Abstract:
In one embodiment of the invention, a programmable logic block within a programmable logic device includes: a plurality of lookup tables, each lookup table providing a combinatorial output signal; and a plurality of registers, each register being adapted to register a selected one of the combinatorial output signals, wherein the number of registers is less than the number of lookup tables.

Description:
TECHNICAL FIELD 
   The present invention relates generally to programmable logic devices, and more particularly to a programmable logic block architecture. 
   BACKGROUND 
   Programmable logic devices such as field programmable gate arrays (FPGAs) include a number of programmable logic blocks that are interconnected by a programmable interconnect, also referred to as a routing structure. Each programmable logic block generally includes a number of lookup tables (LUTs). During FPGA configuration, a user programs the truth table for each lookup table to implement a desired logical function. The core unit of a programmable logic block is a LUT-register combination often denoted as a “logic cell” as seen in  FIG. 1 . A four-input (16 bit) LUT  100  receives LUT inputs A through D from the routing structure (not illustrated). Based upon the truth table programmed into LUT  100  during configuration of the corresponding FPGA, a combinatorial output  105  is “looked up” as determined by the state of inputs A through D. Output  105  may also be provided as a sequential output  110  after registration in a register  120 . Register  120  may also register a data input  125  through appropriate selection in a multiplexer  130 . 
   The core LUT/register logic cell combination discussed with respect to  FIG. 1  may be organized into what is commonly referred to as a “slice.” The bit size of a slice depends upon the number of LUT/register combinations it contains. For example, slice  200  illustrated in  FIG. 2  contains two LUT/register combinations  205   a  and  205   b  and is thus a two-bit slice. Within slice  200 , registers  120  share common set and reset signals  126 . In addition, clock and clock enable signals  210  are also common to both registers  120   a  and  120   b . A multiplexer  140  selects from combinatorial output signals  105   a  and  105   b  (from LUTs  100   a  and  100   b , respectively) to provide an output signal FX 0   160 . Because both output signals  105   a  and  105   b  are “LUT4” outputs in that LUTs  100   a  and  100   b  are 4-input LUTs, FX 0   160  represents a 5-input LUT (LUT5) output signal. As seen in  FIG. 3 , a programmable logic block  300  may include a plurality of slices  200  such as slice- 0  through slice- 3 . The bit size of the slices is arbitrary—for example, rather than use two-bit slices, programmable logic block  300  may include four-bit slices. As known in the art, various interconnections exist amongst slices  200  within programmable logic block  300 . For example, as seen in  FIG. 2 , slice  200  may include a multiplexer  150  that selects from input signals FXA and FXB to provide an output signal FX 1   170 . In addition, a carry chain  180  couples across LUTs  100   a  and  100   b . Carry chain  180  extends across all the LUTs (not illustrated) within slices  200  of programmable logic block  300  as well. To allow the formation of LUT6, LUT7, and LUT8 output signals (corresponding to the output signal of a 6-input LUT, a 7-input LUT, and an 8-input LUT, respectively), output signals FX 1  and FX 0  from each slice may couple back as inputs FXA and FXB in various fashions. For example, in slice  0 , input signal FXA is received as the FX 0  output signal from slice  1  whereas input signal FXB is received as the slice  0  FX 0  output signal. Because each FX 0  output signal may be a LUT5 output signal as discussed with regard to  FIG. 2 , FX 1  from slice  0  is thus a LUT6 output signal. An analogous situation exists for slice  2  in that its FXA input signal is received as the FX 0  output signal from slice  3  whereas its FXB input signal may be the FX 0  output signal from slice  2 . Thus, the FX 1  output signal from slice  2  may be a LUT6 output signal. But note that the FXA input signal for slice  1  is received as the FX 1  output signal from slice  2 . Similarly, the FXB input signal for slice  1  is received as the FX 1  output signal from slice  0 . Thus, the FX 1  output signal from slice  1  is a LUT7 signal. To allow for the formation of a LUT8 output signal, slice  3  receives as its FXA input signal a LUT7 signal cascaded from another programmable logic block (not illustrated). Slice  3  also receives as its FXB input signal the FX 1  output signal (LUT7) from slice  1 . Thus, the FX 1  output signal from slice  3  may be a LUT8 output signal. It will be appreciated that other types of interconnections exist between slices but are not shown for illustration clarity. 
