Abstract:
A carry-ripple adder having inputs for supplying three input bits of equal significance 2 n  that are to be summed and two carry bits of equal significance 2 n+1  that are also to be summed. A calculated sum bit of significance 2 n  and two calculated carry bits of equal significance 2 n+1  which are higher than the significance 2 n  of the sum bit are provided at outputs. A final carry-ripple stage VMA may be used even after a reduction to three bits.

Description:
PRIORITY AND CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation of International Application No. PCT/DE2004/000796 filed Jan. 29, 2004, which claims priority to German application 103 05 849.4 filed Feb. 12, 2003, both of which are incorporated herein in their entirety by reference.  
       BACKGROUND  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to the field of logic devices, and more particularly, it relates to 3 &amp; 2 to 3 carry-ripple adders.  
         [0004]     2. Description of the Related Art  
         [0005]     Carry-ripple adders have sequential carry logic, and similar carry-save adders, they have a plurality of inputs of equal significance and, during operation, sum the bits applied to these inputs. The sum is provided at outputs of different significance, for example in binary coded numerical notation (BCD).  
         [0006]     In order to add a plurality of bits of equal significance, for example in multipliers, it is known to build carry save adder arrays, for example in accordance with the Wallace tree algorithm, and to finally use a vector merging adder (VMA) to convert the resultant sum, and carry data representation in redundant numerical notation into unambiguous numerical notation. This final stage is often in the form of a carry-ripple adder, two bits of equal significance respectively being summed. In the case of such an approach, it is thus necessary for the carry save adder tree to generally be reduced to two bits for the purposes of addition.  
         [0007]     Consequently, use has only been made of carry-nipple adders that add two input bits and one carry, one sum bit of significance 2 n  and one carry of significance 2 n+1  being generated. This results in the need for multistage approaches such that a carry save adder tree in accordance with the number of input bits is first of all used and finally a 2-bit carry-ripple adder is used.  
         [0008]     Solutions for carry-ripple adders that add up to five input bits of equal significance, for example 2 n , are known. However, these configurations are disadvantageous, both as regards the processing speed and as regards the substrate area required, for an implementation using complementary CMOS gates on account of the resultant high number of transistors.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     By way of introduction only, a carry-ripple adders described, including uses thereof. An exemplary carry-ripple adder enables small layouts, or reduction in the area for the carry-ripple adder, and a reduced power loss during operation. A carry-ripple adder may generate two carries, or carry bits, of equal significance, where the carries, or carry bits, are passed directly to the next stage of a multistage carry-ripple adder and assessed therein.  
         [0010]     An exemplary carry-ripple adder may have three first inputs for supplying three input bits of equal significance 2 n  that are to be summed, two second inputs for supplying two carry bits of equal significance 2 n+1  that are also to be summed, one output for outputting a calculated sum bit of significance 2 n , and two outputs for outputting two calculated carry bits of equal significance 2 n+1  which is higher than the significance 2 n  of the sum bit. A final carry-ripple stage VMA (vector merging adder) may be used even after a reduction to three bits. This makes it possible to save on one carry save stage, which has an advantageous effect on the processing speed and the substrate area of the overall circuit, or to use the third input bit of each carry-ripple adder for the efficient implementation of accumulators, for example in MAC structures.  
         [0011]     Dynamic implementation of carry paths and their logic implementation within a carry-ripple adder additionally make it possible to optimize the area and speed in comparison with complementary or differential CMOS solutions. Simultaneously generating two carries, or carry bits, of equal significance that are assessed in each stage of the carry-ripple adder means that the circuit complexity and the internal wiring complexity are lower than multistage complementary CMOS solutions which are, for example, composed of 3-bit carry save adders and 2-bit carry-ripple adders. This also applies to dynamic carry-ripple adders having three inputs.  
         [0012]     Because of the considerably reduced number of transistors in a carry path, the carry-ripple adder has been optimized in terms of area and power loss. The carry-ripple adder may be used as a final adder in multipliers, adder trees, filter structures, accumulators and arithmetic logic units.  
