Abstract:
A set of logic elements can be configured as a cascadable shift register. In one embodiment, a logic element for an FPGA can be configured as any one of a random access memory, a cascadable shift register and a lookup table. The data-in path to the shift register includes a cascade multiplexer for optionally forming large shift registers using multiple logic elements. Each logic element includes a plurality of memory cells which are interconnected such that the data output of each memory cell can serve as the input to the next memory cell, causing the logic element to function as a shift register. The cascade multiplexer allows the last bit of one logic element to be connected to the first bit of the next logic element, bypassing any decode logic of the lookup table. Variable tap shift registers of arbitrary length can be created by cascading lookup tables of plural logic elements in series. The lookup table decode logic plus additional multiplexers can be used to select any memory cell (not necessarily the last memory cell) of the shift register.

Description:
RELATED APPLICATION 
     This application is divisional application of U.S. patent application Ser. No. 09/253,313 filed Feb. 18, 1999 now U.S. Pat. No. 6,118,298, which is a continuation-in-part of application Ser. No. 08/754,421 filed Nov. 22, 1996, now U.S. Pat. No. 5,889,413 issued Mar. 30, 1999, the contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to means and methods of customizing reprogrammable logic functions in an integrated circuit logic device. 
     BACKGROUND OF THE INVENTION 
     Xilinx, Inc. the assignee of the present application, manufactures FPGAs, the complexity of which continues to increase. Freeman in U.S. Pat. No. Reissue 34,363, incorporated herein by reference, which is a re-issue of original U.S. Pat. No. 4,870,302, describes the first FPGA. An FPGA is an integrated circuit chip which includes a plurality of programmable input/output pads, a plurality of configurable logic elements, and a programmable interconnect structure for interconnecting the plurality of logic elements and pads. Each logic element implements a logic function of the n inputs to the logic element according to how the logic element has been configured. Logic functions may use all n inputs to the logic element or may use only a subset thereof. A few of the possible logic functions that a logic element can be configured to implement are: AND, OR, XOR, NAND, NOR, XNOR and mixed combinations of these functions. 
     One disclosed implementation of the logic element includes a configurable lookup table which is internal to the logic element and which includes 2 n  individual memory cells, where n is the number of input signals the lookup table can handle. At configuration, in this architecture a bitstream programs the individual memory cells of the lookup table with a desired function by writing the truth table of the desired function to the individual memory cells. Although the programming is described as being performed serially, other techniques for parallel programming are also known. 
     One memory cell architecture appropriate for use in the lookup tables is shown in FIG.  1  and described by Hsieh in U.S. Pat. No. 4,821,233, incorporated herein by reference. A memory cell of this architecture is programmed by applying the value to be written to the memory cell on the data input line, “Data,” and strobing the corresponding address line, “ADDR.” Further, although this architecture uses five transistors, other known configurations, e.g., six transistor static memory cells, also are appropriate choices for implementing the memory cells of the lookup table. As shown in FIG. 1, inverter  726  may be included to increase the drive of memory cell  700 . 
     After configuration, to use a lookup table, the input lines of the configured logic element act as address lines which select a corresponding memory cell in the lookup table. For example, a logic element configured to implement a two-input NAND gate would output the corresponding value {1, 1, 1, or 0} contained in the one of the four memory cells corresponding to the current input pair {00, 01, 10, 11}, respectively. 
     This selection is performed by a decoding multiplexer which selects a memory cell from the lookup table on the basis of the logic levels of the input lines. A block diagram of an exemplary four-input lookup table composed of 16 memory cells  700   1  through  700   16  and a decoding multiplexer  200  is shown in FIG.  2 . The multiplexer propagates a value stored in one of the memory cells  700   1 - 700   16  of the lookup table to an output X of the lookup table as selected by the four input signals F 0 -F 3 . 
     FIG. 3 is a schematic diagram of another embodiment of a lookup table. In this embodiment, the lookup table is implemented using four memory cells  700   1 - 700   4  and a two-input decoding multiplexer  200  with two input signals, F 0  and F 1 . The two-input decoding multiplexer  200  is shown in detail as being implemented by a hierarchy of pass transistors which propagate the value stored in the selected memory cell to the output X of the logic element. In FIG. 3, the memory cells may be implemented as shown in FIG.  1 . 
     The above architecture was later augmented to enhance the functionality of the lookup tables. U.S. Pat. No. 5,343,406 to Freeman et al., incorporated herein by reference, describes how additional circuitry can enable lookup tables to behave as random access memories (RAMs) which can be both read and written after configuration of the logic device. When the option of allowing the user to write data to memory cells is available, there also must be provision for entering the user&#39;s data into these memory cells and reading from the memory cells. This capability is provided by including two means for accessing each dual function memory cell, one which is used to supply the configuration bitstream from off the chip, and another which is used during operation to store signals that are routed from the interconnect lines of the FPGA. FIG. 4 shows the memory cell architecture described in U.S. Pat. No. 5,343,406 which allows memory cell  750  to be programmed both during and after configuration. During configuration, memory cell  750  is programmed using the same process for programming the memory cell of FIG.  1 . 
