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Renesas annouces breakthrough circuit tech for embedded STT-MRAM MCUs

Date: 22/02/2024
Renesas Electronics said it has developed cutting-edge circuit technologies for an embedded Spin-Transfer Torque Magnetoresistive Random-Access Memory (STT-MRAM) test chip, boasting fast read and write operations. These advancements, presented at the International Solid-State Circuits Conference 2024 (ISSCC 2024), aim to address the escalating demand for higher performance microcontroller units (MCUs) in IoT and AI applications.

Renesas introduces innovative circuit mechanisms to achieve faster read operations in MRAM. Renesas also enhances the write throughput of embedded STT-MRAM by optimizing the write voltage application process.

Here below is the explanation Renesas provided in achieving better read and write of data into MRAM:

Fast Reading: MRAM reading is generally performed by a differential amplifier (sense amplifier) to determine which of the memory cell current or the reference current is larger. However, because the difference in memory cell currents between the 0 and 1 states (the read window) is smaller for MRAM than for flash memory, the reference current must be precisely positioned in the center of the read window for faster reading. The newly developed technology introduces two mechanisms. The first mechanism aligns the reference current in the center of the window according to the actual current distribution of the memory cells for each chip measured during the test process. The other mechanism reduces the offset of the sense amplifier. With these adjustments, faster read speed is achieved.

Furthermore, in conventional configurations, there is large parasitic capacitance in the circuits used to control the voltage of the bitline so it does not rise too high during read operations. This slows the reading process, so a Cascode connection scheme (Note 1) is introduced in this circuit to reduce parasitic capacitance and speed up reading.

Thanks to these advances, Renesas can achieve the world’s fastest random read access time of 4.2 ns. Even taking into consideration the setup time of the interface circuit that receives the MRAM output data, we can realize the random read operation at frequencies in excess of 200 MHz.

Fast Writing: For the write operation, the high-speed write technologies for embedded STT-MRAM announced in December 2021 improved write throughput by first applying write voltage simultaneously to all bits in a write unit using a relatively low write voltage generated from the external voltage (IO power) of the MCU chip through a step-down circuit, and then used a higher write voltage only for the remaining few bits that could not be written. This time, Renesas takes into account that because the power supply conditions used in test processes and by end product manufacturers are stable, the lower voltage limit of the external voltage can be relaxed. Thus, by setting the higher step-down voltage from the external voltage to be applied to all bits in the first phase, write throughput can be improved 1.8-fold.

Combining the above new technologies, a prototype MCU test chip with a 10.8Mbit MRAM memory cell array was fabricated usinga 22 nm embedded MRAM process. Evaluation of the prototype chip confirmed that it achieved a random read access frequency of over 200 MHz and a write throughput of 10.4 MB/s at a maximum junction temperature of 125°C.

The test chip also contains 0.3 Mbit of OTP (Note 2) that uses MRAM memory cell breakdown to prevent falsification of data. This memory can be used to store security information. Writing to OTP requires a higher voltage than writing to MRAM, making it more difficult to perform writing in the field, where power supply voltages are often less stable. However, by suppressing parasitic resistance within the memory cell array, this new technology also makes writing in the field possible.

The successful fabrication of a prototype MCU test chip featuring a 10.8Mbit MRAM memory cell array using a 22 nm embedded MRAM process gives Renesas ability to push the boundaries of memory access speed and efficiency. These breakthrough technologies not only enable higher-performance MCUs with embedded MRAM but also pave the way for faster and more reliable data processing in IoT and AI-driven applications. Renesas is fully focused to advance its MRAM capabilities, aiming to further enhance memory capacity, speed, and power efficiency to meet the evolving needs of the semiconductor industry.


Pic above: Embedded STT-MRAM Test Chip

Source: Renesas