A SPURIOUS POWER SUPPRESSION TECHNIQUE FOR A LOW-POWER MULTIPLIER ABSTRACT Lowering down the power consumption, reducing the area, increasing the speed of operation and enhancing the processing performance of the circuit designs are the important design challenges of VLSI Design. Multiplication process are frequently used for key computations in low-power wireless multimedia and digital signal processor (DSP) applications, such as fast Fourier transform (FFT), discrete cosine transform (DCT), quantization, and filtering. To save significant power consumption of a VLSI design, it is a good direction to eliminate its dynamic power that is the major part of total power dissipation. Design exploration and application of a technique to suppress the spurious switching power existed in the data-paths of a low power VLSI designs have been proposed here. The proposed technique adopts the design concept of separating the arithmetic units into Most Significant Part (MSP) and Least Significant Part (LSP), and then freezing the MSP whenever this part of circuits does not affect the computation result. This paper explores the realizing of a low power Modified Booth Multiplier and the implementation approaches of a spurious power suppression technique (SPST) design concept. Then this SPST approach has been applied on both the compression tree of multipliers and the modified Booth Encoder to enlarge the power clampdown, for high-speed and low-power purposes. For the low power application a powerful signed-number multiplier called Modified Booth has been proposed. To reduce the power consumption as well as to reduce the multiplication process in to half a Bit-Pair Recoding scheme is used in the Modified Booth multiplier. The partial products are called PP candidates generated with the help of Multipliers in a modified Booth Encoder (MBE).
The output of MBE is fed to SPST-equipped adder to get the final result. The SPST approach leads to a 40% speed improvement when compared with the other power minimization technique. In addition, this paper explores the performance of the proposed design under the conditions of different bit-width input data. The SPST approach not only owns equivalent low-power performance but also leads to a higher maximum speed when compared with the former approaches. Moreover, the proposed SPST-equipped multiplier also provides better power efficiency when compared with the existing modern multipliers. 3 I. INTRODUCTION The existing power reduction techniques are as follows,
Partially Guarded Computation (PGC). Dynamic-range Determination (DRD).
Glitching Power Minimization (GPM). P artially Guarded Computation (PGC) has been proposed to reduce the dynamic power consumption by minimizing the switched capacitance. The technique first divides the input data into two parts – MSP and LSP and disables a part of the functional unit that does not affect the output. This technique will reduce 3% delay overhead in functional unit and reduce 10% to 20% power consumption. I n Dynamic-range Determination (DRD) method before performing an addition operation, the effective dynamic ranges of two input data are determined. Based on a larger effective dynamic range, only selected functional blocks of the adder are activated to generate the desired result . The input bits of the unused functional blocks remain in their previous states. The added result is then recovered to match the required word length. This technique will reduce 30% of the total power consumption. I n g l itching power minimization the power consumption is reduced by replacing some existing gates with functionally equivalent one. These equivalent gates can be frozen the glitching power by asserting a control signal. This technique can only achieve saving of 6.3% in total power dissipation. Before going for the proposed technique we have to go through the Modified Booth Multiplier section for to get better understand of the SPST concept. 4 2. MODIFIED BOOTH MULTIPLIER To speed-up the multiplication process in the Booth’s algorithm a technique called bitpair recoding is used. It is also called modified Booth’s algorithm. It halves the maximum number of summands. In this technique, the Booth-recoded multiplier bits are grouped in pairs. Then each pair is represented by its equivalent single bit multiplier reducing total number of multiplier bits to half. For example pair (+1 -1) is equivalent to the pair (0+1). That is, instead of adding - 1 time multiplication at shifted position i to +1 time the multiplicand at position +1 , the same result is obtained by adding +1 time multiplicand at position i. Similarly, (+10) is equivalent to (0+2), (-1 +1) is equivalent to (0 -1), and so on. By replacing pairs with their equivalents we can get bit- pair
recoded multiplier. But instead of deriving bit- pair recoded multiplier from Booth recoded multiplier one can directly derive it from original multiplier. The bit- pair recoding or multiplier can be directly derived from Table 1. shows the bit-pair code for all possible multiplier bit options. Table 1. Bit-pair recoding scheme. E.g.: By referring the above table 1. the bit-pair code for a multiplier 0000000001101010 can be finding as follows Multiplier bit-pair (i+1) Multiplier bit-pair (i) Multiplier bit-pair(i-1) Bit-pair recoded multiplier bit at position (i) 0000 0 0 1 +1 0 1 0 +1 0 1 1 +2 1 0 0 -2 1 0 1 -1 1 1 0 -1 1110 5 Fig. 1. Bit Pair Recoding of “006A” Consider a Modified Booth Multiplication with two numbers “2AC9” and “006A”. The above Fig. 1. Shows the Bit Pair Recoding of multiplier “006A”. The recoded output 0000+2-1-1-2 has been considered as new multiplier and this will reduce the multiplication process into half of its
original step, this lead to the reduction of power consumption by half. The modified booth multiplied partial products are represented by PP0, PP1, PP2, PP3…etc as shown in the below Fig. 2. Fig. 2. Illustration of multiplication using Modified Booth Encoding. The PP candidates (partial products) generation using MUX in modified booth encoder is shown in Fig. 3. The PP candidates are added using a SPST- equipped adder to get the final result. When an application such as fast Fourier transforms (FFT), discrete cosine transform (DCT), quantization, and filtering are concern multiplication is 16 bit and the speed of operation is depends up on the processor. For DSP applications the input data is sophisticated, at that time the multiplication process is split down to MSP and LSP. Then the operation has been carried out with the help of some latch to switch down some unwanted signals and transients power to get perfect output. 6 Fig.3. Modified Booth Encoder with MUX 3. PROPOSED TECHNIQUE SPURIOUS POWER SUPPRESSION TECHNIQUE (SPST). This paper explores the implementation approaches of an Spurious Power Suppression Technique (SPST) design concept and its application on a low-power (LP) Modified Booth Multiplier (MBM). This SPST approach has been applied on both the compression tree of multipliers and the modified Booth Encoder to enlarge the power clampdown, for high-speed and low-power purposes. To illustrate the influence of the illogical power signal transitions, five cases of a 16-bit addition are explored as an example below (shown in Fig. A.) The 1st case illustrates a transient state in which the illogical transitions of carry signals occur in the MSP though the final result of the MSP are unchanged.
