Abstract:
Logic circuits that support the addition of three binary numbers using hardwired adders are described. In one embodiment, this is accomplished by using a 3:2 compressor (i.e., a Carry Save Adder method), using hardwired adders to add the sums and carrys produced by the 3:2 compression, and sharing carrys data calculated in one logic element (“LE”) with the following LE. In such an embodiment, with the exception of the first and last LEs in a logic array block (“LAB”), each LE in effect lends one look-up table (“LUT”) to the LE below (i.e., the following LE) and borrows one LUT from the LE above (i.e., the previous LE). The LUT being lent or borrowed is one that implements the carry function in the 3:2 compressor model. In another aspect, an embodiment of the present invention provides LEs that include selectors to select signals corresponding to the addition of three binary numbers mode.

Description:
BACKGROUND OF THE INVENTION 
     Programmable logic devices (“PLDs”) (also sometimes referred to as complex PLDs (“CPLDs”), programmable array logic (“PALs”), programmable logic arrays (“PLAs”), field PLAs (“FPLAs”), erasable PLDs (“EPLDs”), electrically erasable PLDs (“EEPLDs”), logic cell arrays (“LCAs”), field programmable gate arrays (“FPGAs”), or by other names), are well-known integrated circuits that provide the advantages of fixed integrated circuits with the flexibility of custom integrated circuits. Such devices typically provide an “off the shelf” device having at least a portion that can be programmed to meet a user&#39;s specific needs. Application specific integrated circuits (“ASICs”) have traditionally been fixed integrated circuits. However, it is possible to provide an ASIC that has a portion or portions that are programmable. Thus, it is possible for an integrated circuit device to have qualities of both an ASIC and a PLD. The term PLD as used herein will be considered broad enough to include such devices. 
     PLDs have configuration elements that may be programmed or reprogrammed. Configuration elements may be realized as random access memory (“RAM”) bits, flip-flops, electronically erasable programmable read-only memory (“EEPROM”) cells, or other memory elements. Placing new data into the configuration elements programs or reprograms the PLD&#39;s logic functions and associated routing pathways. Configuration elements that are field programmable are often implemented as RAM cells (sometimes referred to a “configuration RAM” (“CRAM”)). However, many types of configurable elements may be used including static or dynamic RAM (“SRAM” or “DRAM”), electrically erasable read-only memory (“EEROM”), flash, fuse, and anti-fuse programmable connections. The programming of configuration elements could also be implemented through mask programming during fabrication of the device. While mask programming may have disadvantages relative to some of the field programmable options already listed, it may be useful in certain high volume applications. For purposes herein, the generic term “configuration element” will be used to refer to any programmable element that may be configured to determine functions implemented by other PLD elements. 
     PLDs typically include blocks of logic elements, sometimes referred to as logic array blocks (“LABs”; also referred to by other names, e.g., “configurable logic blocks” (“CLBs”)). Typically, the basic functional block of a LAB is a logic element (“LE”) that is capable of performing logic functions on a number of input variables. LEs, which are sometimes referred to by other names, e.g., “logic cells”, may include a look-up table (LUT) or product term, carry-out chain, register, and other elements. PLDs typically combine together large numbers of such LEs through an array of programmable interconnects to facilitate implementation of complex logic functions. LABs (comprising multiple LEs) may be connected to horizontal and vertical conductors that may or may not extend the length of the PLD. 
     One of the functions implemented by an LE is the addition of binary numbers. It is sometimes desirable to include hardwired adders in the implementation of the adder using the LE. Thus, some LEs include hardwired adders, sometimes also referred to as dedicated adders. Additionally, it is sometimes desirable to add three, rather than only two, binary numbers at once. There are a number of known techniques for adding three or more binary numbers. One of those techniques is the Carry Save Adder method. 
       FIG. 1  illustrates the concept of Carry Save Adder method. As illustrated in  FIG. 1 , in the Carry Save Adder method, three binary words, X, Y, and Z, are compressed into sums and carrys output vectors using an array of full adders. In some cases, arrays of independent adders are used to produce the sums and carrys output vectors. Each bit of the sums vector represents the binary sum result of adding the corresponding bits of the binary numbers X, Y, and Z. Each bit of the carrys vector represents the binary carry result of adding the corresponding bits of the binary numbers X, Y, and Z. Thereafter, the carrys vector is shifted to the left by one bit, thus effectively multiplying it by 2. The sums and the shifted carrys are also referred to as the 3:2 compressor results. The sums vector and the shifted carrys vector are then added to generate the final output, which is also referred to as the total in  FIG. 1 . In  FIG. 1 , the decimal equivalents of the binary numbers X, Y, Z, as well as the sums, carrys, and total are shown to the right of their corresponding binary numbers. 
