Ultra-Low-Power Embedded Flash Memory Designed for Energy-Harvesting-Powered Systems

Renesas Electronics Corporation announced the development of new low-power technology for use in embedded flash memory. The technology is based on the company’s 65nm SOTB™ (Silicon On Thin Buried Oxide) process.

Available with 1.5 MB capacity, Renesas claims that it is the world’s first embedded 2T-MONOS (2 Transistors-Metal Oxide Nitride Oxide Silicon) flash memory based on 65nm SOTB technology.

With the addition of a new circuit technology that reduces the power consumption of the peripheral circuits on flash memory, the new embedded flash memory achieves read energy as low as 0.22 picojoules per bit (pJ/bit) at an operating frequency of 64MHz, which the company says is among the world’s lowest levels for embedded flash memory on an MCU.

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The embedded flash memory leverages the company’s newly developed low-power technology for peripheral circuits comprised of circuit technology that reduces energy consumption when sensing data in memory and also reduces the amount of transmission energy consumed when read data is transmitted externally.

Renesas presented these results on June 12 at the 2019 Symposia on VLSI Technology and Circuits in Kyoto, Japan, June 9-14, 2019.

The company has already implemented its new SOTB-based technology in the R7F0E embedded controller, which it designed specifically for energy harvesting applications.

Renesas says its exclusive SOTB process technology drastically reduces power consumption in both the active and standby states.

Power consumption in these two states had previously been a trade-off in which lower power consumption in one usually meant higher power consumption in the other.

The new technology substantially reduces power consumption when reading data from the flash memory. In contrast to non-SOTB 2T-MONOS flash memory, which requires a memory read current of about 50µA/MHz, the read current is reduced to about merely 6µA/MHz. This is equivalent to a read energy level of 0.22pJ/bit, one of the lowest levels for embedded flash memory on an MCU.

The new technology also contributes greatly to achieving a low-active read current of 20µA/MHz on the R7F0E, among the best in the industry.

Key Features

The 2T-MONOS embedded flash memory using the SOTB process has a two-transistor structure comprised of electrically isolated elements. Unlike a single-transistor structure, the embedded flash memory has no need for negative voltage during read operation, thereby reducing power consumption when reading data.

Also, compared with other memory processes, MONOS uses fewer masks during the production process, and it allows data storage with a discrete charge-trapping scheme, thus enabling low power consumption and high rewrite reliability without increasing the production cost.

Sense amplifier circuit for ultra-low energy consumption

Most of the energy consumption during memory read operations occurs during sensing operation to identify data and transmit operation to output the identified data to an external destination. To address the former, a single-ended sense amplifier substantially reduces the bit line pre-charge energy during sensing operation, employing a new charge-transfer technology that boosts pre-charging speed and energy efficiency.

In addition, a newly developed regulator circuit technology employs leak monitoring to optimize intermittent control of the sense amplifier’s reference voltage, which consumes energy in a constant manner. According to Renesas, these advances enable a speed-up sensing operations while considerably reducing energy consumption.

One characteristic of the SOTB process is minimal variation in the transistor threshold (Vth), and the newly developed circuit technology takes advantage of this low level of variation to achieve data transmission using a minuscule voltage amplitude. This advance results in a substantial decrease in the transmission energy consumed when read data is transmitted to an external destination.

Renesas says that it considers energy harvesting systems, which do not require batteries or battery replacement, to be essential in realizing a smart society.

Renesas Electronics
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