   In certain FPGA designs, the bit size of the slice encompasses the entire programmable logic block in what may be denoted as a block-based approach such that all registers in a block-based programmable logic block receive common control signals. Regardless of the bit size used for the slices, it may be seen from examination of  FIG. 2  that a one-to-one correspondence exists between LUTs  100  and registers  120  within each logic cell. The symmetry resulting from this one-to-one arrangement has obvious advantages such as ease of use. Synthesis, mapper, and placer and router tools have been optimized in view of this one-to-one correspondence. However, it has been observed that a register-to-LUT usage ratio for the vast majority of user designs ranges from 40% to 60%. A fixed one-to-one LUT-register ratio thus often results in a substantial waste of register resources. This waste leads to silicon die inefficiency and thus higher manufacturing costs. 
   Accordingly, there is a need in the art for improved programmable logic block architectures that provide a more efficient use of die area. 
   SUMMARY 
   In accordance with an embodiment of the invention, a programmable logic block within a programmable logic device includes: a plurality of lookup tables, each lookup table providing a combinatorial output signal; and a plurality of registers, each register being adapted to register a selected one of the combinatorial output signals, wherein the number of registers in the plurality of registers is less than the number of lookup tables in the plurality of lookup tables. 
   In accordance with another embodiment of the invention, a programmable logic device includes: a plurality of programmable logic blocks, wherein each programmable logic block includes a plurality of lookup tables, each lookup table providing a combinatorial output signal and wherein each programmable logic block includes a plurality of registers, each register being adapted to register a selected one of the combinatorial output signals, wherein for at least one of the programmable logic blocks the number of registers in the plurality of registers is less than the number of lookup tables in the plurality of registers. 
   In accordance with another embodiment of the invention, a programmable logic block within a programmable logic device includes: a plurality of slices, wherein each slice includes a plurality of lookup tables, each lookup table being adapted to provide a combinatorial output signal, the slices including a plurality of registers adapted to register corresponding ones of the combinatorial output signals, and wherein the number of registers in the plurality of registers is less than the number of lookup tables in the pluralities of lookup tables. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a conventional logic cell architecture. 
       FIG. 2  is a block diagram of a conventional type two-bit slice, which is denoted herein as a type A slice 
       FIG. 3  is a block diagram of a conventional programmable logic block. 
       FIG. 4   a  is a block diagram of a two-bit slice (denoted herein as a type B slice) in which a single register may register either LUT combinatorial output in accordance with an embodiment of the invention. 
       FIG. 4   b  is a block diagram of a two-bit slice (denoted herein as a type C slice) in which a single register may register only a first one of the combinatorial outputs in accordance with an embodiment of the invention. 
       FIG. 4   c  is a block diagram of a two-bit slice (denoted herein as a type D slice) that does not include a register in accordance with an embodiment of the invention. 
       FIG. 5   a  illustrates an eight-LUT programmable logic block including three two-bit type A slices and a single two-bit type D slice in accordance with an embodiment of the invention. 
       FIG. 5   b  illustrates an eight-LUT programmable logic block including two two-bit type A slices and two two-bit type B slices in accordance with an embodiment of the invention. 
       FIG. 5   c  illustrates an eight-LUT programmable logic block including two two-bit type A slices and two two-bit type C slices in accordance with an embodiment of the invention. 
       FIG. 6   a  illustrates an eight-LUT programmable logic block including two two-bit type A slices and two two-bit type D slices in accordance with an embodiment of the invention. 
       FIG. 6   b  illustrates an eight-LUT programmable logic block including four two-bit type B slices in accordance with an embodiment of the invention. 
       FIG. 6   c  illustrates an eight-LUT programmable logic block including four two-bit type C slices in accordance with an embodiment of the invention. 
       FIG. 7   a  illustrates a four-bit type B slice architecture in accordance with an embodiment of the invention. 
       FIG. 7   b  illustrates a four-bit type C slice architecture in accordance with an embodiment of the invention. 