     The addition of three binary numbers requires a larger number of inputs to the LE. Sometimes, an LE does not include enough input terminals to support the addition of three binary numbers using dedicated adders. The present invention addresses this issue. 
     SUMMARY OF THE INVENTION 
     In one aspect, an embodiment of the present invention provides LEs that support the addition of three binary numbers using hardwired adders. In one embodiment, this is accomplished by using a 3:2 compressor (i.e., a Carry Save Adder method), using hardwired adders to add the sums and carrys produced by the 3:2 compression, and sharing carrys data calculated in one LE with the following LE. In such an embodiment, with the exception of the first and last LEs in a LAB, each LE in effect lends one LUT to the LE below (i.e., the following LE) and borrows one LUT from the LE above (i.e., the previous LE). The first LE in a LAB in effect lends one LUT to the LE below, but does not borrow a LUT. The last LE in a LAB in effect borrows one LUT from the LE above, but does not lend a LUT. The LUT being lent or borrowed is one that implements the carry function in the 3:2 compressor model. 
     In another aspect, an embodiment of the present invention provides LEs that include selectors to select signals corresponding to the addition of three binary numbers mode. 
     The ability to add three binary numbers at a time allows converting an adder tree from binary to ternary. Ternary adder trees exhibit lower depth, logarithmic in base 3 rather than 2. Decreasing the depth tends to improve circuit speed. Ternary adder trees also require approximately half as many nodes as equivalent binary adder trees. Thus, the ability to add three binary numbers results in approximately 50% area savings and 33% depth savings over the traditional binary adder tree when implementing adder trees. The reduction in node count translates into a reduction in the chip area required to implement the circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of the invention are set forth in the appended claims. However, for purpose of explanation, several aspects of a particular embodiment of the invention are described by reference to the following figures. 
         FIG. 1  illustrates the concept of the Carry Save Adder method. 
         FIG. 2A  is a block diagram of two LEs of the present invention. 
         FIG. 2B  illustrates an exemplary logic circuit that performs the function of a Sum LUT shown in  FIG. 2A . 
         FIG. 2C  illustrates an exemplary logic circuit that performs the function of a Carry LUT shown in  FIG. 2A . 
         FIG. 3  is a more detailed block diagram of an LE of the present invention. 
         FIG. 4  is a schematic diagram illustrating some benefits of using a ternary adder tree instead of a binary adder tree. 
         FIG. 5  illustrates an exemplary data processing system including an exemplary PLD in which logic circuits in accordance with the present invention might be implemented. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of particular applications and their requirements. Various modifications to the exemplary embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     As noted above, the Carry Save Adder method is a known method. However, it has not yet been implemented in a PLD using hardwired adders to add the sums and shifted carrys to produce the final result. In the present invention, the Carry Save Adder method is implemented using a hardwired adder to add the sums and shifted carrys. Moreover, some carrys from one LE are shared with the following LE. This is illustrated in the following figures. In the present invention, carrys are shifted relative to the sums in the sense that an n-th carry bit is added to an (n+1)-th sum bit, where n is an integer. 
       FIG. 2A  is a block diagram of two LEs of the present invention. In  FIG. 2A , LE  205  includes LUTs  210 ,  215 ,  220 , and  225 . Additionally, it includes adders  216  and  226 . Similarly, LE  255  includes LUTs  260 ,  265 ,  270 , and  275 . Additionally, it includes adders  266  and  276 . In one embodiment, adders  266  and  276  are hardwired adders. 
     LUTs  210  and  215  provide the sums and carrys results for the n-th bit of the binary numbers X, Y, and Z. In other words, they provide the sums and carrys results for the X[n], Y[n], and Z[n] bits. LUTs  220  and  225  provide the sums and carrys results for the (n+1)-th bit of the binary numbers X, Y, and Z. In other words, they provide the sums and carrys results for the X[n+1], Y[n+1], and Z[n+1] bits. LUTs  260  and  265  provide the sums and carrys results for the (n+2)-th bit of the binary numbers X, Y, and Z. In other words, they provide the sums and carrys results for the X[n+2], Y[n+2], and Z[n+2] bits. LUTs  270  and  275  provide the sums and carrys results for the (n+3)-th bit of the binary numbers X, Y, and Z. In other words, they provide the sums and carrys results for the X[n+3], Y[n+3], and Z[n+3] bits. 
