Speaker
Dr
Toshitsugu Sakamoto
(NEC Corporation)
Description
The Internet of Things (IoT) has been envisioned as a fundamental infrastructure that will bring about useful information and knowledge resulting in efficiency and growth in industry and improved comfort and safety in human life. Sensors, networks, and information technology (IT) are designated as key technology elements to make IoT a practical knowledge framework. IoT is to be used for supporting so-called lifeline as energy supply, water works, traffic control, logistics, broadcasting, and telecommunication. Everything is to be connected through Machine to Machine (M2M) network anytime and anywhere to realize the IoT framework.
Space systems, such as satellites, can be identified as sensor nodes and relay nodes among IoT applications. It is integrated with ground systems, and wide range of collected information must be transmitted through the limited transmission capacity of existing network. Thus, it is essential to reduce the data size by computationally intensive algorithms including data compression, data prediction, adaptive sampling, and so on. Field programmable gate arrays (FPGAs) with high performance computation are suitable for this purpose. Especially, a single-event-effect (SEE) free FPGA is the most desirable device because the demanding characteristics on satellites is a continuous operation in harsh environment with background radiation in orbit. Low power consumption is another demanding characteristics for realizing less heat dissipation indispensable to space applications operating in exoatmosphere.
We have developed an atom-switch based FPGA with radiation hardened characteristics. Atom switch provides rewritable capability for FPGAs without static random access memory (SRAM) or electrically erasable programmable read-only memory (EEPROM) to store circuit configurations. The atom switch functions as routing switch and memory bit in look-up table (LUT). The atom switch has a durability against radiation. This results in mitigating SEE in circuit configuration of FPGA. Memory patrol and memory scrubbing functions are not required for the atom-switch FPGA, and that eliminates external peripheral devices used with conventional rewritable FPGAs.
Atom switch is a resistance-change type nonvolatile switch. The novel switch is composed of the solid electrolyte sandwiched between Pt and Cu electrodes. When a positive voltage is applied to Cu electrode, Cu is ionized and precipitated at Pt electrode, and then a conducting metal bridge is formed between Cu and Pt electrodes. The conductance of the switch changes to high (or ON state). When a negative voltage is applied to the ON-state atom switch, the metal bridge is broken and dissolved into the solid electrolyte and the switch turns off. The programming cycles is up to 1,000. The switch’s resistance value is maintained even when the power supply is turned off, resulting in nonvolatile FPGA.
The fabricated atom-switch FPGA includes a 40 x 40 logic cell array, each of which consists of a switch block and a logic block including 4x 4-input LUTs. The 4.38-Mb atom switches are integrated between metals 4 and 5 of Cu interconnect using 40-nm CMOS technology. To achieve a high OFF state reliability, two atom switches are serially connected with opposite direction in each element. Two atom switches are programmed by using the cell transistor connected to the middle node. This switch is named as a complementary atom switch (CAS), since the voltage stress is shared by two atom switches complementarily. Small area and small capacitance (~0.14fF) of CAS contributes to low power consumption and high speed of FPGA. Its non-volatility also allows an immediate wake-up/sleep operation without re-loading the configuration data, and saves the standby power effectively.
We compared the performance of the atom-switch FPGA with the commercial SRAM-based FPGA, which was a low-power one (iCE40) for mobile applications by Lattice Corp. The benchmark circuit of 16-bit Arithmetic Logic Unit (ALU) was mapped on both FPGAs. The atom-switch FPGA operated at 3.8 times faster clock frequency over the conventional one. The dynamic and active power consumptions of the atom switch FPGA were reduced by 67% and 39%, respectively. These improvements are mainly originated from small capacitance of atom switch and smaller logic-cell size, which is a half of the conventional one at same technology node of 40nm.
Radiation resistance of atom switch was evaluated by using a heavy ion cocktail beam. For this evaluation, array of 128k-bit atom switches or atom-switch FPGA were used. The array of atom switches were exposed by Xe and Kr ions. Linear energy transfers (LETs) of Xe and Kr were estimated to be 68.9 and 40.3 MeV/(mg/cm2) at the chip surface, respectively. During the irradiation, we observed no SEE, showing that SEE cross section is at least 100 times lower than that of NAND flash.
The atom-switch FPGA will be evaluated in orbit via the innovative satellite technology demonstration program of JAXA in FY2018. During a whole year, the full-HD image will be compressed in the atom-switch FPGA and transmitted to ground stations.
Primary author
Dr
Toshitsugu Sakamoto
(NEC Corporation)
Co-authors
Dr
Hiroki Hihara
(NEC Space Technologies, Ltd.)
Mr
Hiromitsu Hada
(NEC Corporation)
Mr
Makoto Miyamura
(NEC Corporation)
Dr
Munehiro Tada
(NEC Corporation)
Mr
Tadahiko Sugibayashi
(NEC Corporation)
Peer reviewing
Paper
Paper files:
