Abstract:
A crossbar ( 20 ) circuit with multiplexer ( 22 A,  22 B) circuits implemented in a polygonal form on a chip. The crossbar can be used for implementing a permutation of input bits ( 24 A,  24 B) controlled by a bit vector ( 25 ). Horizontal and vertical wiring lengths in the crossbar ( 20 ) are reduced by stacking the operand latches ( 24 A,  24 B,  25 ) and horizontal or vertical multiplexers ( 22 A,  22 B). This implementation decreases the latency of the crossbar and avoids latches to store intermediated results, thus reducing area and power consumption.

Description:
FIELD OF THE INVENTION 
   The present invention lies within the field of computer hardware and in particular of chip macro design. It relates to an electronic computing circuit for implementing a permutation operation between for a plurality of M input bits controlled by a control vector of a plurality of C control bits, which is implemented in a polygonal form on a chip. 
   DESCRIPTION OF PRIOR ART 
   Under the term “permutation operation” a function shall be understood which generates an output bit vector dependent of an input vector, and which in its most general scope enables an output set of bits to be generated from a respective input bit setting. In contrast to rotation and shift operations the output bits can be generated by a mapping from the input bits without any regularity or similarity between both vectors, when a vector is seen as a whole. A set of input bits may comprise a number of 1 bit, 2 bits, . . . bits, for example, 9 bits (1 byte+parity bit) may be used. Here, we exemplarily use sets of 8 bits, building together one byte, respectively, without delimiting the disclosure. Of course, this operation can be extended to be done in half-bytes, or other multiples of one bit. The selection of an input vector element is determined by a control vector, with a tuneable selection granularity, the details of which are described with reference to  FIG. 1 . 
     FIG. 1  represents a prior art way to implement a crossbar macro  10 . US Patent application US2004/0054879 A1 discloses a table-lookup application for such crossbar implementation without describing details of the architectural design. 
   The height of the crossbar is defined throughout the current disclosure in vertical y-direction of the figure, the width in horizontal x-direction. Let O be the number of operands, S be the number of words of each operand, and W be the number of bits in a word, which are controlled by a single control bit of the control vector C. 
   The crossbar implementation illustrated by way of example in  FIG. 1  for a 2*16-byte case, processes two input operands A and B, each of A and B comprising a 16-bytes bit length and generating a result vector of 16 bytes length in a control granularity of 8 bits. Thus, M is O*S*W=2*16*8=256 bits. N is 1*16*8=128 bits. With a control granularity of W=8 bits a single control bit of control vector C controls W=8 bits concurrently. The control vector has a width of S*log 2 O*S=16*5=80. 
   The A and B operands as well as the control vector C come out of latches  14  that are physically situated at one side of the polygon of the crossbar macro  10 , in  FIG. 1  they are depicted at the top edge of the macro  10 . The A and B operands are located in a stacked form in the input latches  14  for being input into the multiplexer logic. Thus, in the most left input latch the bits A 0 , B 0  are stacked, in the most right bit latch A 127 , and B 127 . The input bit latches of control vector C are spread, such that a number of 5 control bits are arranged in groups adjacent to a byte of data in A and B. 
   The crossbar macro  10  basically is composed of a plurality of multiplexers, which are in total depicted as block  12 . The multiplexers  12  have to select the bytes from operands A and B, which are to be passed through to an output latch arrangement  16 . Thus, these multiplexers  12  determine from which position each output byte is coming, based on the control vector. The overall crossbar structure is a regular arrangement of S=16 independent multiplexers. 
   Imposed by the increasing tendency to continued shrinking of macro implementations, the semiconductor chip wiring and signal runtime thereon must be optimized correspondingly in an increasing manner, as the more a wire cross-section shrinks, the worse becomes its RC resistance. As the clock cycles get shorter in parallel, wire optimization tends to be a must in general, but in particular in circuits like multiplexers  12 , as this circuit has considerable lateral dimensions both in X and Y direction. 
