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1 Low Power Bus Encoding Technique Considering Coupling Effects Hsin-Wei Lin H.W. Lin is with the Graduate Institute of Integrated Circuit Design, National Changhua University of Education, Taiwan. (e-mail: m94662001@mail.ncue.edu.tw)

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2 Outline Introduction Proposed Scheme Experimental Results Conclusion

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3 Introduction Increased coupling effect between interconnects not only aggravate the power consumption but also deteriorates the signal integrity. The power consumption of bus depends on several factors such as: switching activity wire aspect and spacing inter-wire capacitances power supply voltage

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4 Introduction (con.) With shrinking feature sizes, the wire aspect is increasing and the spacing between the bus lines is reducing. In order to reduce the power consumption, many different bus encoding techniques have been presented in the literature. Bus-Invert EXODUS EXNORA

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5 Introduction (con.) Lowering transition-switching activity on the bit lines of bus leads to a significant reduction the bus power consumption.

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6 Bus model with self- and coupling-capacitances

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7 Switching activity Switching activity is described as the transition between different logic levels which divides into self-transition (α s ) and coupling-transition (α c ). Correlated switching is defined as the neighbouring bus lines switch simultaneously in opposite directions.

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8 Example of transition types The ratio of total effective coupling capacitance is 1:2:4 in type A, type B and type C respectively.

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9 Proposed Scheme The encoding technique utilizes XOR and XNOR four kinds of combinations conversion of data. D(t) data on a bus at cycle time t E[D(t)] encoded data of D(t) D n (t) is divided into subsets such that each subset consists of D 4 (t). and are independently encoded.

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10 Encoding rules for 4-bit subset

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11 Encoding example for 4-bit subset Current data: E[D(t)] = 1011 Next data: D(t+1) = 0 00 0 Encoded data: E[D(t+1)]= 1011 Encoding rule: XOR-XOR

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12 Encoding example for 4-bit subset Current data: E[D(t)] = 1011 Next data: D(t+1) = 0 10 0 Encoded data: E[D(t+1)]= 0011 Encoding rule: XNOR-XOR

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13 Encoding examples for 4-bit subset Next data: D(t+1)Encoding rule Example Current data: E[D(t)]=1011 Unencoded data: D(t+1) Encoded data: E[D(t+1)] X00XXOR-XOR 00001011 00011010 10000011 10010010 X01XXOR-XNOR 00101010 00111011 10100010 10110011 X10XXNOR-XOR 01000011 01010010 11001011 11011010 X11XXNOR-XNOR 01100010 01110011 11101010 11111011 Number of correlated switchings80

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14 Illustration for 8-bit encoding data lines The rationale for encoding type selection is to silence the middle two data lines of each subset.

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15 Receiving end Restore original data by control line at the receiving of the bus. The original data can be retrieved by simply applying the same type of decoding, because of the XOR property that, which is also the case for XNOR.

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16 Schematic of codec circuit for 4-bit data lines Receive blockTransmission block

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17 Experimental Results Assumed that the activity on a typical data bus was randomly and uniformly distributed as in the statistical power estimation method. There are 2 2N possible transitions and N-bit changes per transition, there is a total of N×2 2N possible bit changes for N-bit bus lines.

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18 Power dissipation The average power dissipated on the bus is given by: average power number of transitions per bus cycle parasitic capacitances of the bus lines supply voltage clock frequency

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19 Number of switching activities in 4-bit data lines Item 4-bit data lines UnencodedBus-InvertEXODUSEXNORAOur Scheme Total combinations1024 Number of self-transitions 512 ( 1.6 ) 320 ( 1 ) 320 ( 1 ) 320 ( 1 ) 256 ( 0.8 ) Number of silent lines 512 ( 0.73 ) 704 ( 1 ) 704 ( 1 ) 704 ( 1 ) 768 ( 1.09 ) Number of coupling-transitions 384 ( 1 ) 384 ( 1 ) 384 ( 1 ) 400 ( 1 ) 256 ( 0.67 ) Number of correlated switchings 96 ( 4 ) 24 ( 1 ) 16 ( 0.67 ) 0 ( 0 ) 0 ( 0 ) Power dissipation1.610.930.840.54

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20 Number of switching activities in 8-bit data lines Item 8-bit data lines UnencodedBus-InvertEXODUSEXNORAOur Scheme Total combinations524288 Number of self-transitions 262144 ( 1.6 ) 163840 ( 1 ) 163840 ( 1 ) 163840 ( 1 ) 131072 (0.8) Number of silent lines 262144 ( 0.73 ) 360448 ( 1 ) 360448 ( 1 ) 360448 ( 1 ) 393216 (1.09) Number of coupling-transitions 229376 ( 1.02) 224768 ( 1 ) 229376 ( 1.02 ) 230272 ( 1.02 ) 163840 ( 0.73 ) Number of correlated switchings 57344 (3.7) 15488 ( 1 ) 12288 ( 0.79 ) 5920 (0.1) 8192 (0.53) Power dissipation1.610.970.890.69

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21 Conclusion The propose a bus encoding scheme for reducing switching activity and power dissipation. It eliminates correlated switchings in each subset of 4-bit data lines and minimizes the correlated switchings between the neighbouring subsets. It also minimizes number of self-transitions compared to other proposed schemes and reduces the power dissipation by 46% compared to Bus-Invert method.

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22 Thanks for your listening !

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