         [0013]     An carry-ripple adder may also include a precharge input that drives an integrated precharge logic stage, a carry stage, and a summation stage, and combinations thereof. The carry stage may have two carry addition blocks that independently calculate the carry output signals in a temporally parallel manner. The summation stage may have a quintuple XOR function or block.  
         [0014]     A bit addition device may include a parallel circuit that has multiple carry-ripple adders where 3 input bits of equal significance 2 n  being provided for each carry-ripple adder.  
         [0015]     The foregoing summary is provided only by way of introduction. The features and advantages of the carry-ripple adder may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims. Nothing in this section should be taken as a limitation on the claims, which define the scope of the invention. Additional features and advantages of the present invention will be set forth in the description that follows, and in part will be obvious from the description, or may be learned by practice of the present invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a shows a schematic illustration of a 3 &amp; 2 to 3 carry-ripple adder.  
         [0017]      FIG. 2  shows a truth table for a 3 &amp; 2 to 3 carry-ripple adder.  
         [0018]      FIG. 3  shows a schematic illustration of an internal design of a 3 &amp; 2 to 3 carry-ripple adder.  
         [0019]      FIGS. 4, 4A , and  4 B show a schematic illustration of the connection of a carry-ripple adder for three input words having five bits each.  
         [0020]      FIG. 5  shows a schematic illustration of a carry stage.  
         [0021]      FIG. 6  shows a schematic circuit diagram of a block of the carry stage shown in  FIG. 5 .  
         [0022]      FIG. 7  shows a schematic circuit diagram of the second block of the carry stage shown in  FIG. 5 .  
         [0023]      FIG. 8  shows a schematic illustration of a sum block.  
         [0024]      FIG. 9  shows a schematic circuit diagram of a quintuple XOR stage of the sum block.  
         [0025]      FIG. 10  shows a schematic block diagram for carry-ripple adders. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     Exemplary carry-ripple adders will now be described more fully with reference to the accompanying drawings. In each of the following figures, components, features and integral parts that correspond to one another each have the same reference number. The drawings of the figures are not true to scale.  
         [0027]      FIG. 1  shows a schematic illustration of a 3 &amp; 2 to 3 carry-ripple adder  10  having three bit inputs i 0 , i 1  and i 2 , two equivalent carry inputs ci 1 , ci 2 , two equivalent carry outputs co 1 , co 2  and a sum output s.  
         [0028]      FIG. 2  shows a truth, or function, table for one bit in the carry-ripple adder shown in  FIG. 1 . On the basis of the coding selected for the two equivalent carry output signals co 2  and co 1 , input combinations where ci 2 =1 and ci 1 =0 (hashed in  FIG. 2 ) do not occur during operation since ci 2  can only be set if ci 1  has also been set, from which a double carry is deduced. This fact that “don&#39;t care elements” occur is used to minimize the circuit. The simple sum of the five input bits at the inputs i 0 , i 1 , i 2 , ci 1 , ci 2  results at position s in the table, and a carry is generated at the output co 1  if the sum of the input bits is, for example,≧2, a 1 being applied to the output co 2  as soon as the sum of the five input bits is ≧4 but co 1  then already having been set to 1 since the sum is also ≧2.  
         [0029]      FIG. 3  shows a block diagram of an exemplary basic design of a carry-ripple adder  10  having three input bits i 0 , i 1 , i 2 , two equivalent carry inputs ci 1 , ci 2 , two equivalent carry outputs co 1 , co 2  and a sum output s. The adder  10  includes two blocks  11 ,  12 : a carry stage  11 , and a summation stage or circuit  12 . The signals prech_ 1  and prechq_ 1  which are optionally supplied preferably control an integrated precharge logic stage if a dynamic implementation is provided. The three input bits i 0 , i 1 , i 2  and the two carry input bits ci 1  and ci 2  are respectively supplied to the two blocks  11  and  12 , as are a supply voltage vdd and a reference ground potential vss. The carry outputs co 1  and co 2  are operated using the carry block  11 . In the a dynamic implementation, the precharge signals prech_ 1  and prechq_ 1  are applied to complementary inputs of the carry block  11 . The summation block  12  has the sum output s, and the precharge signal prechq_ 1  is applied to an inverting input of said summation block in the case of a dynamic implementation.  