       FIG. 7   c  illustrates a four-bit type D slice architecture in accordance with an embodiment of the invention. 
       FIG. 8   a  illustrates an eight-LUT programmable logic block architecture having a single four-bit type A slice and a single four-bit type B slice in accordance with an embodiment of the invention. 
       FIG. 8   b  illustrates an eight-LUT programmable logic block architecture having a single four-bit type A slice and a single four-bit type C slice in accordance with an embodiment of the invention. 
       FIG. 9   a  illustrates an eight-LUT programmable logic block architecture having a single four-bit type A slice and a single four-bit type D slice in accordance with an embodiment of the invention. 
       FIG. 9   b  illustrates an eight-LUT programmable logic block architecture having two four-bit type B slices in accordance with an embodiment of the invention. 
       FIG. 9   c  illustrates an eight-LUT programmable logic block architecture having two four-bit type C slices in accordance with an embodiment of the invention. 
       FIG. 10   a  illustrates a sixteen-LUT programmable logic block architecture having three four-bit type A slices and a single four-bit type D slice in accordance with an embodiment of the invention. 
       FIG. 10   b  illustrates a sixteen-LUT programmable logic block architecture having two four-bit type A slices and two four-bit type B slices in accordance with an embodiment of the invention. 
       FIG. 10   c  illustrates a sixteen-LUT programmable logic block architecture having two four-bit type A slices and two four-bit type C slices in accordance with an embodiment of the invention. 
       FIG. 11   a  illustrates a sixteen-LUT programmable logic block architecture having two four-bit type A slices and two four-bit type D slices in accordance with an embodiment of the invention. 
       FIG. 11   b  illustrates a sixteen-LUT programmable logic block architecture having four four-bit type B slices in accordance with an embodiment of the invention. 
       FIG. 11   c  illustrates a sixteen-LUT programmable logic block architecture having four four-bit type C slices in accordance with an embodiment of the invention. 
   

   Use of the same reference symbols in different figures indicates similar or identical items. 
   DETAILED DESCRIPTION 
   Reference will now be made in detail to one or more embodiments of the invention. While the invention will be described with respect to these embodiments, it should be understood that the invention is not limited to any particular embodiment. On the contrary, the invention includes alternatives, modifications, and equivalents as may come within the spirit and scope of the appended claims. Furthermore, in the following description, numerous specific details are set forth to provide a thorough understanding of the invention. The invention may be practiced without some or all of these specific details. In other instances, well-known structures and principles of operation have not been described in detail to avoid obscuring the invention. 
   An improved programmable logic block architecture is provided for programmable logic devices such as FPGAs. This improved architecture may be used regardless of the slice bit size. Certain slices within a programmable logic block will not have a one-to-one LUT/register ratio whereas other slices within the programmable logic block may possess a one-to-one LUT/register ratio. For example, in a 2-bit slice architecture, a conventional one-to-one LUT/register slice such as slice  200  of  FIG. 2  may be denoted as an “type A” slice. Slices having a reduced register/LUT ratio may be constructed using a number of alternative embodiments. As seen in  FIG. 4   a , a two-bit slice  400  includes a single register  120  for both LUTs  100 . In this embodiment, a three-to-one multiplexer  405  receives both combinatorial outputs  105  from LUTs  100  as well as data input  125  to select for a data input D to register  120 . As used herein, two-bit slice  400  will be referred to as a “type B” slice. 
   An alternative embodiment for a two-bit slice having a reduced register/LUT ratio is illustrated in  FIG. 4   b . A two-bit slice  410  includes a first LUT  100   a  providing a combinatorial output  105   a  and a second LUT  100   b  providing a combinatorial output  105   b . However, in contrast to slice type B, only combinatorial output  105   a  may be registered in register  120 . Thus, multiplexer  130  selects only between combinatorial output  105   a  and data input  125 . Combinatorial output  105   b  thus cannot be registered in two-bit slice  410 . As used herein, two-bit slice  410  will be referred to as a “type C” slice. 