     After configuration, memory cell  750  is programmed differently. A value to be written to memory cell  750  is applied through the interconnect structure of the FPGA to the second data line  705 , and then the corresponding write-strobe line WS for the memory cell is pulsed. This pulse latches the value on line  705  into memory cell  750 . Like the lookup table of FIG. 2 which uses a series of memory cells from FIG. 1, a series of memory cells from FIG. 4 are combinable into a lookup table. 
     FIG. 5 is a block diagram showing a four-input lookup table with synchronous write capability. There is a write strobe generator  504  which receives a clock signal, CK, and a write enable signal, WE, and creates a single write strobe signal, WS, for the lookup table. To write a value to a desired memory cell, say  750   5 , the value is applied on line D in  and the address of the desired memory cell  750   5  is applied to the input lines F 0 -F 3  of demultiplexer  500 . The value then is latched into the desired memory cell  750   5  by pulsing the write strobe. Conversely, to read a value stored in a different desired memory cell  750   3 , the address of the memory cell  750   3  is applied to the input lines F 0 -F 3  of decoding multiplexer  200  (without pulsing the write strobe), as was described with reference to FIGS. 2 and 3. 
     FIG. 6 is a schematic illustration of a two-input lookup table with synchronous write capability. FIG. 6 includes four memory cells  750   1  through  750   4 . Detail of demultiplexer  500  and multiplexer  200  is shown in FIG.  6 . 
     The implementation and operation of other logic array devices are described in “The Programmable Logic Data Book,” pages 4-1 to 4-372, copyright 1996 by Xilinx, available from Xilinx, Inc., 2100 Logic Drive, San Jose, Calif. 95124. This portion of “The Programmable Logic Data Book” is incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     The present invention provides means and method for programming a configurable logic element so that the logic element can implement any one of a shift register and a combinatorial logic function using a lookup table. In one embodiment, the invention further provides for implementing a random access memory in this same logic element. The lookup table includes a plurality of memory cells which are connected in series so that an output of a first memory cell is configurable as an input to a second memory cell of the same lookup table. Further, by connecting shift registers of plural logic elements in series, larger shift registers can be built from smaller shift registers. Previous architectures built n-bit shift registers out of n flip flops connected in series, thereby wasting interconnect resources and logic while achieving mediocre performance. 
     In one mode, the memory cells which store the lookup table values are used as registers in a shift chain. When the logic element is in shift register mode, the Data-in value is shifted into the first cell and the value in each memory cell is shifted to the next cell. When the logic element is in random access memory mode, the Data-in value is written to a cell addressed by F 3 -F 0 , as discussed above. When the logic element is in pure lookup table mode, no value can be written after configuration and the logic element continues to generate the function loaded in during configuration. 
     According to another aspect of the invention, shift registers formed in a single lookup table can be cascaded together through cascade multiplexers to form larger shift registers. Each cascade multiplexer receives two input signals, the output signal from the last memory cell in a previous lookup table, and an input signal from the interconnect structure (or other selectable source). The output signal from the cascade multiplexer provides the input signal to the first memory cell in the next lookup table. 
     According to yet another aspect of the invention, a hierarchy of multiplexers is provided to generate functions of more inputs than the lookup table can handle. For example, a lookup table having 16 memory cells can generate functions of four input signals. By combining the outputs of two lookup tables in a multiplexer (F 5 ) controlled by a fifth input signal, any function of five input signals can be generated. Using a sixth signal to select between the outputs of two such F 5  multiplexers allows any function of six input signals to be generated, and so forth. In one embodiment, a configurable logic block (CLB) includes four slices, each having two four-input lookup tables (a total of eight lookup tables). The multiplexer hierarchy allows for all functions of eight input signals to be generated by selecting the output signal of one of the 16 lookup tables in a pair of CLBs. In addition to the eight lookup tables that generate functions of four input signals, the CLB includes four F 5  multiplexers, where each F 5  multiplexer receives input signals from two lookup tables and can generate all functions of five input signals when the two lookup tables receive the same four input signals and the F 5  multiplexer is controlled by the fifth input signal. The CLB also includes two F 6  multiplexers where each F 6  multiplexer receives input signals from two of the F 5  multiplexers. The CLB further includes an F 7  multiplexer which receives the two F 6  signals. The CLB also includes an F 8  multiplexer which receives the F 7  multiplexer output signal and an F 7  multiplexer output signal from an adjacent CLB. 
     In one embodiment, this hierarchy of eight multiplexers is controlled by the same lines that provide shift register input signals. In this embodiment, the eight lookup tables are paired into 4 slices so that the downstream lookup table in each slice receives a shift register input signal on the line that also controls the F 5  multiplexer for the slice. The upstream lookup table of the slice receives a shift register input signal on the line that controls an F 6 , F 7  or F 8  multiplexer. This arrangement is advantageous because the structure can be configured as a variable length shift register, where the line carrying the most upstream signal is used for loading shift register data and the more downstream lines all control multiplexers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a first prior art memory cell architecture used in lookup tables in FPGAs where a value of the memory cell is stored during configuration. 