         [0030]      FIGS. 4, 4A , and  4 B schematically show the connection of a carry-ripple adder for three input words i 0 , i 1  and i 2  each having 5 bits &lt;4:0&gt;, 5 carry-ripple adders as shown in  FIG. 2  being coupled to one another, one carry-ripple adder  10  for each bit position &lt;n&gt; (n=0 to 4). The nth stage adds to the three input bits i 0 &lt;n&gt;, i 1 &lt;n&gt; and i 2 &lt;n&gt; having the significance 2 n  two carry input signals ci 1 &lt;n&gt; and ci 2 &lt;n&gt; which likewise have the significance 2 n  and generates a sum signal s_n of equal significance 2 n  and two carry output signals co 1 &lt;n+1&gt;, co 2 &lt;n+1&gt; of the next higher significance 2 n+1  which correspond to the carry input signals ci 1 &lt;n+ 1 &gt;, ci 2 &lt;n+ 1 &gt; of the n+1th stage, n being an integer between 0 and 4, inclusive, in the present example shown in  FIG. 4 .  
         [0031]      FIG. 5  schematically shows a carry stage  11  of a carry-ripple adder as shown in  FIG. 3  and/or  FIG. 4 . The carry stage  11  has two blocks  13  and  14  which each calculate a carry output signal co 2  and co 1  independently of one another and thus in a temporally parallel manner. Both the block  13  for calculating the carry output signal co 2  and the block  14  for calculating the carry output signal co 1  are connected to the inputs i 0 , i 1 , i 2 , ci 1  and ci 2  of the supply voltage vdd and the reference ground potential vss. In the case of a dynamic implementation, the two blocks  13  and  14  are preferably connected to the precharge signals prech and prechq that are supplied in such a manner that they are inverted, or having opposite poloarity, with respect to one another.  
         [0032]      FIG. 6  shows a schematic circuit diagram of a dynamic implementation of the block  13  (shown in  FIG. 5 ) for generating the carry output signal co 2  on the basis of the signals at the three bit inputs i 0 , i 1 , i 2 , the two carry inputs ci 1  and ci 2  and the precharge signals prech and prechq. A p-channel field effect transistor P is driven, on the gate side, by the precharge signal prechq. The p-channel field effect transistor P is also connected between the supply voltage vdd and a node  17 . An n-channel FET N is connected, on the gate side, to the carry input ci 1 . The n-channel FET N is also connected between the node  17  and a node  18 . The node  18  may be connected to the supply voltage vdd via an n-channel FET N that is driven, on the gate side, with the precharge signal prech. A series circuit comprising three n-channel FETs N is located between the node  18  and the reference ground potential vss, one of said n-channel FETs being connected, on the gate side, to i 0 , the next n-channel FET being connected, on the gate side, to i 1 , and the third n-channel FET being connected, on the gate side, to i 2 .  
         [0033]     An n-channel FET is connected, on the gate side, to the carry input ci 2 , and is connected between the node  17  and a node  19 . A series circuit comprising two n-channel FETs N is located between the node  19  and the reference ground potential vss, one of said n-channel FETs in the series circuit of two n-channel FETs between node  19  and the reference ground is connected, on the gate side, to i 1  and the other is connected to i 2 . A parallel circuit of two n-channel FETs N is parallel to said series circuit between the node  19  and a node  20 . One of the n-channel FETs of the parallel cirucit of two n-channel FETs N between node  19  and  20  is connected, on the gate side, to i 1 , the second is connected, on the gate side, to i 2 . The drains of each of the n-channel FETs of the parallel cirucit are combined or connected to node  20  which is connected to the reference ground potential vss via an n-channel FET N to which i 0  is applied on the gate side. The node  19  is optionally connected to the supply voltage vdd via an n-channel FET having a gate connected to the precharge signal prech.  
         [0034]     A series circuit of a p-channel FET P and an n-channel FET N is arranged in a further parallel branch between the supply voltage vdd and the reference ground potential vss, where the p-channel FET P is connected, on the gate side, to node  17  and the precharge signal prech is applied to the n-channel FET N on the gate side. The carry output co 2  is provided at a junction between the p-channel field effect transistor P and the n-channel FET N of the series circuit between the supply voltage vdd and the reference ground potential vss.  