   Thus, the wiring and in particular the wiring length is an increasingly important design issue in such multiplexer macro structure. In the best case, which is depicted as permanent-type line, the byte comes out of a latch situated in the middle of the crossbar  12 , and runs on an equal length to the left and to the right. This configuration has involved minimal horizontal wiring. The disadvantage of this prior art is that only very few bytes benefit from this optimal position. In the classical prior art example, only 2 bytes per operand can use the optimal position. In the worst case, the byte is starting from the extreme left, or extreme right position, respectively and has to travel across the total height, before running along the entire length on a horizontal wire (shown as a broken line in  FIG. 1 ) and through several MUX stages. Even though the vertical wire can be between the several MUX stages as well, this adds up to same vertical wire length. 
   The control signals wiring has a similar problem: they have a large fan-out (equals W) and must be distributed along the entire height of the crossbar, which is represented as a dotted line in  FIG. 1 . The straight-forward way how to manage such situations is certainly to split up the crossbar operations and to execute them in more than one cycle. But this increases significantly the amount of latches in the design and impacts disadvantageously macro area, power consumption and timing. 
   OBJECTIVES OF THE INVENTION 
   It is thus an objective of the present invention to provide a crossbar implementation with reduced horizontal and vertical wire length of the built-in multiplexers. 
   SUMMARY OF THE INVENTION 
   This objective of the invention is achieved by the features stated in enclosed independent claims. Further advantageous arrangements and embodiments of the invention are set forth in the respective dependent claims. Reference should now be made to the appended claims. 
   In the present invention various implementations of a crossbar circuit are disclosed which reduce the length of the horizontal and/or vertical wires. This is particularly advantageous, when an architectural constraint is present which requires producing the crossbar output in a single-clock cycle or less, without intermediate latches other than the operand latches and the result latches. 
   On the other hand the present invention includes embodiments not explicitly described in more detail, which may implement intermediate latches motivated by any other reasons. Those embodiments just do not exploit the full degree of signal path optimization, but may be useful in any other technical regard. 
   According to the broadest aspect of the invention it is disclosed to provide 
   a) two separate input register sets for said input bits being arranged at opposite sides of the polygonal macro crossbar circuit, wherein the input bits can be freely assigned to a plurality of O=2, 3, . . . , N_op operands, 
   wherein 
   b) an output register set is arranged in a central region of said polygon, 
   c) a first multiplexer set is connected to the first input register set for selecting bits there from and extends from a respective first outer region of the circuit to the central region, 
   d) a second multiplexer set is connected to the second input register set for selecting bits there from and extends from a respective second outer region of the circuit to the central region, and 
   e) a third multiplexer&#39;s inputs are connected to the outputs of said first and second multiplexer sets for selecting bits there from and said third multiplexer&#39;s output is connected to the output register set. 
   Advantageously, the third multiplexer is arranged as close as possible to the output register set. 
   The permutation is controlled by a control vector C. Each bit thereof may control a group of a plurality of W bits concurrently. If W=8, then the permutation is controlled on a byte wise level. The control vector C is divided in a number of S (S=16 in the case of  FIG. 2 ) subvectors, wherein each subvector controls one MUX “slice”. Each subvector contains log 2 (O*S) bits (5 bits in the example of  FIG. 2 ). Each control bit controls W multiplexers of log 2 (O*S) levels within a respective MUX slice. 
   Preferred are cases, where N_op is an equal number N_op=2, 4, 6, 8, . . . as the implementation of the crossbar will be quite symmetric, as a number of N_op/2 is evaluated from top to center (in  FIG. 2 ) and the other half from bottom to center. 
   Further preferably, each of above mentioned multiplexer sets is connected to a respective input of at least one N_op:1 multiplexer or a respective group of a multiplexer plus a subsequent multiplexer, for selecting between one of said N_op sets of input bits. 
   The detailed embodiment described later shows two input operands A and B, thus, N_op equals 2, and there is a single 2:1 MUX, which output is connected to the result register (output register) set. 