     FIG. 2 is a block diagram of a prior art programmable 4-input look-up table implemented by a sixteen-to-one decoding multiplexer and a series of sixteen memory cells. 
     FIG. 3 is an expanded view of a schematic illustration of a prior art two-input lookup table and a decoding multiplexer implemented by a hierarchy of pass gates. 
     FIG. 4 is a schematic illustration of a second prior art memory cell architecture used in lookup tables where the value of the memory cell is stored at configuration and remains dynamically readable and writable after configuration. 
     FIG. 5 is a block diagram of a prior art logic element that is configurable to implement either a sixteen-by-one random access memory or a four-input lookup table. 
     FIG. 6 is a schematic illustration of a prior art logic element that is configurable to implement either a four-bit random access memory or a two-input lookup table. 
     FIG. 7 is a schematic illustration of a memory cell architecture according to the present invention which can alternatively be configured as a shift register or a lookup table. 
     FIGS. 7A and 7B are waveform diagrams showing non-overlapping signals Phi 1  and Phi 2  which cause a bit value to shift from a preceding memory cell into the current memory cell when Phi 2  is asserted. 
     FIG. 8 is a block diagram of a logic element according to the invention that can implement either a four-input lookup table or a 16-bit shift register. 
     FIG. 9 is a circuit diagram of a logic element according to the invention that can implement either a 2-input lookup table or a 4-bit shift register, where the mode of the logic element controls the operation of the control logic, and may be stored in configuration memory. 
     FIG. 10 is a schematic illustration of a memory cell for implementing any of a lookup table, a shift register, or a RAM. 
     FIG. 11 is a block diagram of a logic element that is configurable to implement any one of a four-input lookup table, a sixteen-bit shift register, and a sixteen-bit random access memory. 
     FIG. 12 is a schematic diagram of a logic element according to the present invention that is configurable to implement any one of a two-input lookup table, a four-bit shift register, and a four-bit random access memory. 
     FIG. 13 comprising FIGS. 13A through 13H shows waveform diagrams of the operation of the logic element when configured in shift-register mode. 
     FIG. 14 is a block diagram of a logic element which includes both a shift register and a flip-flop. 
     FIG. 15 is a block diagram of an FPGA. 
     FIG. 16 shows a 64-bit variable length shift register formed by combining structures such as shown in FIG.  8 . 
     FIG. 17 shows a 64-bit variable length shift register formed using an architecture with an advantageous modification to the structure of FIG.  8 . 
     FIG. 18 shows a logic slice structure from which the 64-bit variable length shift register of FIG. 17 can be formed. 
     FIG. 19 shows a layout of wiring for cascading adjacent lookup table slices by which interiors of adjacent lookup table slices can be identically laid out. 
     FIG. 20 shows more detail of the structure of FIG. 19, illustrating the lookup table structures. 
    
    
     DETAILED DESCRIPTION 
     With an increase in logic gate density, a shift register can now be implemented as one element of a larger user-configurable integrated circuit logic array. In a first embodiment of the present invention, a logic element is configurable to implement both an n-bit shift register and a (log 2  n)-input lookup table. FIG. 7 shows a schematic illustration of a memory cell  770   2  of the logic element architecture according to the present invention which, when configured to be in shift register mode, advantageously enables a value to be shifted from a preceding memory cell  770   1  into the memory cell  770   2 . Memory cell  770   2  includes a pass transistor  706 . The configuration value is written into memory cell  770   2  by pulsing configuration control line  702  of transistor  706 , while applying the configuration value to the data line  704 . 
     The output of memory cell  770   2  is programmably connected to the input of a next memory cell  770   3  by pass transistors  720   2 , inverter  726   2 , and a next pass transistor  708   3  not shown in FIG.  7 . As shown by the timing diagrams in FIGS. 7A and 7B, during most of each cycle the clocking signal Phi 1  on output control line  724  remains high, and thus the output signal  734   2  of memory cell  770   2  is applied through inverter  726   2  to shift input line  714   2  leading to the next memory cell  770   3 . When Phi 1  goes low at time t 1 , pass transistor  720   2  is turned off. Inverter  726   2  continues for a short time to hold as an output signal the logic level previously asserted by memory cell  770   2 . In this way, the combination of transistor  720   2  and inverter  726   2  serves as a temporary latch. When a second clocking signal, Phi 2 , is asserted at time t 2  on input control line  716 , inverter  701  receives both the output of inverter  703  of memory cell  770   2  and the output of inverter  726   1  of the previous memory cell  770   1 . Each inverter  726  is designed to overpower the inverter  703  so that values can be shifted between adjacent memory cells. Therefore, the current value stored in memory cell  770   2  is overwritten by the output of the previous memory cell  770   1 . When Phi 2  returns low at time t 3 , memory cell  770   2  is once again latched, holding its current value independent of changes in shift input line  714   1 . At time t 4 , Phi 1  goes high, thus applying the new value to inverter  726   2 . Thus in one clock cycle, a bit shifts one cell. In contrast, if Phi 1  and Phi 2  mistakenly overlapped, the value of the output  734  of each memory cell  770  would propagate from preceding memory cell  700   1  through memory cell  770   2  to the next memory cell  770   3 . This would not produce the desired single bit shift. However, by using non-overlapping two-phase clocking, as shown in FIGS. 7A and 7B, the memory cells shift one bit per cycle of Phi 1  and Phi 2 . 
