Power aware RTL design

With the progress in technology, the designs are moving into deeper sub-micron technology nodes. There is an ever-increasing concern about power dissipation within the SoC. But this should not come at the cost of performance. So, along with less power dissipation, there is need for maximum power efficiency, that is maximum proportion of available power should be used for useful purposes rather than just to keep the device awake. Now the question arises: Whether to start planning from power perspective at the RTL Design level or wait for the problems to be fixed in the backend flow of the design cycle. The answer is former. Efforts are made to achieve maximum power efficiency along all the stages of the design. But the backend flow can only implement the changes at physical level. It cannot fix the micro-architecture which has a significant impact on the dynamic power dissipation within the SoC.

Figure showing Impact of design change on performace

Power aware design is achieved at several levels of abstraction. System design starts from system requirements and specification and goes through design at architecture design, RTL design, gate level design and finally, layout design. At all these stages, techniques are adopted to meet the design power and performance requirements. It has been found that any effort that is made to improve the power efficiency along all the design stages has maximum impact, if it is done at the RTL level. But, the impact is measured most immediately if it is done at the layout level. So, it is very difficult to measure the impact of any architectural change at RTL level. Improvements are needed for power estimation methods at the RTL level. But, it does not mean that backend techniques should not be adopted.

Power aware design is often misunderstood as low power design. But, these are not the same. By low power design, we mean minimizing the power consumption with or without any performance constraint. But by power aware design, is meant the minimizing the power dissipation without any impact on power. Power aware design refers to maximizing some other performance constraint without any significant impact on power efficiency. Achieving maximum performance being constrained to a particular power budget is the aim of power aware design. 

As said earlier, there is an ever increasing demand for low power devices. As these devices run on batteries having limited supply, and the requirement for them is to operate the maximum they can on a single battery. There are long phases of device idle time. In between, the device is active for very small periods of time. And during the active time, high performance is the requirement. One such example is digital energy meters where there is requirement to keep record of the total kWh used. The power may be available in patches, or may be continuously available. There may be long periods when there is no power. Since the power is available, we can afford to have chargeable batteries, but the watts consumed by the controller itself should be very less as compared to the total power consumed so as to minimize the overhead. During the idle periods, device may go to sleep mode so as to save power. As long as power is available, it should wake up immediately. In other words, average power is less but variance in power consumption is very high.  Hence, it requires a provision in RTL to sense incoming signal levels and to change the gears accordingly. There are many techniques adopted for power aware RTL designsuch as performance throttling, judicious module selection, incorporation of power information in RTL, voltage and power islands and power aware design of memories.

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