The famous quote from the movie 'Spiderman' – “With great power comes great responsibility” gets convoluted when applied to the semiconductor industry where “With LOW power comes great responsibility”. The design cycle that once focused on miniaturization, shifted gears to achieve higher performance in the PC era and now in the world of connected devices it is POWER that commands most attention. Energy consumption today drives the operational expense for servers and data centers so much so that companies like Facebook are setting up data centers close to Arctic Circle where the cold weather reduces cooling expense. On the other hand, for consumer devices ‘meantime between charging’ is considered as one of the key factors defining user experience. This means that environment friendly green products resulting from attention to low power in the design cycle can bring down the overall cooling/packaging cost of a system, reduce probability of system failure and conserve energy.
DESIGN FOR LOW POWER
The existing hardware description languages like Verilog & VHDL fall short of semantics to describe the low power intent of a design. These languages were primarily defined to represent the functional intent of a circuit. Early adopters to the low power design methodology had to manually insert cells during the development phase to achieve the desired results. This process was error prone with limited automation and almost no EDA support. Archpro, an EDA startup (acquired by Synopsys in 2007) provided one of the early solutions to this space. Unified Power Format (UPF – IEEE 1801) and Common Power Format (CPF from Si2) are the two TCL based design representation approaches available today to define a low power design intent. All EDA vendors support either or both formats to aid development of power aware silicon.
VERIFYING LOW POWER DESIGN
Traditional simulators are tuned to HDLs i.e. logic 1/0 and do not have a notion of voltage or power turning on/off. For the first generation of low power designs, where cells were introduced manually, the verification used to be mostly script based by forcing X (unknown) on the internal nodes of the design and verifying the outcome. Later, when power formats were adopted for design representation, the verification of this intent demanded additional support from the simulators such as –
- Emulating the cells like isolation, state retention etc. during the RTL simulations to verify the power sequencing features of the design. These cells are otherwise inserted into the netlist by the synthesis tool taking the power format representation as the base
- Simulating power ON/OFF scenarios such that the block that is turned off has all outputs going X (unknown)
- Simulating the voltage ramp up cycle which means once the power is turned ON, it takes some time for the voltage to ramp up to the desired level and during this period the functionality is not guaranteed
- Simulating multi voltage scenarios in the design and in absence of level shifter cells the relevant signals are corrupted
- Simulating all of the above together resulting into a real use case scenario
Tools from Archpro (MVRC & MVSIM) worked with industry standard simulators through PLI to simulate power aware designs. Today all industry standard simulators have the feature to verify such design with limited or complete support to the UPF & CPF feature list. Formal/Static tools are available to perform quick structural checks to the design targeting correct placement of cells, correct choice of cells, correct connections to the cells and ensuring power integrity of the design based on the power format definition at RTL and netlist level. Dynamic simulations further ensure that the power intent is implemented as per architecture by simulating the power states of the design as functional scenarios.
In the last decade, the industry has collaborated at different levels to realize power aware design for different applications. Low power was adopted for the products targeting the consumer and handheld markets initially but today it is pervasive across all segments that the semiconductor industry serves. The key is to define the low power intent early and incorporate tools to validate that the intent is maintained all throughout. As a result, low power lead to greater responsibility for all stakeholders to the design cycle in general and for the verification engineers in particular!
DVCON 2014 issue of Verification Horizons has an article “Taming power aware bugs with Questa” co-authored by me on this subject.
Drop in your questions and experiences with low power designs in the comment section or write to me at email@example.com