     FIG. 8 shows a logic element which implements a 16-bit shift register and 4-input lookup table according to a first embodiment of the invention. For simplicity, in FIG. 8 the structures within memory cells  770  of FIG. 7 have not been explicitly illustrated. 
     In FIG. 8, when in shift register mode, a first memory cell  770   1  of the memory is programmed with an initial value. The memory cell&#39;s value may be over written with a new value by applying the new value to the D in  terminal of the first memory cell  770   1  and strobing the clock line, CK. The strobing of CK in turn invokes the two-phase clocking cycle of FIGS. 7A and 7B. As data is moved synchronously from left to right in the shift register, i.e., from the first memory cell  700   1  to a last memory cell  700   16 , the logic element can continue to act as a lookup table though the function changes with every clock cycle. As in the prior art lookup tables, the decoding multiplexer  200  outputs on output line X the contents of the memory cell selected by the user inputs, i.e., F 0 -F 3 . 
     FIG. 9 shows a structure for implementing a 2-input lookup table or a 4-bit shift register, and shows internal structure of multiplexer  200  and memory cells  770   1  through  770   4 . FIG. 9 is oriented on the page the same way as FIG. 8, and thus assists in understanding the relationship between the elements that make up the lookup table/shift register embodiment. 
     In a second embodiment of the present invention, a logic element is configurable to implement an n-bit shift register, an n-bit random access memory, and a (log 2 n)-input lookup table. FIGS. 10-12 illustrate this embodiment. FIG. 10 illustrates the memory cell. The memory cell of FIG. 10 can be loaded from three different sources. During configuration, memory cell  790   2  is loaded by applying configuration data to line  704  and strobing control line  702  of transistor  706 . When memory cell  790   2  is in shift register mode, it is loaded through transistor  708 , as discussed above. When memory cell  790   2  is in RAM mode, it is loaded through demultiplexer  500  on line  705   2 . Write strobe line WS is pulsed, turning on transistor  707 , and thus applying a data signal to node  730 . 
     FIG. 11 shows a logic element which implements any one of a 16-bit shift register, a 16-bit random access memory, and 4-input lookup table according to the second embodiment of the present invention. In this embodiment, a memory cell, say  790   5 , of the lookup table is programmed with an initial value during configuration, as discussed above. Subsequently, the initial value may be replaced in either of two ways, depending on the mode of the logic element: shift or RAM. 
     When the lookup table including memory cells  790  is being used in RAM mode, each memory cell  790  receives its data input on RAM input line  705 . To write to any memory cell  790 , the write strobe line WS pulses, thereby driving the value of Din through demultiplexer  500  into the addressed memory cell via input line  730 . 
     The operation of the logic element in each of these modes is controlled by control logic  1000 . Control bits which specify whether the logic element is in RAM mode, shift mode, or neither are inputs to control logic unit  1000 . Control logic unit  1000  also receives the user clock signal and the write enable signal. From these inputs, control logic unit  1000  outputs Phi 1 , Phi 2  and write strobe signal WS to either shift data between memory cells, to write to a particular memory cell, or to leave the memory cell data untouched. When in shift register mode, as in FIG. 8, data is moved synchronously from left to right in the shift register, i.e., from the first memory cell  790   1  to a last memory cell  790   16 , as described above, by invoking a two-phase clocking cycle when CK is strobed. On the other hand, when the logic element is configured as a random access memory (RAM), the addressing lines F 0 -F 3  select one of the memory cells ( 790   1  through  790   16 ) to be written to and read from by using the demultiplexer  500  and the decoding multiplexer  200 , respectively. When in shift register mode, the first memory cell  790   1  receives as its input the signal applied to line D in . When in RAM mode, memory cell  790   1  receives an input signal on line  705   1  from demultiplexer  500 . 
     In RAM mode, to write to a given memory cell, say  700   5 , the write enable line WE must be active. When the user clock signal CK is asserted in conjunction with the active WE signal, control logic unit  1000  generates a write strobe WS. When the write strobe WS is high, memory cell  700   5  addressed by address lines F 0 -F 3  of the demultiplexer  500  receives the value from data input line D in . This value overwrites the previous contents of the memory cell  700   5 . No other memory cells receive the value applied to D in  since they are not addressed and therefore separated from D in  by high impedance connections from the demultiplexer  500 . 