     Adder  216  receives data from LUT  210 . If LE  205  is the first LE in a LAB, then adder  216  also receives ground signals. Otherwise, if LE  205  is not the first LE in a LAB, then adder  216  receives the output signals of a carry LUT (i.e., a LUT that determines the carrys for the (n−1)-th bit). Additionally, if n is not the first bit to be output as a result of adding X, Y, and Z, then adder  216  also receives a carry over signal from the previous LE. The carry over signal is received on line  290 , which is part of the carry chain for adders  216 ,  226 ,  266 , and  276 . If n is the first bit to be output as a result of adding X, Y, and Z, then adder  216  would receive a ground signal on line  290 . Adder  216  outputs the final result for the n-th bit. It also outputs a carry over signal that is sent to adder  226  via line  290 . 
     Adder  226  receives data from LUTs  215  and  220 . In other words, it receives the carrys for the n-th bit and the sums for the (n+1)-th bit. Moreover, adder  226  receives the carry over signal from adder  216  via line  290 . Adder  226  outputs the final result for the (n+1)-th bit. It also outputs a carry over signal that is sent to adder  266  via line  290 . 
     Adder  266  receives data from LUTs  225  and  260 . In other words, it receives the carrys for the (n+1)-th bit and the sums for the (n+2)-th bit. Moreover, adder  266  receives the carry over signal from adder  226  via line  290 . Adder  266  outputs the final result for the (n+2)-th bit. It also outputs a carry over signal that is sent to adder  266  via line  290 . 
     Adder  276  receives data from LUTs  265  and  270 . In other words, it receives the carrys for the (n+2)-th bit and the sums for the (n+3)-th bit. Moreover, adder  266  receives the carry over signal from adder  266  via line  290 . Adder  276  outputs the final result for the (n+3)-th bit. It also outputs a carry over signal that is sent to the first adder in the next LE via line  290 . 
     As can be seen in  FIG. 2A , the output of LUT  275  is not used by either LE  205  or LE  255 . Instead, the output of LUT  275 , which is the carrys for the (n+3)-th bit are shared with the LE following LE  255 . 
     Each of the Sum LUTs, such as LUTs  210 ,  220 ,  260 , and  270 , receives one bit of data from each of the binary numbers X, Y, and Z, and outputs a one bit signal that represents the sum of the three bits received. For example, LUT  210  receive the n-th bit of the binary numbers X, Y, and Z and outputs the sum of those three bits. In other words, it receives the bits X[n], Y[n], and Z[n] and outputs X[n](XOR)Y[n](XOR)Z[n], where XOR represents the Boolean exclusive OR function. 
       FIG. 2B  illustrates an exemplary logic circuit that performs the function of a Sum LUT that receives the binary numbers X, Y, and Z, and outputs X(XOR)Y(XOR)Z in response thereto. As can be seen in  FIG. 2B , inputs signals X, Y, and Z are XOR-ed by the XOR gate  291 , which outputs the signal X(XOR)Y(XOR)Z. It is to be noted that other logical circuits may also perform the function of receiving three binary bits and outputting the sum thereof. 
     Each of the Carry LUTs, such as LUTs  215 ,  225 ,  265 , and  275 , receives one bit of data from each of the binary numbers X, Y, and Z, and outputs a one bit signal that represents the carry resulting from adding the three bits received. For example, LUT  215  receive the n-th bit of the binary numbers X, Y, and Z and outputs the carry resulting from adding those three bits. In other words, it receives the bits X[n], Y[n], and Z[n] and outputs (X[n](AND)Y[n])(OR)(X[n](AND)Z[n])(OR)(Y[n](AND)Z[n]), where AND represents the Boolean AND function, and OR represents the Boolean OR function. 
