Power Reduction Techniques and Flows at RTL and System Level

Abstract

Power reduction is becoming a critical design criterion for ASIC/SOC designers. Reducing both dynamic and leakage power is imperative to meet power budgets for portable devices as well as to ensure that the systems that these ASICs meet their packaging and cooling costs. In addition, the power of an ASIC has a significant impact on its reliability and manufacturing yield. Traditionally, most automated power optimization tools have focused at gate-level and physical level optimizations. However, major power reductions are only possible by addressing power at the RTL and system levels. At these levels, it is possible to make the sequential modifications needed to reduce power and energy consumption via techniques like sequential clock gating, power gating, voltage/frequency scaling and other micro-architectural techniques. The focus of this tutorial will be on techniques for power reduction at the RTL and system level. It will also focus on expressing power intent at system and RTL levels and the flows needed to use that power intent in tools for functional verification, RTL-level optimization, logic synthesis and physical design. The following sections describe the key focus areas in the tutorial. We will start by discussing the key trends in the semiconductor industry and in CMOS technology and relate them to the need for power-aware design flows all the way from systemlevel design, through micro-architecture definition and RTL design and implementation. We will then present different power and energy metrics that are used at different points in the design cycle and for different purposes such as average power of a system, peak power of a system, energy per cycle etc. We will relate these metrics to their typical use and discuss when a metric should be used and optimized. State-of–the-art techniques for estimation of power and energy metrics will be presented including those for software power estimation, energy estimation for applications on a system, RTL and gate-level power estimation etc. We will discuss both simulation-based and statistical techniques for estimating switching activity in a design. Since, creation of system-level models is becoming a standard part of the design flow in most design teams, we will present typical flows from system-level models to RTL and the kinds of power/energy tradeoffs done at these levels such as power islands and mode identification, memory and bus architectures, voltage scaling and scheduling, and identification of clocking schemes and clock domains. Since power of a design is a function of how it performs a computation over time, almost all the major transformations that have significant impact on the power of a design are sequential in nature – they change the sequence of values generated at key internal registers or memories in time. We will discuss the sequential optimizations like, sequential clock gating, power gating, dynamic voltage scaling and memory banking. The impact of these optimizations on verification and implementation flows will be highlighted and solutions to verification and implementation issues will be presented. In the last few years, standards have started emerging to allow designers to express power intent such as voltage islands and power modes in a design. These are allowing for the same power intent to be seen by all the tools in the RTL flow: RTL simulation, logic synthesis, place and