利用動態輸入選擇控制之低功率多工器樹設計

Abstract

可攜式設備的需求增加已經導致數位系統設計更為複雜，設計這類的數位系統，功率消耗是必須考量的要點。多工器是近代VLSI設計中很常被使用的基本元件。然而，標準元件庫中卻多只提供2對1和4對1多工器，因此多對1的多工器樹必須由2對1和4對1多工器元件來組成。在STUMP處理器中，多工器樹的功率消耗佔了34%。而目前的研究中，有使用演算法層級及電路層級技術來降低多工器樹的功率消耗，但是使用暫存器傳送階層（RTL）來降低多工器樹的功率消耗卻很少被提出。在此論文裡，讓多工器樹中每一個多工器元件有各自的選擇訊號，當多工器樹在做資料選擇時，利用控制器使多工器樹中大部分的選擇訊號都維持不動，只有選擇路徑上的選擇訊號會改變，藉此減少多工器樹的功率消耗。實驗結果顯示，對於256選一且輸入資料寬度為128位元的多工器樹，我們所提出的架構，在最遭的情況下，仍比傳統的多工器樹減少將近58%的功率消耗而在最佳的情況下，可比傳統的多工器樹減少高達93%的功率消耗。

The increasing demand of portable devices has greatly raised the complexity of digital systems. To develop these systems, the designers must take power consumption into account. A multiplexer (MUX) is a basic component often used in modern VLSI designs. However, there are only 2-to-1 and 4-to-1 MUX cells in most standard cell libraries. An n-to-1 MUX tree must be composed of 2-to-1 and 4-to-1 MUX cells. In the STUMP processor, the power consumption of MUX trees occupies 34% of the total power. Many techniques have been proposed to reduce power consumption of MUX trees. They have been performed at the algorithmic level and also at the circuit level. But the technology at Register Transfer Level (RTL) has seldom been addressed. In this thesis, we let each MUX cell in the MUX tree have its own selection signal. With operations of the MUX tree, a controller is used to keep most of selection signals hold their previous values. Only the selection signals on the output signal propagation path have transitions. Thus power consumption of MUX trees can be reduced. For a 256-to-1 MUX tree with 128-bit width input data, the experiment results show that our proposed MUX tree design reduces about 58% power dissipation in the worst case and reduces up to 93% power dissipation in the best case.