Low-Power Variation-Tolerant Nonvolatile Lookup Table Design

Abstract:

Emerging nonvolatile memories (NVMs), such as MRAM, PRAM, and RRAM, have been widely investigated to replace SRAM as the configuration bits in field-programmable gate arrays (FPGAs) for high security and instant power ON. However, the variations inherent in NVMs and advanced logic process bring reliability issue to FPGAs. This brief introduces a low-power variation-tolerant nonvolatile lookup table (nvLUT) circuit to overcome the reliability issue. Because of large ROFF/RON, 1T1R RRAM cell provides sufficient sense margin as a configuration bit and a reference resistor. A single-stage sense amplifier with voltage clamp is employed to reduce the power and area without impairing the reliability. Matched reference path is proposed to reduce the parasitic RC mismatch for reliable sensing. Evaluation shows that 22% reduction in delay, 38% reduction in power, and the tolerance of variations of 2.5× typical RON or ROFF in reliability are achieved for proposed nvLUT with six inputs. The proposed architecture of this paper area and power consumption analysis using tanner tool.

Enhancement of the project:

Changes the parameter for improve the efficiency of the circuit.

Existing System:

Emerging NVMs, such as MRAM, PRAM, and RRAM, has been verified with better scalability and logic compatibility. Based on the logic-in-memory concept, lookup table, which is the core building block in FPGAs, has been proposed with non-volatility. First, various nonvolatile SRAM (nvSRAM) structures with MRAM and RRAM were proposed to directly replace SRAM in the traditional lookup table to acquire non-volatility. However, the size of nvSRAM cell is remarkably larger than that of SRAM, and the write disturbance is also difficult to avoid for half-select RRAM cells. For MRAM, Suzuki et al. proposed a two-input nonvolatile lookup table (nvLUT) based on MRAM in the current-mode logic for low power. Suzuki et al. also proposed a six-input nvLUT with serial/parallel magnetic junctions to acquire enough sensing margin. Zhao et al. proposed another MRAM-based nvLUT for run-time reconfiguration. Ren proposed a third type of MRAM-based nvLUT named hybrid-LUT2. However, the ROFF/RON of MRAM is smaller compared with PRAM or RRAM, resulting in less sense margin or larger area due to serial/parallel magnetic junctions.

Moreover, the first three MRAM nvLUTs have a mismatch in parasitic RC between the selected path in the multiplexer and the reference path, which may cause nvLUT to fail. For hybrid-LUT2, the configuration of MRAM cells shares the same decoding circuit with logic operation, whose inputs may be wired to other logic blocks and cannot be used as the address inputs during configuration. For RRAM, Sakamoto et al. proposed an nvLUT based on nanobridge. However, the programming path of nanobridge shares the same multiplexer with the logic path for selection, making the size of transistors in the multiplexer considerably large to satisfy the reset voltage for RON. Chen et al. proposed another RRAM-based nvLUT using crossbar array. However, the sneaking paths inherent in crossbar array bring considerable leakage and poor sensing margin of only 10 mV. To sum up, none of the previous work has achieved high reliability against memory and logic variations, low power, high-area efficiency, and low leakage at the same time.

Disadvantages:

high parasitic RC mismatch
high power consumption

Proposed System:

To illustrate the proposed design, a two-input nvLUT is presented, as shown in Fig. 1. The input count can also be easily extended to six, which is prevailing in current main-stream FPGA products. The overall architecture of nvLUT consists of an SSAVC, a tree multiplexer (TMUX), an MRP, a RRAM slice, and a footer transistor. The RRAM slice constitutes of four 1T1R RRAM cells at the left for configuration and a dummy RRAM cell at the right-most as a reference resistor.

Fig. 1. Overall architecture of the proposed low-power variation-tolerant nvLUT based on RRAM.

RRAM as a Configuration Bit and a Reference Resistor

The 1T1R RRAM cell is employed as a configuration bit and a reference resistor to provide sufficient sense margin, as shown in Fig. 1. Different from crossbar array, a 1T1R RRAM cell can eliminate the sneaking current and the disturbances during write and read, thus saving power and acquiring high yield.

Fig. 2. 1T1R RRAM cell integration process and structure. (a) Cross-sectional view. (b) Schematic.

SSAVC

SSAVC converts the resistance state of RRAM into a rail-to-rail logic voltage. As shown in Fig. 1, transistors M3–M6 constitute of a latch amplifier. Transistors M1 and M2 are used to precharge the output nodes OUT and OUTB to VDD when CLK is low and transistor MF is used to initiate the conversion when CLK is high.

MRP

Although trimming Rref by SAWM can help to disabuse the parasitic resistance mismatch between the selected path in TMUX and the reference path, their parasitic capacitance mismatch cannot be easily estimated and compensated. The MRP is devised to minimize the parasitic RC mismatch between the above-mentioned two paths.

Fig. 3(a) and (b) compares the parasitic RC of MRP with P01. And (c) and (d) also gives the previous reference resistor tree (RRT) and its corresponding RC equivalent circuit.

Fig. 3. Parasitic RC equivalent circuits of (a) P01 and (b) Pref in Fig. 1. (c) Schematic and (d) parasitic RC equivalent circuit of RRT

Advantages:

reduce the parasitic RC mismatch
reduce the power

Software implementation:

Tanner tools

Low-Power Variation-Tolerant Nonvolatile Lookup Table Design