   Using this inventional approach a classical permutation of the input bit setting can be performed, regardless from where the input bits originate, what they encode, etc. 
   Of course, the inventional concept can be applied to other forms of multiplexing, for example a 3:1, a 4:1 multiplexer, etc., as in general, the inventional approach means to take into account the importance of wire delay during development. Base of this design technique is that the total wire is distributed over functional cycles and reduced in the function-critical part of the design. While doing so the focus is to minimize the horizontal and vertical wire length while keeping in mind the complexity of the function. 
   A vertical multiplexer, for example a 32:1 MUX, fitting in 1 bit slice, is implemented for each output bit. Preferably, this vertical multiplexer is divided in three parts combined with the input/output latch arrangement as described next in the preferred embodiments, i.e., the A operand placed on top, the B placed on the bottom and the result and control latch placed at the center. This arrangement aims at reducing the length of the vertical wires. 
   A second feature discloses a “folding” of the input latches in order to reduce the horizontal wiring length. This can be applied byte-wise or bit-wise. In the optimum case, all the data signals originate from the center of the crossbar. By stacking the operand latches, the horizontal wiring length can thus be reduced. By stacking 2 latch rows, the wiring length is reduced to ¾ of the crossbar width; by stacking 4 rows of latches, the horizontal wiring length is reduced to ⅝ of the total crossbar with. Since the vertical wiring length increases with the amount of latch rows that are stacked, an optimum can be found where the decrease of the horizontal wiring length and the increase of the vertical wiring length result in a total, i.e., horizontal and vertical minimum wiring length. The amount of horizontal wire tracks needed is not changed by the folding. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and is not limited by the shape of the figures of the drawings in which: 
       FIG. 1  is a schematic block diagram illustrating a prior art crossbar implementation in a rough macro view, 
       FIG. 2  is a schematic block diagram illustrating a crossbar implementation in a rough physical view according to a first embodiment of the invention, 
       FIG. 3  is schematic block diagram of the circuit of  FIG. 2  in logical view, and 
       FIG. 4  is a schematic block diagram illustrating a crossbar implementation in a rough physical view according to a second embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   With general reference to the figures and with special reference now to  FIG. 2 , a first embodiment of the invention comprising a 2*16-byte input (for operand A and B), 16-byte output crossbar implementation is described next in more detail. Where applicable in an obvious way the introductory description of  FIG. 1  can be applied also in the rest of the figures. 
   The crossbar implementation denoted with reference sign  20  as a whole, has still a rectangle form and comprises two separate input register sets  24 A and  24 B, which are arranged at opposite margins of the rectangle. There are two separate multiplexer sections  22 A and  22 B for operand A and B, respectively, comprising a 32:1 multiplexer per output bit, details see in  FIG. 3 . 
   The register set for the control vector and that of the result vector  26  are arranged in the middle of the crossbar, together with a last 2:1 multiplexer—provided per output bit—which is implemented as a MUX “slice”—vertical in  FIG. 2  and not depicted in order to improve clarity of the drawing—in a distributed form around the control register set  25  thus building a joining connection of the before-mentioned multiplexer sections  22 A,  22 B. This multiplexer set is denoted as “third” multiplexer set in the appended claims. 
   A multiplexer slice of this third multiplexer set comprises preferably three NAND gates. 
   Operands A and B are 16 bytes large. The control vector C depicted with reference sign  25  is composed of 80 bits. For each output byte, 5 of these control bits are used to control the multiplexers  22 A and  22 B. 1 bit selects A or B operand, 4 bits select one of the 16 bytes of the selected operand. 
   As a skilled reader may appreciate, this first embodiment is used to reduce the length of the vertical wires in the crossbar implementation. According to this embodiment the placement of the latches for input register set for operands A, B, C and that of the result is changed compared to the prior art implementation of  FIG. 1 . 