     FIG. 12 is a schematic illustration which shows more detail of a logic element according to the second embodiment of the present invention. Collectively, demultiplexer  500 , decoding multiplexer  200 , pass transistors  708  and  720 , inverters  726 , and RAM mode pass transistors  707  form an interconnection network and are combined with memory cells ( 790   1  through  790   4 ) and control logic unit  1000  to implement the logic element according to the second embodiment. If the logic element of the second embodiment is not configured as a shift register, then the logic element acts as either a random access memory or a lookup table. In either non-shift register mode, Phi 2  is maintained at a low level, deactivating pass transistors  708 , thereby blocking data from one memory cell  790   i  from affecting the next memory cell  790   i+1 . Also, in the non-shift register modes, Phi 1  is maintained at a high logic level, thereby feeding the outputs of the memory cells ( 790   1  to  790   4 ) through to the decoding multiplexer  200 . As before, the output of the logic element is selected by the decoding multiplexer  200  according to the user inputs F 0  and F 1 . 
     When the logic element of FIG. 12 is configured as a shift register, the RAM mode pass transistors  707  are turned off because WS is held low, isolating the memory cells from the outputs of demultiplexer  500 . Memory cell  790   1  is programmably connected to D in  through transistor  708   1 . To shift values, control logic unit  1000  produces control signals Phi 1  and Phi 2 , triggered while the write enable signal is active by a rising edge of the User Clock signal CK applied to control logic unit  1000  such that values are shifted from one memory cell to next memory cell, i.e., from memory cell  790   i−1  to memory cell  790   i , and from memory cell  790   i  to memory cell  790   i+1 . When control logic unit  1000  receives a rising edge of the user clock signal, control logic unit  1000  first pulls Phi 1  low, then pulses Phi 2  high long enough to overwrite the contents of the memory cells ( 790   1  to  790   4 ), and lastly reasserts Phi 1  after Phi 2  has fallen. It is important for extremely low clocking frequencies that Phi 2  be only a pulse since Phi 1  must be off while Phi 2  is on. To accomplish this, the control logic is designed so that Phi 1  and Phi 2  do not rely on the falling edge of the User Clock signal  1008 , but rather are self-timed. 
     FIG. 13 comprising FIGS. 13A through 13H are waveform diagrams of the operation of the logic element of FIG. 12, When the logic element of FIG. 12 is configured in shift-register mode, setting F 1  to  1  and F 0  to  0  makes it function as a three-bit shift register. As shown in FIG. 13E, the input, D in , to the three-bit shift register is maintained continuously at a high logic level throughout the example. Upon receiving a rising edge  1104  of a first user clock pulse  1108 , control logic unit  1000  pulls Phi 1  to a low logic level, as shown in FIG. 13G, to deactivate pass transistors  720  (FIG.  12 ). After temporarily having isolated the outputs  734   1  through  734   4  of the memory cells ( 790   1  through  790   4 ) from inputs of inverters  726   1  through  726   4 , the control logic unit  1000  asserts Phi 2 , which propagates outputs of inverters  726   1  through  726   4  to their corresponding next memory cells, i.e., memory cells  790   2  through  790   4 . When Phi 2  is asserted, the value on D in  is written to first memory cell  790   1 . The non-overlapping Phi 2  pulse is shown in FIG.  13 F. As shown in FIG. 13D, the value stored in first memory cell  790   1  (corresponding to  734   1 ) changes shortly after Phi 2  is asserted. This change is indicated by reference  1112 . The new value of output  734   1  of the first memory cell  790   1  does not affect the second memory cell  790   2  (corresponding to  734   2 ) because Phi 1  is temporarily inactive. After asserting Phi 2  long enough for the memory cells ( 790   1  to  790   4 ) to reach their new states, Phi 2  is lowered, thereby latching the data values. Only after Phi 2  has been lowered does control logic unit  1000  raise Phi 1 . 
     On receiving the rising edge of Phi 1 , the values of outputs  734   1  through  734   4  again pass through pass transistors  720   1  through  720   4 . Reference numeral  1116  shows that the change in the output X of the three-bit shift register is synchronized with the rising edge of Phi 1 . As seen in FIGS. 13G and 13H, the reassertion of Phi 1  and the lowering of the User Clock are independent, thus logic designers need not depend on exact timing relationships between these two edges. Of course, Phi 1  must be reasserted before the inputs of inverters  726   1  through  726   4  float to an invalid voltage. 
     FIG. 14 is a block diagram of a logic element which includes both a logic element  1200  and a flip-flop  1204 . The purpose of the flip-flop is to improve the clock-to-out delay of the output of the logic element  1200 . This is simple and efficient in Xilinx FPGAs because function generators are historically paired with flip-flops in Xilinx logic elements. Further, when an n-bit, synchronous shift register is required, the logic element can be configured so that the shift register  1200  is an (n−1)-bit shift register and flip-flop  1204  is the final register of the n-bit shift register. When configured in this alternative fashion, the final bit XQ is available upon the rising edge  1104  of the User Clock pulse  1108 , rather than on the rising edge  1116  of Phi 1 . This provides a faster clock-to-out time for the overall n-bit shift register. 
     By configuring the logic element to route XQ back to D in , the present invention can also perform circular shifts. 
