Low Power Bus Encoding Technique Considering Coupling Effects

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)

2. Outline • Introduction • Proposed Scheme • Experimental Results • Conclusion

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

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

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

6. Bus model with self- and coupling-capacitances

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

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.

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) • Dn(t) is divided intosubsets such that each subset consists of D4(t). • and are independently encoded.

10. Encoding rules for 4-bit subset

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

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

13. Encoding examples for 4-bit subset

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

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.

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

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 22N possible transitions and N-bit changes per transition, there is a total of N×22N possible bit changes for N-bit bus lines.

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

19. Number of switching activities in 4-bit data lines

20. Number of switching activities in 8-bit data lines

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.

22. Thanks for your listening !