         [0035]      FIG. 7  illustrates a schematic circuit for dynamically implementing the block  14  shown in  FIG. 5 . A p-channel FET P having a gate to which the precharge signal prechq is applied, is connected between a supply voltage vdd and a circuit node  21 . A series circuit of two n-channel FETs N is provided between the node  21  and a reference ground potential vss. The carry input ci 1  is applied to the gate of one of the n-channel FETs and i 2  is applied to the gate of the second n-channel FET of the series circuit of two n-channel FETs N provided between the node  21  and the reference ground potential. A parallel circuit of two n-channel FETs N is parallel to the series circuit between the node  21  and a node  22 , where one of the n-channel FETs is connected, on the gate side, to i 2  and the other n-channel FETs is connected, on the gate side, to the carry input ci 1 . The node  22  is connected in turn, via a parallel circuit of two n-channel FETs N, to the reference ground potential vss in a manner dependent on i 0  or i 1 . One of the n-channel FETs of the parallel circuit between node  22  and the reference ground Vss is connected, on the gate side, to i 0  and the other n-channel FETs is connected, on the gate side, to i 1 .  
         [0036]     The circuit node  22  may be connected, via an n-channel FET N, to the supply voltage vdd in a manner dependent on the precharge signal prech, where the precharge signal prech is connected to the gate of the n-channel FET N connected between the supply voltage and node  22 .  
         [0037]     Provided as further parallel branches between the circuit node  21  and the reference ground potential vss is a series circuit of two n-channel FETs N, where i 1  is applied to one of the n-channel FETs on the gate side, and i 0  is applied to the other n-channel FET on the gate side. In addition, an n-channel FET N to which ci 2  is applied on the gate side, is connected parallel to the series circuit between the circuit node  21  and the reference ground potential vss. A series circuit of a p-channel FET P and an n-channel FET N is connected, as a parallel branch, between the supply voltage vdd and the reference ground potential vss. The p-channel FET P of the series circuit connected between the supply voltage vdd and the reference ground potential vss is connected, on the gate side, to the node  21 . The n-channel FET N of the series circuit connected between the supply voltage vdd and the reference ground potential vss is connected, on the gate side, to receive the precharge signal prech. The carry output signal co 1  is provided at the junction of the p-channel FET P and n-channel FET N of the series circuit connected between the supply voltage vdd and the reference ground potential vss.  
         [0038]      FIG. 8  shows a schematic illustration of the sum block  12  shown in  FIG. 3  and/or  FIG. 4  and shows (on the left hand part) a possible implementation of the input stage. A series circuit comprising a p-channel FET P and an n-channel FET N is arranged between a supply voltage vdd and a reference ground potential vss, where the precharge signal prechq is applied to the p-channel field effect transistor P on the gate side, and the signal at the carry input ci 1  is applied to the n-channel FET N on the gate side. The circuit node  23  at which the signal i 1   q  is tapped off is located between the p-channel FET P and the n-channel FET N. The signal i 1   q  at the node  23  is converted into a signal i 1  using an inverter  1  which is connected to both the reference ground potential vss and the supply voltage vdd. A similar input stage is provided for each input signal ci 1 , ci 2 , x 1  (which corresponds to i 0 ), x 2  (which corresponds to i 1 ) and x 3  (which corresponds to i 2 ) (see  FIG. 4 ). The signals i 2   q  and i 2  are generated, for the sum block, from the carry input ci 2 . The signals i 3  and i 3   q  are generated from the input signal x 1 . The signals i 4  and i 4   q  are generated from the input signal x 2 . The signals i 5  and i 5   q  are generated from the input signal x 3 .  
         [0039]      FIG. 8  shows (on the right hand part) a schematic illustration of the sum block, with resorting likewise being carried out again in this case since i 3  shown in  FIG. 8  (left-hand part) becomes x 1 , i 3   q  becomes x 1   q , i 4  becomes x 2 , i 4   q  becomes x 2   q , i 5  becomes x 3 , i 5   q  becomes x 3   q , i 2  becomes x 4 , i 2   q  becomes x 4   q , i 1  becomes x 5  and i 1   q  becomes x 5   q . In addition, the summation device shown in  FIG. 8  (right hand part) has a precharge access having the signal prechq, an enable input EN (the signal prechq also being applied to the enable input EN), a sum output s and a connection to the reference ground potential vss and the supply voltage vdd. The input stage shown in  FIG. 8  (left hand part) is used to synchronize the sum stage with dynamic circuit parts of the overall circuit.  