     As discussed above (FIGS.  13 A- 13 H), a shift register having fewer stages than the number of memory cells in a lookup table can be formed by directing a bit other than the last bit to output terminal X. Lookup tables likewise may be cascaded to create shift registers of a greater size than supported by a single lookup table. For example, it is possible to create a 20-bit shift register in a logic array composed of 16-bit lookup tables by cascading two logic elements. A first full 16-bit shift register  1200  and a second full 16-bit shift register  1200  combine to produce a 32-bit shift register. Thus, to achieve a 20-bit shift register, user input lines F 0 -F 3  of the first logic element are set to 1111 and user input lines F 0 -F 3  of the second logic element are 0011, i.e., the second 16-bit shift register  1200  is programmed to pass the output of the fourth memory cell  790   4 , which is the final output of the 20-bit shift register. Additionally, in order to improve the clock-to-out delay of the cascaded shift registers, an alternate embodiment uses a first full 16-bit shift register  1200  addressed to 1111, a second full 16-bit shift register  1200  addressed to 0010 and the flip-flop  1204  . The output, X, of the second shift register feeds the input of flip-flop  1204  of the second shift register. If desired, the flip-flops  1204  can also be used to extend the number bits that can be shifted within a logic element. Fully utilizing both 16-bit shift registers  1200  and their flip-flops  1204 , cascaded shift registers can be built which are 17-bit, 34-bit, 51-bit, etc. 
     The novel shift register logic element is typically implemented in an FPGA such as the FPGA of FIG. 15 having logic blocks 101, each comprising a portion of an interconnect structure and a logic element. The FPGA of FIG. 15 is further discussed by Tavana et al. in the application Ser. No. 08/618,445 incorporated herein by reference. 
     FIG. 16 shows a 64-bit variable length shift register formed by combining structures such as shown in FIG.  8 . Variable length shift registers are desired when building FIFOs (first-in-first-out storage devices). 
     Conventional FIFOs are commonly composed of a block of RAM addressed by READ and WRITE pointers which each increment through the block and cycle to the bottom upon reaching the top. When a word is written (pushed) into the FIFO, it is written to the address pointed to by the WRITE pointer, and the WRITE pointer is then incremented to point to the next address. When a word is read (popped) from the FIFO, it is taken from the address pointed to by the READ pointer and the READ pointer is incremented to the next address. Thus the data in a RAM based FIFO are never shifted. Rather, the READ and WRITE pointers are incremented independently. 
     In the present case using a shift register, whenever a WRITE command is received, data are always written to one location in a shift register and all other data are shifted one step through the shift register. In response to a WRITE command, a READ pointer is incremented. In response to a READ command, the READ pointer is decremented. There is no WRITE pointer. (The READ address represents the end of the string of stored data.) Such a shift register can be used to implement a variable length FIFO. If a shift register FIFO is desired that is no more than 16 words deep, then such a FIFO can be built in an FPGA using only one lookup table configured as a shift register for each bit of the word to be stored. If a FIFO is desired that can store more than 16 words, a structure such as shown in FIG. 16 must be built for each bit of the word. For example, a 64-word FIFO with 8-bit words would require 8 of the structures shown in FIG.  16 . The structure of FIG. 16 can store up to 64 bits, the DATA bits being written from the left on data input line Din and being read out on the line OUT. 
     However, because the architecture of FIG. 8 provides only a single output from each LUT, (outputs are labeled X and Y), it is necessary to duplicate the data, an upper bank being used to store data for writing to subsequent lookup tables, and a lower bank being used for providing the particular data bit that has been addressed during a READ operation. A long shift register requires that the last sequential bit ( 770   16 ) of each 16-bit shift register be shifted to the first bit of the subsequent shift register, and that every bit be addressable by the READ address applied to the LUT output multiplexers  200 . (If the FIFO is nearly empty, the READ address points to a memory cell near the left of the picture, for example cell  770   1  of LUT-G of slice S 63 . If the FIFO is nearly full, the READ address points to a memory cell near the right of the picture, for example cell  770   16  of LUT-F of slice S 64 .) Data bits are routed from one slice to another using the general interconnect routing lines. (These lines are illustrated using dotted lines to indicate that they are programmably connectable and to distinguish from the routing lines that are part of the slice itself.) 
     Using the architecture of FIG. 8, five slices S 1  through S 5  are used. A slice includes two lookup tables LUT-F and LUT-G, each comprising  16  memory cells  770   1  through  770   16 , a multiplexer  200 -F or  200 -G, four LUT input lines F 1  through F 4  or G 1  through G 4  and a LUT output line X or Y. The slice also includes a clocking structure  800  receiving write enable signal WE, clock input signal CK, and a shift control signal from, for example, a configuration memory cell. Clocking structure  800  generates two non-overlapping clocking signals Phi 1  and Phi 2 , as discussed earlier (See FIGS.  7 A and  7 B). These clocking signals Phi 1  and Phi 2  operate to shift bits to the right in response to clock signal CK when the shift memory cell contains a logic 1 and when the write enable signal WE is logic 1. In order to provide that the last bit  770   16  of lookup table LUT-G of slice S 61  is fed to lookup table LUT-F of slice S 63 , while simultaneously allowing an addressed bit to be read from any of four lookup tables (two in slice S 63  and two in slice S 64 ), it is necessary to duplicate three of the four lookup tables and to configure the lookup tables so that in one lookup table the last bit is always routed out through multiplexer  200 -F or  200 -G to the first bit of the next shift register, and in the duplicate lookup table, the addressed bit is read. Thus, the addressed bit is read from the addressed lookup tables LUT-G of slice S 63 , LUT-F of slice S 63 , LUT-G of slice S 64 , or LUT-F of slice S 64  while the last bit of lookup table LUT-G of slice S 61 , LUT-F of slice S 61 , or LUT-G of slice S 62  is shifted in to the first bit of lookup table LUT-F of slice S 63 , LUT-G of slice S 64  of LUT-F of slice S 64 , respectively, regardless of which address is being read out. Since lookup table LUT-F of slice S 64  is the last in the chain, it is not necessary to form a duplicate in lookup table LUT-F of slice S 62 . (Recall that the data stored in slice S 61  is identical to the data stored in slice S 63 , and the data stored in LUT-G of slice S 62  is identical to the data stored in LUT-G of slice S 64 .) 