       FIG. 2C  illustrates an exemplary logic circuit that performs the function of a Carry LUT that receives the binary numbers X, Y, and Z, and outputs (X(AND)Y)(OR)(X(AND)Z)(OR)(Y(AND)Z) in response thereto. As can be seen in  FIG. 2C , AND gate  292  receives X and Y and outputs the result X(AND)Y. The AND gate  293  receives X and Z and outputs the result X(AND)Z. The AND gate  294  receives Y and Z and outputs the result Y(AND)Z. The OR gate  295  receives the outputs of AND gates  292 ,  293 , and  294 , and outputs the signal {X(AND)Y}(OR){X(AND)Z}(OR){Y(AND)Z} in response thereto. It is to be noted that other logical circuits may also perform the function of receiving three binary bits and outputting the carry resulting from adding those bits. 
     It is to be noted that the carry over signals that are determined by adders  216 ,  226 ,  266 , and  276  and carried on line  290  are not the same as the carry signals determined in LUTs  215 ,  225 ,  265 , and  275 , which are also herein referred to as a share carry signals. The carry over signal is the carry signal resulting from adding the signals input to the adder. For example, the carry over signal output by adder  226  is the carry signal resulting from adding the signals received from LUTs  215  and  220  and from adder  216  via line  290 . The share carry signal is the carry signal in the Carry Adder Save process. It is the carry result of adding the binary numbers X, Y, and Z. 
     In one embodiment, each of adders  216 ,  226 ,  266 , and  276  may be implemented using logic circuits such as those shown in  FIGS. 2B and 2C . The three input signals to the adder would be provided to both of the logic circuits. One logic circuit, such as that shown in  FIG. 2B , would output the sum of the three input signals. That sum would represent the one bit sum of the corresponding bits of the numbers X, Y, and Z and would be provided as an output of the LE. The other logic circuit, such as that shown in  FIG. 2C , would output the carry resulting from adding the three input signals. The carry signal would be provided to the following adder on line  290  as a carry over signal. It is to be noted that when logic circuits, such as those shown in  FIGS. 2B and 2C , are used in an adder, such as for example, adder  216 , the input signals to the logic circuits are not X, Y, and Z. Instead, they are the three input signals that adder  216  receives as shown in  FIG. 2A  and described above. It is also to be noted that other logic circuits, besides those shown in  FIGS. 2B and 2C , may be used to perform the function of adding three bits of numbers and providing their sum and carry results. 
       FIG. 3  is a more detailed block diagram of an LE  205  of the present invention. In  FIG. 3 , LUTs  312  and  313  in conjunction with multiplexer  314  correspond to LUT  210 . Those skilled in the art know that two 3 input LUTs (such as LUTs  312  and  313 ) in combination with a 2:1 multiplexer (such as multiplexer  314 ) are functionally the same as a 4 input LUT (such as LUT  210 ). LUTs  317  and  318  in conjunction with multiplexer  319  correspond to LUT  215 . LUTs  322  and  323  in conjunction with multiplexer  324  correspond to LUT  220 . LUTs  327  and  328  in conjunction with multiplexer  329  correspond to LUT  225 . Multiplexers  314  and  319  receive the signal D 0  as a select signal. Multiplexers  324  and  329  receive the signal D 1  as a select signal. 
     LE  205  in  FIG. 3  also includes multiplexers  331 ,  332 ,  341 ,  342 ,  381 ,  386 , and  391 . The input terminals of multiplexer  331  are coupled to the output terminals of multiplexers  314  and  319 . Multiplexer  331  receives the signal E as a select signal. Using select signal E, multiplexer  331  selects as an output signal one of the two input signals that it receives, i.e., the output signals of multiplexers  314  and  319 . The input terminals of multiplexer  332  are coupled to the output terminals of LUTs  317  and  318 . Multiplexer  332  receives the signal E as a select signal. Using select signal E, multiplexer  332  selects as an output signal one of the two input signals that it receives, i.e., the output signals of LUTs  317  and  318 . 
     The input terminals of multiplexer  341  are coupled to the output terminals of multiplexers  324  and  329 . Multiplexer  341  receives the signal E as a select signal. Using select signal E, multiplexer  341  selects as an output signal one of the two input signals that it receives, i.e., the output signals of multiplexers  324  and  329 . The input terminals of multiplexer  342  are coupled to the output terminals of LUTs  327  and  328 . Multiplexer  342  receives the signal F as a select signal. Using select signal F, multiplexer  342  selects as an output signal one of the two input signals that it receives, i.e., the output signals of LUTs  327  and  328 . 
