Pulse-latch approach reduces dynamic power

Satoshi Shibatani and Albert H.C. Li
(07/17/2006 8:00 PM EDT), EE Times
 
Recently, many methodologies have been introduced for reducing dynamic power for systems-on-chip (SoCs). These methodologies, however, impose restrictive physical constraints which have schedule impact or which are heavily dependent on logic functions such as clock gating.

This article presents an elegant methodology using pulsed latch instead of flip-flop without altering the existing design style. It reduces the dynamic power of the clock network, which can consume half of a chip's dynamic power. Real designs have shown approximately a 20 percent reduction in dynamic power using the methodology described below.

Introduction

Dynamic power is consumed across all elements of a chip. The clock network is one of the large consumers of dynamic power. According to a recent IBM study [1], half of dynamic power is dissipated in the clock network.

Therefore, reducing power in the clock network can impact the overall dynamic power significantly. Designers already use a variety of techniques to reduce the clock power using smaller clock buffers, reducing the overall wiring capacitance, employing clock gating to reduce the dynamic power [2], and de-cloning to move the clock buffers at higher levels of hierarchy.

Even with these techniques, the dynamic power of clock network can be large since registers are used as state elements in the design. In general, a flip-flop is used as the register. A conventional flip-flop is composed of two latches (master and slave) triggered by a clock signal.

Flip-flop synchronization with the clock edge is widely used because it is matched with static timing analysis (STA). Timing optimization based on STA is must for SoCs. On the other hand, designers may choose to use a latch for storing the state. A latch is simple and consumes much less power than that of the flip-flop. However, it is difficult to apply static timing analysis with latch design because of the data transparent behavior.

A methodology has been developed which uses latches triggered with pulse clock waveforms. With this methodology, designers can apply static timing analysis and timing optimization to a latch design while reducing the dynamic power of the clock networks. The following describes this pulsed latch design methodology in detail and gives some guidelines as to how designers can apply this methodology in their designs.

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