   Another alternative embodiment for a two-bit slice having a reduced register/LUT ratio is illustrated in  FIG. 4   c . A two-bit slice  420  includes two LUTs  100  wherein each LUT provides a combinatorial output  105  as discussed above. However, two-bit slice  420  does not include a register. Thus, two-bit slice  420  can only provide combinatorial outputs  105  and cannot provide a sequential output as would be the case if it included a register. Because no register is included, two-bit slice  420  does not receive a data input  125 . As used herein, two-bit slice  420  will be referred to as a “type D” slice. 
   Given these illustrative two-bit slice embodiments having reduced register/LUT ratios, their inclusion within a programmable logic block architecture will now be addressed. For example, consider a programmable logic block that includes eight LUTs. An eight-LUT programmable logic block may include 4 two-bit slices. If conventional type A slices were used, this eight-LUT programmable logic block would include eight registers. To save die space and eliminate little or seldom-used registers, eight-LUT programmable logic blocks are described herein that do not include eight registers. The number of registers may range from seven to zero in an eight-LUT programmable logic block having a reduced register/LUT ratio. Including no registers strongly impacts design flexibility. However, including seven registers provides only minor die space savings. It has been observed that a set of six registers provides a sufficient design flexibility vs. die space savings tradeoff. Thus, the following discussion will show various embodiments for a six-register-eight-LUT programmable logic block. It will be appreciated, however, that the number of registers within a programmable logic block having a reduced register/LUT ratio is not limited to a particular value such as six. 
   Turning now to  FIG. 5   a , an eight-LUT programmable logic block  500  includes three type A slices  200 . Because three type A slices  200  will provide six registers  120  (as can be seen from inspection of  FIG. 2 ), the remaining slice in programmable logic block  500  is a type D slice  420  so that the total number of registers  120  equals six. An alternative embodiment is illustrated in  FIG. 5   b  for a programmable logic block  510  that includes two type A slices  200 . Because two type A slices  200  will provide four registers  120 , programmable logic block  510  also includes two type B slices  400  so that the total number of registers  120  equals six. Another alternative embodiment is illustrated in  FIG. 5   c  for a programmable logic block  520  that also includes two type A slices  200 . Rather than use two type B slices, programmable logic block  520  includes two type C slices  410  so that the total number of registers  120  equals six. It will be appreciated that other alternative embodiments may also be constructed. For example, as an alternative to two type C slices in programmable logic block  520 , a single type B and a single type C slice could have been included. For illustration clarity, the interconnections described previously within and between the slices in programmable logic blocks  500 ,  510 , and  530  are represented by a dotted line  530 . 
   Rather than use six registers within each eight-LUT programmable logic block, alternative embodiments may be constructed using a different number of registers. For example,  FIG. 6   a  illustrates an eight-LUT programmable logic block  600  that includes two type A slices  200  and two type D slices  420 . Thus, programmable logic block  600  includes just four registers  120 . An alternative embodiment for a four-register architecture as illustrated in  FIG. 6   b  for which an eight-LUT programmable logic block  620  includes four type B slices  400 . As seen in  FIG. 6   c , a four-register slice architecture may also be achieved using four type C slices  410  in an 8-LUT programmable logic block  630 . A four-register slice architecture may also be achieved using a mixture of type B and C slices. For illustration clarity, the interconnections within and between the slices in programmable logic blocks  600 ,  620 , and  630  are represented by a dotted line  640 . 
   As discussed above, the number of bits within each slice need not be limited to two bits. A conventional four-bit slice has a one-to-one register-to-LUT ratio and thus has a type A architecture as discussed analogously with respect to  FIG. 2 . This ratio may be reduced as seen in  FIG. 7   a  for a four-bit slice  700  having a type B slice architecture. Thus, a combinatorial output  105   a  from a LUT  100   a  or a combinatorial output  105   b  from a LUT  100   b  may be registered in a register  120   a  through appropriate selection in a multiplexer  405   a  that may also select for a data input  125   a . Similarly, a combinatorial output  105   c  from a LUT  100   c  or a combinatorial output  105   d  from a LUT  100   d  may be registered in a register  120   b  through appropriate selection by a multiplexer  405   b  that may also select for a data input  125   b.    