         [0040]      FIG. 9  shows a schematic circuit diagram, of an exemplary quintuple XOR function stage, or circuit, as the sum block shown in  FIG. 8 . The two time critical carry signals ci 1 , which are converted into i 1  and i 1   q , and thus into x 5  and x 5   q  (see  FIG. 8 ), and the carry input signal ci 2 , which is converted into i 2  and i 2   q , and thus into x 4  and x 4   q , are preferably connected to n-channel field effect transistors N located next to the outputs Z and ZQ of the XOR circuit. The quintuple XOR stage  15  shown in  FIG. 9  is connected to the supply voltage vdd by means of an upstream connection  24  in a manner dependent on the precharge signal prechq and, in addition, can be connected to the reference ground potential vss via an enable signal EN at the gate of an n-channel field effect transistor N. This enable signal EN is supplied via the enable input shown in  FIG. 8  (right hand part).  
         [0041]      FIG. 10  illustrates carry-ripple adders B 1 , B 2 , B 3  where the output carry bits are of unequal significance.  
         [0042]     Although the present invention has been described above with reference to a preferred exemplary embodiment, it is not restricted thereto but rather can be modified multifariously. The circuit principle of the carry path, which is based on calculating and forwarding two carries of equal significance, can therefore also be used for two carry signals which are interchangeable. In addition, the blocks which are used to generate the two carry signals are not necessarily independent of one another. In the case of an implementation using complementary CMOS gates, it is possible to make joint use of subblocks. However, separation is advantageous for a high-performance application.  
         [0043]     In addition, the n-channel transistors N which are located in the evaluation part of the carry gates (see  FIG. 6  and  FIG. 7 ) and to whose gate the precharge signal prech is applied are not required for a basic implementation of the logic function. They reduce the charge sharing problem that can arise depending on the technology and layout. They are therefore optional, may also be in the form of p-channel FETs with inverted driving, and constitute advantageous optimization. Any static or dynamic quintuple XOR gate may, in principle, be used as the sum stage. In addition, other carry-ripple adder may be utilized without any restriction.  
         [0044]     The above described embodiments are given as illustrative examples only. It will be readily appreciated that many deviations may be made from the embodiments disclosed in this specification without departing from the invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.  
       LIST OF REFERENCE SYMBOLS  
       [0000]    
       
          i 0 , i 1 , i 2  Inputs for input bits  
          x 1 , x 2 , x 3  Inputs for input bits  
          i 0 &lt; 0 &gt; i 0 &lt; 4 &gt;,  
          i 1 &lt; 0 &gt; i 1 &lt; 4 &gt;,  
          i 2 &lt; 0 &gt; i 2 &lt; 4 &gt; Input bits at corresponding inputs  
          ci 1 , ci 2  Inputs for carry bits  
          s, s 0  s 4  Summation outputs  
          cot, cot Outputs for carry bits  
          2n Significance of a bit (n=natural number)  
          2n+1 Significance of a bit that has been increased by one  
          prech, prechq Precharge inputs  
          prech  1 , prechq  1  Precharge inputs  
          vdd Supply voltage  
          vss Reference ground potential  
           10  Carry-ripple adder/bit summation device  
           11  Carry stage (carry summation)  
           12  Summation stage (normal summation or carry)  
           13  Carry addition block  
           14  Carry addition block  
           15  Quintuple XOR stage  
           16  Multibit carry-ripple adder  
           17 ,  18 ,  19 ,  20  Circuit nodes  
           21 ,  22 ,  23  Circuit nodes  
           24  Upstream connection of the quintuple XOR stage  
          B 1 , B 2 , B 2  Carry-ripple adders based on the prior art in which the output carry bits are of unequal significance  
          P, N p-channel FET, n-channel FET  
          en Enable signal