Multi Threshold (MVT) Voltage Technique

Multiple threshold voltage techniques use both Low Vt and High Vt cells. Use lower threshold gates on critical path while higher threshold gates off the critical path. This methodology improves performance without an increase in power. Flip side of this technique is that Multi Vt cells increase fabrication complexity. It also lengthens the design time. Improper optimization of the design may utilize more Low Vt cells and hence could end up with increased power!

Ruchir Puri et al. [2] have discussed the design issues related with multiple supply voltages and multiple threshold voltages in the optimization of dynamic and static power. They noted several advantages of Multi Vt optimization. Multi Vt optimization is placement non disturbing transformation. Footprint and area of low Vt and high Vt cells are same as that of nominal Vt cells. This enables time critical paths to be swapped by low Vt cells easily.

Frank Sill et al. [3] have proposed a new method for assignment of devices with different Vth in a double Vth process. They developed mixed Vth gates. They showed leakage reduction of 25%. They created a library of LVT, mixed Vt, HVT and Multi Vt. They compared simulation results with a LVT version of each design. Leakage power dissipation decreased by average 65% with mixed Vth technique compared to the LVT implementation.

Meeta Srivatsav et al. [4] have explored various ways of reducing leakage power and recommended Multi Vt approach. They have carried out analysis using 130 nm and 90 nm technology. They synthesized design with different combination of target library. The combinations were Low Vt cells only, High Vt cells only, High Vt cells with incremental compile using Low Vt library, nominal (or regular) Vt cell and Multi Vt targeting Hvt and Lvt in one go. With only Low Vt highest leakage power of 469 µw was obtained. With only High Vt cells leakage power consumption was minimum but timing was not met (-1.13 of slack). With nominal Vt moderate leakage power value of 263 µw was obtained. Best results (54 µw with timing met) obtained for synthesis targeting Hvt library and incremental compile using Lvt library.

Different low leakage synthesis flows are carried out by Xiaodong Zhang [1] using Synopsys EDA tools are listed below:

• Low-Vt --> Multi-Vt flow: This produces least cell count and least dynamic power. But produce highest leakage power. It takes very low runtime. Good for a design with very tight timing constraints

• Multi-Vt one pass flow: It takes longest runtime and can be used in most of designs.

• High-Vt --> Multi-Vt flow: Produce least leakage power consumption but has high cell count and dynamic power. This methodology is good for leakage power critical design.

• High-Vt --> Multi-Vt with different timing constraints flow: This is a well balanced flow and produces second least leakage power. This has smaller cell count, area and dynamic power and shorter runtime. This design is also good for most of designs.

Optimization Strategies

The tradeoffs between the different Vt cells to achieve optimal performance are especially beneficial during synthesis technology gate mapping and placement optimization. The logic synthesis, or gate mapping phase of the optimization process is implemented by synthesis tool, and placement optimization is handled physical implementation tool.

Synthesis

During logic synthesis, the design is mapped to technology gates. At this point in the process optimal logic architectures are selected, mapped to technology cells, and optimized for specific design goals. Since a range of Vt libraries are now available and choices have to be made across architectures with different Vt cells, logic synthesis is the ideal place to start deploying a mix of different Vt cells into the design.

Single-Pass vs. Two-Pass Synthesis –with multiple threshold libraries

Multiple libraries are currently available with different performance, area and power utilization characteristics, and synthesis optimization can be achieved using either one or more libraries concurrently. In a single-pass flow, multiple libraries can be loaded into synthesis tool prior to synthesis optimization. In a two-pass flow, the design is initially optimized using one library, and then an incremental optimization is carried out using additional libraries.

About multi vt optimization in his paper Ruchir Puri[2] says: “The multi-threshold optimization algorithm implemented in physical synthesis is capable of optimizing several Vt levels at the same time. Initially, the design is optimized using the higher threshold voltage library only. Then, the Multi-Vt optimization computes the power-performance tradeoff curve up to the maximum allowable leakage power limit for the next lower threshold voltage library. Subsequently, the optimization starts from the most critical slack end of this power-performance curve and switches the most critical gate to next equivalent lower-Vt version. This will increase the leakage in the design beyond the maximum permissible leakage power. To compensate for this, the algorithm picks the least critical gate from the other end of the power-performance curve and substitutes it back with its higher-Vt version. If this does not bring the leakage power below the allowed limit, it traverses further from the curve (from least critical towards more critical) substituting gates with higher-Vt gates, until the leakage limit is satisfied. Then we jump back to the second most critical cell and switch it to the lower-Vt version. This iteration continues until we can no longer switch any gate with the lower vt version without violating the leakage power limit.”

But Amit Agarwal et al. [5] have warned about the yield loss possibilities due to dual Vt flows. They showed that in nano-scale regime, conventional dual Vt design suffers from yield loss due to process variation and vastly overestimates leakage savings since it does not consider junction BTBT (Band To Band Tunneling) leakage into account. Their analysis showed the importance of considering device based analysis while designing low power schemes like dual Vt. Their research also showed that in scaled technology, statistical information of both leakage and delay helps in minimizing total leakage while ensuring yield with respect to target delay in dual Vt designs. However, nonscalability of the present way of realizing high Vt, requires the use of different process options such as metal gate work function engineering in future technologies.

References

[1] Xiaodong Zhang, “High Performance Low Leakage Design Using Power Compiler and Multi-Vt Libraries”, Synopsys, SNUG, Europe, 2003, www.synopsys.com, 10/9/2007

[2] Ruchir Puri, “Minimizing Power Under Performance Constraint”, International Conference on Integrated Circuit Design and technology, IEEE, pp.159-163, May 17-20 2004

[3] Frank Sill, Frank Grassert and Dirk Timmermann, “Reducing Leakage with Mixed-Vth (MVT)”, 18th International Conference on VLSI Design, IEEE, pp.874-877, January 2005

[4] Meeta Srivatsav, S.S.S.P. Rao and Himanshu Bhatnagar, “Power Reduction Technique Using Multi-vt Libraries”, Fifth International Workshop on System-on-Chip for Real Time Applications, IEEE, pp. 363-367, 2005

[5] Amit Agannral, Kunhyuk Kang, Swarup K. Bhunia, James D. Gallagher, and Kaushik Roy, “Effectiveness of Low Power Dual-Vt Designs in Nano-Scale Technologies Under Process Parameter Variations”, ACM, ISLPED’O5, August 8-10,2005, San Diego, California, USA. 2005.