   Further, in each section  22 A and  22 B a vertical 16:1 multiplexer is introduced. The operand latches  24 A for operand A thus are placed at the top of the crossbar  20 , whereas the operand latches for B are placed to the bottom of the macro and the result latches are now placed in the middle of the crossbar together with a 2:1 multiplexer which inputs are driven by  22 A and  22 B respectively. 
   There is one vertical 32:1 multiplexer  30  per output bit, which is shown only in  FIG. 3  in order to increase the clarity of  FIG. 2 . The multiplexer  30  is composed of three parts. First, the 16:1 multiplexer  22 -A is provided which selects the A operand in the top part, in the bottom part there is a second 16:1 multiplexer  22 -B selecting the B-operand, and finally the before-mentioned third 2:1 multiplexer  28  selects which one from A or B is the final result. 
   The output of this 2:1 multiplexer  28  is input directly into the result latch. The vertical data wires length is reduced since the data has to travel only one half of the crossbar height in the worst case. This is achieved due to the placement of the A and B operand latches at the top and the bottom of the crossbar, respectively. The vertical output wire is minimal thanks to the fact that the last 2:1 multiplexer  28  is placed in close vicinity of the output latch  26 . 
   Of course additional wiring for one operand (here operand B) of about the crossbar height relative to prior art is needed, if the both input operands come from the same direction, e.g., from the top of  FIG. 2 . This is, however in most cases not harmful as to processing time as the signals for B for example can be fed to the input register set in the previous cycle before entering the crossbar implementation. In addition the height of the crossbar is wire-dominated and each portion has about half the height of a prior art implementation. In total the height of the crossbar is equal. 
   With reference to  FIG. 4 , a second embodiment of the invention is described, which is aimed to obtain additionally a reduction of the length of the horizontal wires. This second embodiment includes the features of the first embodiment and introduces some new feature on-top. Of course, these new features can also be implemented in a crossbar implementation according to prior art  FIG. 1 . 
   The width (horizontal direction in  FIG. 4 ) of the crossbar is determined by the amount of vertical 32-bit multiplexers  30 , and thus by the width of the output vector. In this exemplary embodiment, there are a number of 128 vertical 32:1 multiplexers  30 . In the ideal case, the operands should enter in the center of the crossbar implementation in order to travel the minimum length, i.e. half of the crossbar width. This optimal case can be approximated, if the operand latches are stacked. This is proposed as the basic on-top feature. 
   In the actual example of  FIG. 4 , a stack height of two is chosen for the input A latches  24 A. For the A-operand, the latches for bytes  0  to  3 , i.e., A[ 0  . . .  31 ], are stacked above the latches for bytes  4  to  7 , i.e. A[ 32  . . .  63 ]. The latches for bytes  12  to  15  (A[ 96  . . .  127 ]) are stacked above the latches for bytes  8  to  11  (A[ 64  . . .  95 ]). The same principle is applied for the B-operand latches  24 B. An implementation of this feature involves certain, but tolerable increase of the vertical wire length by the height of one latch row and an increased vertical wire channel usage. 
   Further, in order to ensure a proper signal transmission across the crossbar, inverters are placed at the center of each 4-byte crossbar “slice” for regeneration of the signal. Note that in the worst case three regenerating inverters are needed. Depending of the used chip technology the amount of inverters may vary. 
   In the worst case, the signal must travel ¾ of the crossbar width, instead of the entire crossbar width in the classical case. 
   The before-described disclosure of the two embodiments can be varied in a number of architectural design issues, such as bit length of operands, number of operands, and stack height. For example, in order to come nearer to the optimum wire length, further stacking of the operand latches can be used. If a stacking height of 4 is applied, the latches for byte  0  to  3 ,  4  to  7 ,  8  to  11  and  12  to  15  can be stacked. The worst case signal run in this case is ⅝ of the total crossbar width. Depending on the height of the available latches, an optimum can be found, balancing the reduction of horizontal wire length and the increase of the vertical wire length due to the stacking of the latches. This configuration enables to have maximum two regenerating inverters on the signal path, thus having a positive impact on the delay.