1.1 Motivation of the Thesis
Wireless microsensor network (WSN) technology creates enormous possibility to have a positive impact on our near future life [1.1]-[1.2]. Advances in ultra-low voltage (ULV) circuit design have recently demonstrated capabilities compatible with wireless body area sensor networks (WBASNs) needs. An important application of WBAN is the vital sensor network shown in Fig1.1. Sensor nodes that measure biomedical signals such as electrocardiogram, blood pressure, and etc, are small pieces either attached on or implanted into a human body. They use a battery with as thin and light characteristics as possible. Most of them do not have the ability to last for a long time. As a result, the demand for low power has been critical in a WBAN.
For the health-care purpose, it makes sense when the observation period could last for days to weeks. Therefore, the ultra-low power wireless sensor node (WSN) is the most crucial design target to achieve.
Figure 1.1. Sensor network of WBAN.
The power reduction is an important design issue for the WBAN. For ultra-low power circuit design, transistors will operate in near/sub-threshold region [1.3].
Lowering the supply voltage and frequency is one of the attractive approaches to reduce power consumption.
Furthermore, dynamic voltage and frequency scaling (DVFS) shown in Fig.1.2, achieves extremely efficient energy saving by adjusting system supply voltage and frequency depending on workload monitor [1.4]. Because if this reason, there are many previous researches about DVFS power management for digital systems such as RISC, DSP and Video Code.
Figure 1.2 Dynamic voltage and frequency scaling systems.
As we continue to reduce the voltage until the transistor get into the near/sub-threshold voltage, circuits will become more sensitive to PVT variations than super threshold. Thus, minimizing energy dissipation and improving variation immunity are far more important rather than operating frequency. Thus, a process,
Since wearable sensors are intended to be worn on the body, miniaturization and minimal weight are important [1.5], [1.6]. Energy harvesting and 3D integration have the potential to solve above mentioned challenges. Energy harvesting [1.7] can exploit the external environment as a source of energy for sensor nodes operating over a full lifetime; while 3D integration [1.8], [1.9] can stack more die connected with a very high packing density of one chip. Through-silicon via (TSV) technology, however, led to higher power density that is much worse hot spot issues. Thus, a PVT-aware micro-watt DVFS system for energy harvesting is also essential.
1.2 Research Goals and Major Contributions
The goal of this research is to design and implement PVT sensors for micro-watt DVFS system shown in Fig.1.3. It includes the design of self-calibration all-digital PVT sensors, MTCMOS switched capacitor (SC) DC-DC converter, temperature compensation for low-voltage digital assist PLL, and dynamic voltage and frequency scaling system.
The major contributions of this thesis are list as follow:
1. A near-/sub-threshold all-digital process, voltage, and temperature sensor is proposed and integrated to the micro-watt DVFS system. The sensor is proposed for high accuracy, ultra-low voltage, low power, and self-calibration portable applications.
2. A self-calibration near-/sub-threshold all-digital temperature sensor with adaptive pulse width generator is proposed. The sensor is proposed for high accuracy, ultra-low voltage, low power, and self-calibration portable applications.
3. A novel connect scheme for improving power efficiency and reducing voltage variation of switched capacitor(SC) DC-DC converter which generates ultra low voltage is proposed.
4. To proposed a temperature compensation for low voltage digital assist PLL can modulate output frequency via divider. Use the temperature sensor to compensate the frequency can not lock in the worth case.
VDDCORE
VDD
VDDH=0.5V
VDDL=0.4V~0.2V DVFS CTRL
MTCMOS SC DC-DC
Converter
workload
DVFS Module
XTAL
Digital assisted
PLL CLK
N
TS
volt_level
PVT Sensors
Figure 1.3 Proposed PVT sensors for micro-watt DVFS system.
1.3 Thesis Organization
The organization of the thesis is as bellow: Previous of conventional process,
shortcomings of conventional PVT sensors are introduced. The applications of PVT sensors are also introduced.
The organization of the thesis is as below. The Chapter 1 is an introduction and motivations for research. Then, in chapter 2, introduces an overview of conventional process, voltage, and temperature sensors. The detail circuits of conventional process, voltage, and temperature sensors are introduced in this chapter and which also includes the shortages of conventional PVT sensors and the applications of PVT sensors.
In chapter 3, we have as following ideas. First, proposed fully on chip and fully digital PVT sensors. Second, we presented novel FDC technique for PVT sensors, and it can reduce power and area. Also at the same time, it improves temperature linearity in near/sub-threshold region.
The self-calibration all-digital PVT sensors and the self-calibration temperature sensor with adaptive pulse width compensation are proposed in Chapter 4. We use all-digital circuit to replace the traditional PVT sensors which can reduce power consumption and area. And to propose self-calibration technology does not require additional circuitry to achieve high accuracy.
PVT sensors for micro-watt DVFS system is introduced in the final chapter. We combine with previous proposed PVT sensors and DVFS system to control the output voltage of SC DC-DC converter for achieving the reduction of power consumption in different PVT variation. And use PVT sensors to compensate the output frequency of low voltage PLL can not lock in the worth case.