Toshiba Memory Corporation Develops New Bridge Chip Using PAM 4 to Boost SSD Speed and Capacity

Memory Corporation
, the world leader in memory solutions, today
announced the development of a bridge chip that realizes high-speed and
large-capacity SSDs. Using developed bridge chips with a small occupied
area and low-power consumption, the company has succeeded in connecting
more flash memory chips with fewer high-speed signal lines than with the
conventional method of no bridge chips. This result was announced in San
Francisco on February 20, at the International Solid-State Circuits
Conference 2019 (ISSCC 2019).

In SSDs, multiple flash memory chips are connected to a controller that
manages their operation. As more flash memory chips are connected to a
controller interface, operating speed degrades, so there are limits to
the number of chips that can be connected. In order to increase
capacity, it is necessary to increase the number of interfaces, but that
results in an enormous number of high-speed signal lines connected to
the controller, making it more difficult to implement the wiring on the
SSD board.

The company has overcome this problem with the development of a bridge
chip that connects the controller and flash memory chips (Fig. 1), three
novel techniques: a daisy chains*1 connection including the
controller and bridge chips in a ring shape; a serial communication
using PAM 4*2; and a jitter*3 improvement
technique for eliminating a PLL circuit*4 in the bridge
chips. By using these techniques overhead of the bridge chips is
reduced, and it is possible to operate a large number of flash memory
chips at high speed with only a few high-speed signal lines (Fig. 2).

The ring-shape configuration of the bridge chips and the controller
reduces the number of transceivers required in the bridge chip from two
pairs to one pair, it achieves chip area reduction of the bridge chip.
In addition, adopting PAM 4 serial communication between the controller
and the daisy-chained bridge chips lowers the operating speed in the
bridge chips’ circuits and relaxes their required performance. A new CDR*5
that utilizes the characteristics of PAM 4 to improve jitter
characteristics eliminates the need for a PLL circuit in the bridge
chip, which also contributes to a smaller chip area and lower power

The prototype bridge chips were fabricated with 28nm CMOS process, and
results were evaluated by connecting four bridge chips and a controller
in ring-shape daisy chain. This confirmed satisfactory performance of
PAM 4 communication by all of the bridge chips and the controller at
25.6 Gbps, and also that it is possible to obtain a BER*6 of
less than 10-12.

Moving forward, the company will continue development work toward
achieving high-speed, large-capacity storage at levels not yet seen by
further enhancing bridge-chip performance while reducing the chip’s area
and power consumption.

*1 Daisy chain: a connecting scheme in which
multiple chips are wired together in sequence
*2 PAM 4:
4-level Pulse Amplitude Modulation (it contains a 4-value data)
Jitter: Fluctuation in the time domain of the clock or signal waveforms
PLL: Phase Locked Loop (a circuit that generates an accurate reference
*5 CDR: Clock Data Recovery (a circuit that
recovers the data and clock from the received signal)
BER: Bit Error Rate (the lower value is the better performance)

About Toshiba Memory Corporation

Toshiba Memory Corporation, a world leader in memory solutions, is
dedicated to the development, production and sales of flash memory and
SSDs. In June 2018, Toshiba Memory was acquired by an industry
consortium led by Bain Capital. Toshiba Memory pioneers cutting-edge
memory solutions and services that enrich people’s lives and expand
society’s horizons. The company’s innovative 3D flash memory technology,
BiCS FLASH™, is shaping the future of storage in high density
applications including advanced smartphones, PCs, SSDs, automotive and
data centers. For more information on Toshiba Memory, please visit


Toshiba Memory Corporation
Kota Yamaji
Business Planning
Phone: +81-3-6478-2319

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