   A four-bit slice  705  having a type C architecture is illustrated in  FIG. 7   b . As discussed analogously with respect to  FIG. 4   b , combinatorial output  105   a  from a LUT  100   a  may be registered in a register  120   a  through appropriate selection by a multiplexer  130   a . Multiplexer  130   a  may also select for a data input  125   a . Similarly, a combinatorial output  105   c  from LUT  100   c  may be registered in a register  120   b  through appropriate selection by a multiplexer  130   b . Multiplexer  130   b  may also select for a data input  125   b . A combinatorial output  105   b  from a LUT  100   b  and a combinatorial output  105   d  from a LUT  100   d  cannot be registered in slice  705 . 
   A four-bit slice  710  having a type D architecture is illustrated in  FIG. 7   c . In a type D slice architecture, no registers are provided to register the LUT combinatorial outputs. Thus, combinatorial outputs  105   a  through  105   d  from respective LUTs  100   a  through  100   d  cannot be registered in slice  710 . 
   The type A through type D four-bit slices discussed above may be organized in various ways to form eight-LUT programmable logic blocks. For example, as seen in  FIG. 8   a , a programmable logic block  800  may include a four-bit type A slice  805  and a four-bit type B slice  700 . Thus, programmable logic block  800  includes six registers  120 . An alternative embodiment for an eight-LUT, 6-register programmable logic block is illustrated in  FIG. 8   b  with respect to a programmable logic block  810 . Programmable logic block  810  includes a 4-bit type A slice  805  and a four-bit type C slice  705 . For illustration clarity, the interconnections between the slices in programmable logic blocks  800  and  810  are represented by a dotted line  820 . 
   Other eight-LUT programmable logic block architectures that incorporate 4-bit slices may be constructed with less than six registers  120 . For example, as seen in  FIG. 9   a , an eight-LUT programmable logic block  900  includes a single four-bit type A slice  805  and a single four-bit type D slice  710 . Alternatively, as seen in  FIG. 9   b , an eight-LUT programmable logic block  905  includes two four-bit type B slices  700 . In yet another alternative embodiment as seen in  FIG. 9   c , an eight-LUT programmable logic block  910  includes two four-bit type C slices  705 . For illustration clarity, the interconnections between the slices in programmable logic blocks  900 ,  905 , and  910  are represented by dotted lines  920 . 
   Four-bit slices may also be used to construct sixteen-LUT programmable logic blocks (which could also be constructed with two-bit slices). Turning now to  FIG. 10   a , a sixteen-LUT programmable logic block  1000  includes three four-bit type A slices  805  and a single four-bit type D slice  710 . Thus, programmable logic block  1000  includes twelve registers  120 . Other sixteen-LUT programmable logic block architectures having 12 registers and four-bit slice granularity may be constructed. For example, as illustrated in  FIG. 10   b , a programmable logic block  1010  includes two four-bit type A slices  805  and two four-bit type B slices  700 . Another alternative embodiment is shown in  FIG. 10   c  for a programmable logic block  1020  that includes two four-bit type A slices  805  and two four-bit type C slices  705 . For illustration clarity, the interconnections between the slices in programmable logic blocks  1000 ,  1010 , and  1020  are represented by dotted lines  1030 . 
   Turning now to  FIG. 11   a , a sixteen-LUT programmable logic block  1100  having eight registers  120  and a four-bit slice granularity is illustrated. Programmable logic block  1100  includes two four-bit type A slices  805  and two four-bit type D slices  710 . Similarly, as seen in  FIG. 11   b , a sixteen-LUT programmable logic block  1100  includes four four-bit type B slices  700 . In an alternative architecture illustrated in  FIG. 11   c , a sixteen-LUT programmable logic block  1120  includes four four-bit type C slices  705 . For illustration clarity, the interconnections between the slices in programmable logic blocks  1100 ,  1110 , and  1120  are represented by dotted lines  1130 . 
   The above-described embodiments of the present invention are merely meant to be illustrative and not limiting. It will thus be obvious to those skilled in the art that various changes and modifications may be made without departing from this invention in its broader aspects. Accordingly, the appended claims encompass all such changes and modifications as fall within the true spirit and scope of this invention.