On-Going Research on Address Bus Encoding for Low Power: A Status Report

The above observations have motivated a deeper inveatiption of PO8aible bU8 encodiq tedmiques that may improYe the emciency of the existiq approadles. In tbil paper, we report OD the sta&us of our cunen& re8eard1 in tbil area. We tint consider &he case of microp~f'obued, pneral purpose 8YStem., and we dilCUU posaible schemes that adlieve be&ter performance &han &he aimple Gray code. The encodiDl mecbani.m. we propoae rely on &he idea of avoidin, &he &ransfer of consecutive addreuea on the bus by uaiDl' a redundant line, /NC, &0 &ransfer &0 &he receiving .u~tem &he infOlmation on &he sequentiali&y of &he addressea. When two addresaes in the stream are conHCU&ive, the /NC line is set &0 1, the bU8lines are frosen (to avoid unn~ .witd1inp) , and &he new addrea ia compu&ed irec&ly by the receiver. On &he other hand, when &wo addresaes are no& conaecu&ive, the /NC line ia driven to 0 and &he bU8 linea operate normally. The 8d1eme above, called TO code [5], guarantees an asymptotic performan~ of zero t~tiona under &he hypothesis of infinite "reams of co.-ecutive addresaes. Several variants of the TO code are available [6), aome of whim incorporate the Bu.-Invert principle &0 exploit diatinc&ive .pec&raI maracteristica of &he streama beiD& transmi&ted. We then move to special purpose syatema, where &he UN of codes .uch as &he Gray and &he TO are ineffec&ive, due &0 &he reduced sequentiality of typical add.."reama. In apiu of tbil, it may still be the cue that o&her &ypes of temporal conelation. exis& be&ween the patterns that are beinc transmi&&ed. In particuIar, we have DOted &hat &ime-adj_& addr-. usually ahow remarkably hiCh block correJatjona. Therefore, we propoae to exploi& aud1 coneIationa to come up with a adteme, called in the foUowinC &he Beach code [7), which minimithe averace addreu bU8 switdljng activity. Startinc from &ypical &races of &he N -bit addreu bus of &he 'yaUm being deaigned, we collect ata.tistical information identifyinc pouibJe block correlationa. We then group bU8 lines in clU8&en accordinc &0 &heir conelationa, that i., lines belongiq to &he aame clu8&er are highly correlated. For each cluster of size k we au&omaticaUy generate an encoding function, namely, a one-&o-one Boolean func&ion E : Bi -+ Bi. EadJ configuration of bits in &he original cluster is tranalaud in&o a new bi& configuration. The algori&hm which finda function E ~ta the minimization of the swi&chinc 8C&ivi&yj thua, well establiahed temnology, initially developed for low-power FSM state aasipment and re-encodinc, can be auccesafully exploited. The output of the transformation i. an encoded stream for whim the average number of bua line tranaitiona between two successive k-bit patterns is minimised. At the nceivinC end of the bus, the oripnal encoding i. obviou.ly required. Then, &he inverae func&ion, E-l, must aIao be calcuJated. Abstract Thu ,aper ducri6e. our mearch on bw encoding for low ,ower di..i,ation. We "..ent tVlo encoding .cherne., namel" the TO code and the Beach code, that .eMibly reduce the .Ulitching activit, on addm. bw.". The TO code eqloiU the high .equen. tialit, of the .~m. traveling on the add,... bu..e. of general ,urpo.e, micro,roce..or-ba.ed .y.tefn$. The Beach code, on the other hand, tarpet. the reduction of the number of traMitioM on the addre.. bu. line. of .p«ial purpo.e .,.tefn$, where .tream. ~ua/l, haw reduced .equentia/it" and eqloiU the concept of block co~ation that ma, ezi.t between ,attern. being traMmitted over the communication channel.

[1]  Nikil D. Dutt,et al.  Reducing address bus transition for low power memory mapping , 1996, Proceedings ED&TC European Design and Test Conference.