Leona Okamura

A new 7-transistor SRAM cell design with high read stability

The conventional SRAMs, namely four-transistor SRAM (4T) and six-transistor SRAM (6T), suffered from the external noise, because they have direct paths through bit-line(BL) to their storage nodes. This paper proposes seven-transistor (7T) SRAM which has no direct path through BL to the data storage nodes and has higher endurance against external noise. The proposed cell is composed of two separate data access mechanisms; one is for the read operation and another is for the write one. Based upon our SRAM design, data destruction never occurs in the read operation. Simulation result shows that the read Static-Noise-Margin (SNM) of the proposed cell is enhanced by 1.6X and 0.31X with the conventional 4T and 6T SRAM cell respectively. We also manufactured a chip and confirmed its performance.

Low-Power On-Chip Charge-Recycling DC-DC Conversion Circuit and System

A charge-recycling circuit and system that reuses the energy between two or more stacked CPUs is proposed in order to double the life of a battery. In this architecture, CPUs are divided into upper and lower load groups, and electrical charges are shared among the stacked CPUs and a tank capacitor. Charges are temporarily stored in the tank capacitor and are then reused. To control divided loads, a high-speed and energy-efficient regulator is needed. Internal circuit voltage variation between the upper and lower modules is determined by seven low-drop-out (LDO) regulators, a voltage-boosting capacitor circuit, and the tank capacitor. As a result, stable voltage can be supplied to each CPU, even if the upper and lower loads are different or a battery is being used. The LDOs improve the margin of collection in the tank capacitor or task schedule operation, and power efficiency is raised even further. The circuit can be implemented on silicon without a large external control circuit and inductor such as a switching regulator. This circuit was applied to an in-vehicle lock-step system because the upper and lower loads and tasks are the same. Additionally, by using the proposed task scheduling to maximize efficiency, this circuit can be applied not only to lock-step systems but also to general systems. Test chips were fabricated using 90-nm standard CMOS technology. Although the maximum power efficiency of a conventional circuit with a simple LDO is 44.4%, efficiency of the proposed charge-recycling circuit turned out to be as high as 87.1% with the test chips.

A macro-layer level fully parallel layered LDPC decoder SOC for IEEE 802.15.3c application

In this paper, we propose an ultra high-throughput LDPC decoder SOC to fulfill the requirement of IEEE 802.15.3c standard. By implementing a macro-layer fully parallel architecture, our proposed decoder takes only 4 clock cycles to finish one layered decoding iteration. Interconnection complexity problem introduced by high-parallel decoding is nicely solved by proposed reusable message permutation networks utilizing the features of code PCM. Critical path is shortened by applying frame-level pipeline decoding. A 65nm CMOS chip is fabricated to verify the proposed architecture. Measured at 1.2V, 400MHz and 10 iterations the proposed decoder achieves a data throughput 6.72Gb/s and consumes a power 537.6mW with an energy efficiency 8.0 pJ/bit-iter.

A novel structure of energy efficiency charge recovery logic

A novel charge-recovery logic structure called Pulse Boost Logic (PBL) is proposed in this paper. PBL is a high-speed low-energy-dissipation charge-recovery logic with dual-rail evaluation tree structure. It is driven by 2-phase non-overlap clock, and requires no DC power supply. To demonstrate the performance of PBL structure, 4-bit counters designed with both PBL gates and conventional static CMOS. Post-layout simulation is applied to compare energy dissipation performance between PBL and conventional static CMOS in a frequency range of 500MHz to 1GHz. The simulation result indicates that PBL dissipates only 15% energy per cycle at 1GHz, and for sequential circuits such as counter, PBL is less area consuming than static CMOS.

Charge sharing clock scheme for high efficiency double charge pump circuit

A charge sharing clock scheme is proposed to feed a 5-stage double charge pump circuit. By reusing the charges in charging or discharging the parasitic capacitance during the pumping process, dynamic power loss is able to be reduced by nearly a half. Under 1V supply, simulation results show a maximum 10% efficiency increase, and the ripple noise is also reduced by a half comparing to the conventional charge pumps.

