 32. With the R bit set, Look Up Table 30A generates the function F1 (D6, D3, D2A, D1A) and 30B generates the function F2 (D5, D4, D2B, D1B). When the R bit is reset, then the 30A and 30B implement two five-input Look Up Tables with two common inputs respectively.

[0039] FIG. 6D is similar to FIG. 6B, except it includes a swap multiplexer 68 coupled to receive the DX or D1A inputs. With this arrangement, the Look Up Table 30B receives either DX or D1A, instead of D1B as illustrated in FIG. 6B. By providing the swap multiplexer 68, it allows the DX input to be substituted for the otherwise shared input D1, allowing the 30A and 30B to be fractured into two four-input Look Up Tables. F1 is therefore (D5, D3, D2A, D1A) and F2 is (D5, D4, D2B, DX).

[0040] FIG. 6E is the combination of FIGS. 6C and 6D. With this arrangement, 30A and 30B can be configured as two five-input Look Up Tables with common inputs, or two fractured Look Up Tables generating F1(D6, D4, D3, D2A, D1A) and F2(D5, D3, D4, D2B, DX) respectively.

[0041] FIG. 7 is another embodiment of a logic cell of the present invention having register feedback and an input swap multiplexer. This embodiment is essentially the combination FIG. 5A including register feedback 60 and the swap multiplexer 68 of FIG. 6D. In this case, the swap multiplexer receives three inputs, including D2A, DX and the register feedback 60. With this arrangement, 30A and 30B are capable of generating two 5-LUT functions, the lower of which is able to use the feedback register driven by DX as one of it's inputs.

[0042] FIG. 8A is a cell having an arithmetic logic circuit 24 capable of generating two sum outputs, SUM1 and SUM2. The first SUM1 signal is derived by the addition from two signals from the first Look Up Table 30A, whereas the second SUM2 is generated from two signals from the second Look Up Table 30B. As is well understood in the art, a five-input Look Up Table is essentially two four-input multiplexers and a select multiplexer that is controlled by the fifth input. For this reason, the diagram of FIG. 8 shows two four-input Look Up Tables and a multiplexer for elements 30A and 30B respectively. For the sake of simplicity, the non-registered output and output multiplexer of the cell are not shown.

[0043] FIG. 8B is a logic diagram illustrating the arithmetic logic circuit 24 of FIG. 8A. As illustrated, the SUM1 signal is generated by a first adder 82A from the sum of two four-input Look Up Tables, and the signal SUM2 is generated by a second adder 82b from the sum of a second two four-input Look Up Tables. A carry signal 84 is provided between the two adders 84B and 84A.

[0044] While the invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that changes in the form and details of the disclosed embodiments may be made without departing from the spirit or scope of the invention. It is therefore intended that the invention be interpreted to include all variations and equivalents that fall within the true spirit and scope of the present invention.