     Multiplexer  381  receives the output signal of multiplexer  332  on an original input terminal and a shared signal from the previous LE on a new input terminal. Multiplexer  391  receives the output signal of multiplexer  319  on a new input terminal and the output signal of multiplexer  342  on an original input terminal. When the LE is set to operate in the addition of three binary numbers mode, each of multiplexers  381  and  391  is set to select the input signal that it receives on its new input terminal. In other words, multiplexer  381  selects as an output signal the shared signal it receives from the multiplexer of the previous LE and multiplexer  391  selects as an output signal the signal that it receives from the multiplexer  319 . When the LE is not set to operate in the addition of three binary numbers mode, as, for example, when it is set to operate in the addition of two binary numbers mode or a non-arithmetic mode, then each of multiplexers  381  and  391  is set to select the input signal that it receives on its original input terminal. In other words, multiplexer  381  selects as an output signal the signal that it receives from multiplexer  332 , and multiplexer  391  selects as an output signal the signal that it receives from the multiplexer  342 . 
     The output terminals of multiplexers  381  and  391  are coupled to adders  216  and  226 , respectively. Thus, adder  216  receives the output signal of multiplexer  381  as an input signal and adder  226  receives the output signal of multiplexer  391  as an input signal. Adder  216  also receives the output signal of multiplexer  314  and a signal on line  290 . The signal that adder  216  receives on line  290  is either a carry over signal from a previous LE or a ground signal if LE  205  outputs the first bit resulting from adding the binary numbers X, Y, and Z. Adder  226  also receives the output signal of multiplexer  324  and a signal on line  290 . The signal that adder  226  receives on line  290  is the carry over signal from adder  216 . 
     When multiplexer  381  selects the input signal that it receives on the new input terminal, then the output signal of adder  216  is Fn(A,B,C 0 ,D 0 )+Fn(A,B,C 0 ,E). In other words, the output signal is the sum of (1) a function of the input signals received on terminals A, B, C 0 , D 0  and (2) a function of the input signals received on terminals A, B, C 0 , E. When multiplexer  381  selects the input signal that it receives on the original input terminal, then the output signal of adder  216  is A,B,C 0 ,D 0  and prvA,prvB,prvC 1 ,prvD 1 . In other words, the output signal is the sum of (1) the signals received on terminals A, B, C 0 , D 0  and (2) the signals received on the A, B, C 1 , and D 1  terminals of the LE preceding LE  205 . 
     When multiplexer  391  selects the input signal that it receives on the new input terminal, then the output signal of adder  226  is Fn(A,B,C 1 ,D 1 )+Fn(A,B,C 1 ,F). In other words, the output signal is the sum of (1) a function of the input signals received on terminals A, B, C 1 , D 1  and (2) a function of the input signals received on terminals A, B, C 1 , F. When multiplexer  391  selects the input signal that it receives on the original input terminal, then the output signal of adder  226  is A,B,C 1 ,D 1  and A,B,C 0 ,D 0 . In other words, the output signal is the sum of (1) the signals received on terminals A, B, C 1 , D 1  and (2) the signals received on the terminals A, B, C 0 , and D 0  of LE  205 . 
     It is to be noted that in one embodiment, LE  205  may be used for the addition of three binary numbers without including multiplexers  381  and  391 . For example, the terminal on which the shared input signal is received may be hardwired to an input terminal of the adder  216 . Similarly, the output terminal of multiplexer  319  may be hardwired to an input terminal of the adder  226 . Such an embodiment would allow saving the die area that would otherwise be occupied by multiplexers  381  and  391 . It would also save the die area that would otherwise by occupied by the 1-bit RAM for providing the select signal to multiplexers  381  and  391 . Finally, it would save the die area that would otherwise by occupied by multiplexers  332  and  342 . Also, in such an embodiment, the output signals of adders  216  and  226  are the same as those described above when multiplexers  381  and  391  select the signals received on the new input terminals. 
     The input terminals of multiplexer  386  are coupled to the output terminals of multiplexers  331  and  341 . Multiplexer  386  receives the F signal as a select signal. Using the F signal, multiplexer  386  selects as an output signal one of the two signals that it receives as input signals (i.e., the output signals of multiplexers  331  and  341 ). The output signal of multiplexer  386  is Fn(A,B,C,D,E,F). In other words, it is a function of the signals received on terminals A, B, C, D, E, and F of LE  205 . 