Case (1): 7 The 2nd and 3rd cases describe the situations of one negative operand adding another positive operand without and with carry from LSP, respectively. Moreover, the 4th and 5th cases respectively demonstrate the conditions of two negative operands addition without and with carry-in from LSP. In those cases, the results of the MSP are predictable, therefore the computations in the
MSP are useless and can be neglected. Eliminating those illogical computations will not only save the power consumed inside the SPST adder/subtractor but also decrease the glitching noises which will affect the next arithmetic circuits. The main contribution of SPST is to exploring two implementing approaches and comparing their efficiency to get different material for the clampdown of power and to increase the speed of operation. In fig.4. Shows the simplified implementation approach for the addition � subtraction of two bit numbers. The adder/ subtractor is divided in to two parts. i.e., most significant bit (MSP) and Least significant bit (LSP). The MSP of the original adder/subtractor is modified to include detection logic circuit, data controlling circuits (The data controlling circuits mainly consists of latch A and latch B),sign extension circuit and some glue logics for carry in and carry out signal. 8 Fig.4. low-power adder � substractor example based on proposed SPST. 4. IMPLEMENTATION OF ISPCT The proposed low-power multiplier is designed by equipping the SPST on a tree multiplier. There are two distinguishing design considerations in designing the proposed multiplier and SPST, as listed in the following. 4.1. APPLYING THE SPST ON THE MBE In Fig. 2, the shadow denotes that the numbers in this part of Booth multiplication are all zero so that this part of the computations can be neglected. Saving those computations can significantly clampdown the power consumption caused by the transient signals. According to the analysis of the multiplication shown in Fig. 5, we propose the ISPCT-equipped modified-Booth encoder (MBE), which is controlled by a detection unit. The detection unit has one of the two operands as its input to decide whether the Booth encoder calculates redundant computations. As shown in Fig. 5, the latches can, respectively, freeze the inputs of MUX-4 to MUX-7 or only those of MUX-6 to MUX-7 when the PP4 to PP7 or the PP6 to PP7 are zero, to reduce the transition power dissipation. Such cases occur frequently in e.g., FFT/IFFT, DCT/IDCT, and Q/IQ which are adopted in encoding or decoding multimedia data.
9 4.2 APPLYING THE ISPCT ON THE COMPRESSION TREE The proposed SPST-equipped multiplier is illustrated in Fig.6. The PP generator generates five candidates of the partial products, i.e., f2A; A; 0; A; 2Ag, which are then selected according to the Booth encoding results of the operand B. Moreover, when the operand besides the Booth encoded one has a small absolute value, there are opportunities to reduce the illogical power dissipated in the compression tree. According to the redundancy analysis of the additions, we replace some of the adders in compression tree of the multiplier with the SPST - equipped adders, which are marked with oblique lines in Fig. 5. The bit-widths of the MSP and LSP of each SPST -equipped adder are also indicated in fraction values nearing the corresponding adder in Fig.5. Fig.5. SPST-equipped MBE 10 Fig.6. Proposed Low-Power SPST-Equipped Multiplier, Where The Fraction Values Denote The Bit-Width of The MSP And LSP of The SPST-Equipped Adders. In this circuit Carry section, it is used to determine the carry of the output data. The carry section made by simple AND and OR gates. Sign extension is the operation, in computer arithmetic, to preserving the number's sign (positive/negative). The sign digit is placed in the most significant side of the number. Sign- Extension circuits can be intuitively implemented by multiplexers to compensate the sign signals of the MSP. 11 CONCLUSION In this paper, The proposed multiplier adopting the new SPST implementing approach, leads to a 40% speed improvement when compared with the former technique. This paper explores the performance of the proposed design under the conditions of different bitwidth input data. The SPST approach not only owns equivalent low-power performance but also leads to a higher maximum speed when compared with the former techniques. Moreover, the proposed SPST -
equipped multiplier also has better power efficiency when compared with the existing modern multipliers. REFERENCES 1. Kuan-Hung Chen and Yuan-Sun chu .,”A Low Power Multiplier With The Spurious Power Suppression Technique”, 1063-8210 2007 IEEE 2. A.P. Godse, D.A. Godse, “Computer Architecture”, First Edition, Technical Publications ,2003
