Low Power: A multi-layered system approach

The continuous scaling of technology over the last decades has led to a multitude of issues that we have to face today when designing integrated circuits (ICs). Of these, power consumption is one of the most challenging issues in current system design. Thereby, the exponential increase of power dissipation in central processing units (CPUs) is just one obvious example. However, this introductory example can be taken to make the influences on various aspects for developers, vendors and customers clear:

  • Economical: Direct costs for power consumption but also for cooling due to thermal dissipation loss
  • Ecological: Efficient and eco-friendly usage of natural resources
  • Functional: Weight or runtime of mobile devices; Noise of fans for cooling
  • Technical: Appropriate materials, devices and solutions required to avoid performance loss or degraded reliability

Against the background of these aspects, research efforts have to be intensified so that hitting the virtual power barrier can be circumvented. Otherwise, such a barrier would endanger the future performance increase of ICs and thereby limit the access and the opening up of markets and new business opportunities, respectively.

Theoretical background

The total power consumption Ptot can be divided into subparts that have diverse causes and influence other aspects of the system design (e.g. reliability). Formula 1 depicts the basic correlation that includes the three main components of power dissipation:

     Ptot =  α CL VDD2 f  +  VDD Ileakage  +  tSC VDD Ipeak f     (1)

The first, dynamic component describes the loss due to charging the load capacitance CL to the supply voltage VDD. Thereby, the occurrence is dependent on the frequency f and the switching probability α. In contrast to both other components, leakage currentsIleakage also occur during the idle state of the circuit. They are the result of different technological causes but are mainly dominated by sub-threshold currents. The third component arises when a gate connects the supply rail with ground for a short period of time tSC during switching of the gate. This results in a short-circuit currentIpeak that is, thus, dependent on the frequency f.

The given formula gives a first insight into the theoretical denotation of power consumption. However, further elements and impacts have to be considered, e.g. glitches or static power.

Multi-layered system approach

The different components do not just add to the total dissipation but also have an impact on other system characteristics. Power density, IR-drop, thermal stress and electromigration are just some of the related issues that result in design overhead to cope with them or in reliability concerns. Therefore, power consumption can and has to be faced at all levels of the system and during all steps of production and development.

This is what makes it a multi-layered system approach that ranges from technology via all architectural levels to design tools, methodology and application Software.