A 1.5V four phase switched polarity charge pump

A four phase switched polarity charge pump using 0.13µm triple well CMOS technology is presented. The architecture takes advantage of the threshold voltage cancellation scheme in the conventional four phase Dickson charge pump. With the body control technique, the body effect is eliminated. The charge transfer unit is shared in positive and negative operation, which makes the design more compact. Simulation results show that the proposed 5-stage charge pump can reach +8.22V/−8.05V (ideal: +9V/−9V) with 10% of total capacitance as estimated parastics, and the output indicates good linearity with respect to the increase of stages.

Error rate decrease through Hamming weight change for NAND Flash

NAND Flash memory is widely used in recent SoCs. High density NAND Flash requires Error Correcting Code (ECC) mechanism to guarantee data integrity. We propose an efficient ECC model which decrease multi bit errors by considering the Flash memorys characteristic. According to the Flash memory mechanism, 0s error is more likely to happen than 1s error. The proposed error control code counts the number of ‘1’ in a word and inverts all bits to keep the number of 1 is more than that of 0s, which signify a high quantity of Hamming weight. We confirm that the proposed method is not only effective for single error but also dramatically effective for multi bit error.

High efficiency Autonomous Controlled Cascaded LDOs for Green Battery system

Low DropOut Converter (LDO) is commonly employed as voltage down converter (VDC) because of its high compatibility with System on Chips (SoCs), but in general, LDO is not so high power efficient. This paper presents an Autonomous Controlled Cascaded LDOs power system which achieves about doubled power efficiency of the conventional LDO systems. In proposed architecture, existing logic circuitries are divided into two load group, and electrical charges are shared among the blocks and a tank circuit. We confirmed that our architecture makes effective use of 88.9% of the battery energy by simulation which is comparable to that of switching regulators whereas ours is easier to implement in SoCs.1

An Automatic Source/Body Level Controllable 0.5V level SOI Circuit Technique for Mobile and Wireless Network Applications

SOI device has the better potential of high speed, low operating voltage and RF functions like as mobile and wireless network applications. Gate body directly connected SOI MOSFET without historical effects is one of the promised technologies that let the logic circuitry work in ultra low voltage. Compared to Bulk-Si MOSFET, GBSOI can reduce its power supply voltage by 30%, its current by 26% and its power dissipation by 47%. However sub-Gbps level clocking circuits with ultra low voltage require the smaller PVT (process, voltage and temperature) variation. This paper presents an architecture to stabilize SOI logic circuitry against PVT variation especially under ultra low power supply voltage. Deviation of gate delay caused by PVT variation is reduced to 1.6%, while 40% with Bulk-Si. This system realizes the cell libraries whose gate delay is constant despite PVT variation. They greatly help designing circuitry especially under ultra low voltage

Green semiconductor technology with ultra-low power on-chip charge-recycling power circuit and system

For low power consumption which makes more than doubles a battery life, the charge-recycling system by reuse the energy between the two or more CPUs and the task scheduling technique for high efficiency are proposed. In this architecture, CPUs are divided into upper and lower load groups, and electrical charges are shared among the stacked CPUs and a tank capacitor. To control divided loads, a high speed and efficient regulator are needed. The internal circuit voltage variation between upper and lower modules is solved by seven LDO regulators, boosting switched capacitor and the tank capacitor. As a result, the stable voltage can be supplied to each CPU, even if upper and lower loads are different or battery is used. The LDOs improve the margin of accumulation of tank capacitor or task schedule operation, and the power efficiency is raised further. The system can be on-chip without external large control circuit and inductor like switching regulator. The test chips were fabricated using 90nm standard CMOS technology. Although the power efficiency of the conventional system with a simple LDO is 44.4% at the maximum, that of the proposed charge-recycling system improves to 88.9%.