     As another aspect of the particular architecture of FIG. 8, discussed by Young, Chaudhary, and Bauer in pending U.S. patent application Ser. No. 08/806,997, the content of which is incorporated herein by reference, multiplexers are included for generating five (F 5 ) and six (F 6 ) input functions by combining the outputs of the four-input lookup tables LUT-F and LUT-G. But in that described embodiment, the same input signal that feeds the Din signal also serves as the control signal on the F 5  multiplexer. Thus, it is not possible to use an address signal for controlling the F 5  multiplexer when also using that signal for supplying data. Thus a fifth slice S 65  is used. The LUT-F and LUT-G lookup tables and an F 5  multiplexer of slice S 65  are configured to implement a four-to-one multiplexer, the output signal from this multiplexer being the addressed bit. 
     FIG. 17 shows a 64-bit variable length shift register formed using an architecture with an advantageous modification to the structure of FIG.  8 . By changing the architecture to add a two-to-one multiplexer to the data input of each shift register and feeding the output signal of the last memory cell of the previous shift register to that multiplexer (in addition to the signal from the interconnect structure that exists in FIG.  8 ), a variable length shift register can be formed using no more than half the number of lookup tables of FIG.  16 . The structure of FIG. 17 is configured as a 64-bit variable length shift register, just as is the structure of FIG.  16 . But since the structure of FIG. 17 includes multiplexers M 71  and M 72  as inputs to the respective lookup table shift registers, each lookup table has both a variable-tap output through multiplexer  200  and a fixed output from cell  770   16 . This is advantageous for making a FIFO because each lookup table now has the two outputs required when cascading together logic elements to build a long variable-tap shift register, so no duplication of logic is required. And the READ address dynamically addresses one of the 64 memory cells via the four lookup table input signals and the F 5  and F 6  multiplexers. Note that using the shift input of the newly added multiplexer M 71  or M 72  allows the BY or BX input of the newly added multiplexer to be used for another function, in this case controlling an F 5  or F 6  multiplexer. 
     FIG. 18 shows a logic slice structure from which the 64-bit variable length shift register of FIG. 17 can be formed, and in particular shows connections of the F 5  multiplexer and another multiplexer labeled FX. A preferred architecture combines four of these slices into one configurable logic block (CLB). The FX multiplexer can be an F 6 , F 7 , or F 8  multiplexer, depending upon the position of the illustrated slice in the CLB, where an F 6  multiplexer selects between outputs of two F 5  multiplexers, an F 7  multiplexer selects from two F 6  multiplexers, and an F 8  multiplexer selects from two F 7  multiplexers. FIG. 18 illustrates that the BX input signal goes two places: to multiplexer M 72  and to the control terminal of the F 5  multiplexer. Similarly, the BY input signal goes to multiplexer M 71  and to the control terminal of the FX multiplexer. Note that the input signals to the FX multiplexer are labeled FXin 0  and FXin 1 . These input signals come from other F 5  or FX multiplexers within the CLB, and they are most conveniently illustrated in FIG.  19 . In a preferred embodiment, a logic slice structure such as that of FIG. 18 will include additional elements, for example flip flops, fast carry circuits, and routing structures (see, for example, U.S. Pat. Nos. 5,267,187 to Hsieh et al., and 5,349,250 to New, as well as U.S. patent application Ser. No. 08/806,997 referenced above). However, to avoid obscuring the present invention, these additional structures have not been shown here. 