     The output signal of multiplexer  329  is provided to the next LE, i.e., LE  255  (shown in  FIG. 2A ). More specifically, it is provided as an input signal to hardwired adder  266  in LE  255  (both of which are shown in  FIG. 2A ). The output signal of multiplexer  329  is a shared carry signal. 
     As shown in  FIG. 2A , each LE outputs two bits of data resulting from adding the binary numbers X, Y, and Z. As further shown in  FIG. 3 , in addition to the two adder outputs, i.e., the outputs of adders  216  and  226 , LE  205  also outputs a signal Fn(A,B,C,D,E,F) that is a logical function of the input signals A, B, C, D, E, and F. 
       FIG. 4  is a schematic diagram illustrating, by way of example, the benefits of using a ternary adder tree  405  (where each adder adds three binary numbers) instead of a binary adder tree  410  (where each adder adds two binary numbers). In the example of  FIG. 4 , there are 128 binary numbers that are to be added. The addition of 128 binary numbers may, for example, occur in a large finite input response (“FIR”) filter. In the example shown in  FIG. 4 , in the case of the binary adder tree  410 , there are seven levels of adders and 127 adders  411  (not all of which are shown in  FIG. 4 ) required to produce a result. By contrast, in the case of the ternary adder tree  405 , there are 5 levels of adders and 64 adders  406  (not all of which are shown in  FIG. 4 ) required to produce a result. Thus, using a ternary adder tree, instead of a binary adder tree, results in an approximately 50% reduction in the number of adders needed. This reduces the chip area required to implement the adder tree by approximately 50%. The reduction in the number of adder levels increases the speed with which the 128 binary numbers can be added. In the example of  FIG. 4 , the ternary adder tree  405  provides an approximately 33% improvement in speed over the binary adder tree  410 . 
     The adder tree accounts for the bulk of digital signal processing (“DSP”) applications such as FIR filters as well as appearing in multipliers and general arithmetic logic. This makes the area savings attractive for common classes of circuits. 
     Those skilled in the art will recognize that adders  406  or  411  are not the same as adders  216 ,  226 ,  266 , or  276  (shown in  FIGS. 2 and 3 ). Instead, each of adders  406  includes a combination of LUTs, multiplexers, and hardwired adders used to implement an adder for adding three binary numbers. Similarly, each of adders  411  includes LUT(s), multiplexer(s), and/or hardwired adders needed to implement an adder for adding two binary numbers. 
       FIG. 5  illustrates, by way of example, a PLD  510  in a data processing system  500 . As one example, logic circuits of this invention may be implemented in LEs of PLDs such as PLD  510 . PLD  510  includes a plurality of LABs such as LAB  512  (only one LAB is shown to avoid overcomplicating the drawing). LAB  512  includes a plurality of LEs such as LE  205  (only one LE is shown to avoid overcomplicating the drawing). In one embodiment, LE  205  and LAB  511  are on the same die/chip as PLD  510 . Data processing system  500  may include one or more of the following components: a processor  540 ; memory  550 ; input/output (I/O) circuitry  520 ; and peripheral devices  530 . These components are coupled together by a system bus  565  and are populated on a circuit board  560  which is contained in an end-user system  570 . A data processing system such as system  500  may include a single end-user system such as end-user system  570  or may include a plurality of systems working together as a data processing system. 
     System  500  can be used in a wide variety of applications, such as computer networking, data networking, instrumentation, video processing, DSP, or any other application where the advantage of using programmable or reprogrammable logic is desirable. PLD  510  can be used to perform a variety of different logic functions. For example, PLD  510  can be configured as a processor or controller that works in cooperation with processor  540  (or, in alternative embodiments, a PLD might itself act as the sole system processor). PLD  510  may also be used as an arbiter for arbitrating access to a shared resources in system  500 . In yet another example, PLD  510  can be configured as an interface between processor  540  and one of the other components in system  500 . It should be noted that system  500  is only exemplary. 
     In one embodiment, system  500  is a digital system. As used herein a digital system is not intended to be limited to a purely digital system, but also encompasses hybrid systems that include both digital and analog subsystems. 
     While the present invention has been particularly described with respect to the illustrated embodiments, it will be appreciated that various alterations, modifications and adaptations may be made based on the present disclosure, and are intended to be within the scope of the present invention. While the invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the present invention is not limited to the disclosed embodiment but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.