## 2 Dynamic Power Management
### Dynamic Frequency and Voltage Scaling
#### 2.2.4 Dynamic Voltage and Threshold Scaling (DVTS)  
The energy efficiencies provided by DVS/AVS can be further improved by adjusting the threshold voltage in addition to the supply voltage [9]. From Equation 2, it is apparent that a constant performance can be obtained by scaling the numerator and the denominator in an identical fashion. Thus the locus of constant performance is approximately a straight line, in the \(V_{DD}-V_{TH}\) plane as shown in Figure 5.  
The DVS technique, drives down supply voltage to point A on the constant performance curve, which is at the intersection of the nominal \(V_{TH}\) line with the constant performance locus. However, the point of minimum total power can be elsewhere on this locus and its exact location depends on the ratio of the leakage power to the dynamic power. For instance, when leakage power dominates, it is more optimal to increase threshold voltage to reduce leakage power, and increase supply voltage (and dynamic power) to maintain performance. Conversely, when dynamic power dominates, it is beneficial to reduce supply voltage and also the threshold voltage. Using simple models for power and performance, analytical results for the optimum power point have been derived [9]. Figure 6 shows power savings of DVTS over that of DVS for a 27 stage ring oscillator with 1% switching activity in a UMC 90 nm process node. The power savings of using DVTS over DVS become larger with increasing contribution of sub-threshold leakage power to total power consumption. Figure 5: Locus of constant frequency under Dynamic Voltage and Threshold Scaling. The threshold voltage can be controlled by changing the body bias of the transistors and is called the adaptive body bias (ABB) technique. For fast silicon, reverse body bias is applied to increase threshold and reduce leakage, while for slow silicon, forward body bias is applied to reduce threshold and improve performance [4]. Here the body bias adjustment is done sparingly as needed based on the chip's process condition, and is not a continuous variable like the supply voltage. However the point of minimum power depends on the ratio of dynamic to leakage power, with the former modulated by the activity factor. For media applications, the activity factor can vary a lot across different applications and hence, the optimal settings for threshold and supply also vary [13]. Analogous to table based DVS, a table based DVTS scheme has been proposed where for different activity factors, the corresponding supply and body bias values are stored in the look up table. The activity factors are estimated for different applications off-line. During run-time, based on the application being run, the appropriate activity factor is used to consult the look up table to obtain the settings for supply and body bias, which will minimize power [13]. Adaptive voltage and threshold scaling aims to arrive at the optimal voltage and threshold values at run time, by not only adapting to the process and temperature like in AVS, but also to the dynamically varying activity conditions. One approach to doing this is shown in Figure 7[14]. The critical path monitor, a control system and the supply voltage regulator are common with an AVS system. In addition, the DVTS system has a power monitor (both dynamic and leakage power) as well as a regulator to generate the body bias voltage. The control system is enhanced now to include readings from the power monitor and hunts for the optimal supply and body bias voltage which minimizes power while meeting target frequency. Dynamic and leakage power are monitored on replica circuits, much like the replica delay path. So it is essential to tune the replica circuits to match with the characteristics of the load circuit, across different process, temperature and activity conditions. Figure 8 shows an alternative approach to monitor the total power of the load circuit. Figure 6: Savings provided by DVTS beyond that of DVS in a 90 nm process node for a 27 stage ring oscillator with 1% switching activity.