     FIG. 19 shows a layout of wiring for cascading adjacent lookup table slices by which interiors of adjacent lookup table slices can be identically laid out and by which a single input line BX or BY can serve a function in an earlier architecture as well as a new function discussed here (so the new architecture discussed here can implement designs that have been implemented in the previous architecture illustrated in FIG.  16 ). FIG. 19 illustrates one configurable logic block (CLB) comprising four slices, each having two lookup tables (LUTs). Each slice is equivalent to that of FIG.  18 . Whereas FIG. 18 shows one F 5  multiplexer and one FX multiplexer (in addition to the two M 71  and M 72  multiplexers discussed earlier), FIG. 19 shows the different interconnections to the FX multiplexer in different parts of one CLB. These wide function multiplexers are now labeled F 6 , F 7 , and F 8  to show the number of input signals they can provide all function of. Thus, the F 8  multiplexer selects from the output signals of two F 7  multiplexers and an F 7  multiplexer selects from two F 6  multiplexers and so on. The lookup tables themselves provide all functions of four input signals. Note that the F 8  multiplexer receives one input signal from the F 7  multiplexer of its own CLB and another input signal from the F 7  multiplexer of an adjacent CLB. Note also that one CLB includes four F 5  multiplexers, two F 6  multiplexers, one F 7  multiplexer, and one F 8  multiplexer. 
     The novel and advantageous placement of these wide function multiplexers always allows the control signal BX or BY to serve the dual function of providing shift-in data and controlling a corresponding multiplexer. This is because only one of the BX or BY terminals will be used for shifting in data to a shift register, and the sharing is arranged so that the highest order multiplexer is placed at the beginning of the shift register for that length. In the case of a 64-bit shift register, two slices will be used (see FIG.  17 ). The address will be six bits long and will use two F 5  multiplexers and one F 6  multiplexer. Looking at FIG. 19, this can be accomplished in either the upper two slices S 3  and S 2  or in the lower two slices S 1  and S 0 . In either case, data will be shifted in on line BY of slice S 3  or S 1 , and multiplexer M 71  of the slice will be set to receive the BY signal. The F 7  or F 8  multiplexer will not be used since the desired output signal is provided by the F 6  multiplexer of slice S 2  or S 0 . Thus there is no conflict that the line used for controlling the F 7  or F 8  multiplexer is used in this case as a data input line to the shift register. 
     If a 128-bit shift register is desired, the entire CLB of FIG. 19 will be used. Data will be shifted in on the BY line of slice S 3  and the output signal will be taken from the F 7  multiplexer. The F 8  multiplexer will not be used. Thus, again, there is no conflict in the fact that the line used for controlling multiplexer F 8  is used to provide data to the shift register. Similarly, if a 256-bit shift register is desired, two CLBs of the type shown in FIG. 19 will be used, data being shifted in to the upper of the two CLBs and the output signal taken from the F 8  multiplexer of the lower CLB. So again there is no conflict. Knowing this relationship, architectures can be provided having longer patterns of multiplexers for providing larger functions. All this is possible because for n-input lookup tables we need (n−1) lines for controlling multiplexers and 1 line for shifting in data to a shift register. The (n−1) multiplexer control signals plus 1 data-in signal exactly match the n lines provided. 
     Shift registers of sizes other than powers of two can also be formed by combining the appropriate number of slices. For example, if a user wanted a 200-bit variable length shift register, this could be implemented in seven slices using 13 LUTs, seven F 5  multiplexers, four F 6  multiplexers, two F 7  multiplexers, and one F 8  multiplexer. The three LUTs not needed in the eight slices that feed the F 8  multiplexer could be used for other functions. To avoid generating an erroneous output signal if one of the unused lookup tables is addressed, the control inputs for the F 5  and F 6  multiplexers associated with partially used slices are preferably tied to a constant value. 
     FIG. 20 shows more detail of the structure of FIG. 19, illustrating the lookup table structures and clocking structures discussed earlier. Since the additional details of FIG. 20 have been discussed earlier, they are not discussed again here. 
     Numerous modifications and variations of the present invention are possible in light of the above teachings. Although FIGS. 7 and 10 show a memory cell programmed through only one node of the latch, the invention can also be used with memory cells in which some data signals are inverted and applied to both nodes of the latch, or in which different control signals are applied to different nodes of the latch. Further, in FIG. 10 the three transistors  706 ,  708 , and  707  can be implemented as a multiplexer receiving input signals on lines  704 ,  714 , and  705 . And transistors  706 ,  708 ,  707 , and  720  can be replaced by transmission gates. While particular multiplexer and demultiplexer implementations are shown, the invention can use other implementations as well. And, of course, different structures and methods for generating signals such as Phi 1 , Phi 2 , and WS can be used with the invention. Further, although the above embodiments show a single multiplexer with a single output terminal for selecting one signal from a plurality of memory cells, other embodiments can select more than one memory cell from which to provide an output signal. And although FIGS. 19 and 20 show a CLB with lookup tables and multiplexers for generating functions of up to 8 input signals, other embodiments can use CLBs with more lookup tables and higher order multiplexers, for example CLBs with 16 or 32 lookup tables with F 9  and F 10  multiplexers. A lookup table can have fewer or more than the 16 memory cells shown. For example, a 6-input lookup table would use 64 memory cells (configurable as a shift register) and the combining multiplexers would start with F 7 . Further, although the cascading aspect of the invention has been discussed in comparison to FIG. 8, this aspect also applies to structures with demultiplexing, such as shown in FIG.  11 . More fundamentally, although the above invention has been described in connection with an FPGA, a shift register with cascade multiplexers can be formed in other structures than FPGAs, and formed not in connection with lookup tables. 
     It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.