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Journal of Semiconductors
Vol. 43, Issue 3, 031401 (2022)

A review on SRAM-based computing in-memory: Circuits, functions, and applications

Zhiting Lin, Zhongzhen Tong, Jin Zhang, Fangming Wang, Tian Xu, Yue Zhao, Xiulong Wu, Chunyu Peng, Wenjuan Lu, Qiang Zhao, and Junning Chen

Author Affiliations

School of Integrated Circuits, Anhui University, Hefei 230601, China

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DOI: 10.1088/1674-4926/43/3/031401 Cite this Article

Zhiting Lin, Zhongzhen Tong, Jin Zhang, Fangming Wang, Tian Xu, Yue Zhao, Xiulong Wu, Chunyu Peng, Wenjuan Lu, Qiang Zhao, Junning Chen. A review on SRAM-based computing in-memory: Circuits, functions, and applications[J]. Journal of Semiconductors, 2022, 43(3): 031401 Copy Citation Text

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(Color online) Overall framework of static random-access memory (SRAM)-based computing in-memory (CIM) for the review: (a) various functions implemented in CIM, (b) operation functions realizable with CIM, and (c) application scenarios of CIM.

Fig. 1. (Color online) Overall framework of static random-access memory (SRAM)-based computing in-memory (CIM) for the review: (a) various functions implemented in CIM, (b) operation functions realizable with CIM, and (c) application scenarios of CIM.

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(Color online) (a) Standard 6T SRAM cell, (b) dual-Split 6T SRAM cell, and (c) 4+2T SRAM cell.

Fig. 2. (Color online) (a) Standard 6T SRAM cell, (b) dual-Split 6T SRAM cell, and (c) 4+2T SRAM cell.

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Fig. 3. (Color online) SRAM cells with separated read and write: (a) standard 8T SRAM bit-cell, (b) 7T SRAM cell, (c) 9T SRAM cell, and (d) 10T SRAM cell.

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Fig. 4. (Color online) SRAM cells based on capacitive coupling: (a) C3SRAM bitcell and (b) M-BC bitcell.

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Fig. 5. (Color online) (a) Transposable bitcell contains two pairs of access transistors and (b) separated read–write transposable bit cell. (c) Schematic of the transposable 10T bit bitcell.

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Fig. 6. (Color online) Compact coupling structure: (a) 12T cell and (b) two-way transpose multibitcell.

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Fig. 7. (Color online) (a) Asymmetric differential sense amplifier (SA), (b) flash ADC, and (c) successive approximation ADC.

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Fig. 8. (Color online) (a) Weighted array with different capacitor sizes and (b) multi-period weighting technique using capacitors of the same size.

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Fig. 9. (Color online) Schematic of the column-wise GBL_DAC circuit: (a) Circuit of the constant current source, (b) two-stage MUX, and (c) waveform of the column-wise GBL_DAC circuit. (d) Schematic and waveform of the pulse height modulation circuit.

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Fig. 10. (Color online) Redundant reference column technology.

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Fig. 11. (Color online) (a) In-/near-memory computing peripherals and (b) a bit-tree adder.

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Fig. 12. (Color online) Signed 4-b × 8-b least significant bit (LSB) multiplier: (a) timing diagram and (b) circuit schematic.

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Fig. 13. (Color online) Boolean operation: (a) Boolean logical operations using an SRAM array, (b) histogram of AND and NOR operation voltages, and (c) schematic of the 8T-SRAM for implementing the IMP and XOR operations.

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Fig. 14. (Color online) Column-wise BCAM: (a) search example in 3D-CAM and (b) 4+2T. Row-wise TCAM: (c) organization based on 10T and (d) organization based on 6T.

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Fig. 15. (Color online) Schematic and truth table of the binary dot product: (a) 6T-SRAM binary dot product and (b) 8T-SRAM binary dot product. (c) Ternary dot product: operation of ternary multiplication and XNOR value mapping table.

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Fig. 16. (Color online) Row of 9T SRAM cells for calculating the Hamming distance.

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Fig. 17. (Color online) (a) Precharge weighting technology, (b) pulse width weighting, (c) pulse height weighting, and (d) pulse number weighting.

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Fig. 18. (Color online) (a) 8T-SRAM memory array for computing dot products with 4-bit weight precision and (b) Twin-8T cell.

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Fig. 19. (Color online) (a) Schematic of SAD circuit and (b) sequence diagram.

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Fig. 20. (Color online) Implementation of (a) CNN and (b) AES on multiple SRAM arrays.

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Fig. 21. (Color online) Application in the k-NN algorithm.

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Fig. 22. (Color online) Application in classifier algorithms.

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Fig. 23. (Color online) Read disturb issue.

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Fig. 24. (Color online) (a) Single row activation during normal SRAM read operation, (b) multirow read and nonlinearity during CIM, and (c) inconsistent CIM calculation.

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Fig. 25. (Color online) Approach of mapping from the common operator set to the actual circuits.

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Fig. 26. (Color online) Architecture of the bidirectional CIM system, including a reusable and reconfigurable module.

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Fig. 27. (Color online) Multithreaded CIM macro based on a pipeline processor.

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Parameter	Structure of the 6T cell				Cell structure with additional devices
	Standard 6T	Dual-split 6T	4+2T		Read and write separating				Capacitive coupling			Transposable				Compact coupling
	Ref. [19]	Refs. [1, 2]	Ref. [20]		Ref. [18]	Ref. [67]	Ref. [57]		Refs. [31, 58]	Refs. [59, 60, 61]		Refs. [12, 13]	Refs. [14, 55]	Ref. [11]		Ref. [17]	Ref. [43]
¹Input precesion/Weight precesion/Output precesion. TWT-MC: Two-way transpose multibit accumulation; DDC: deeply depleted channel; FDSOI: full depleted silicon on insulator
Cell type	6T	6T	6T		8T	9T	10T		8T1C	10T1C		8T	8T	10T		12T	TWT-MC
Process technology	28-nm FDSOI	65 nm	55-nm DDC		45 nm	65 nm	28 nm		65 nm	65 nm		28 nm	7 nm	28 nm		65 nm	28 nm
Added circuit	No	No	Two read ports		One read port	One read port	Two read ports		Two transistors one cap	Four transistors one cap		Two read ports	One read port	Two read ports		Pull-up/ down circuits	Multiply cell
Read write disturb	Yes	Yes	No		No	No	No		No	No		No	No	No		No	Yes
Area efficiency	High	High	High		Med.	Med.	Med.		Low	Low		Med.	High	Low		Low	Low
TOPS/mm²	NA	33.13	NA		NA	NA	170		20.2	0.6		27.3	NA	NA		5.461	NA
TOPS/W	NA	30.49– 55.8 1/1/5b¹	41.4 1/1/1b¹		NA	NA	1002 1/1/1b¹		671.5 1/1/5b¹	192– 400 1–8b		0.56/5.27 Arbitrary	6.02 8/1/ 11b¹	66.7 1/1/1b¹		403 1/1/ 3.46b¹	7.2–61.1 2,4,8/4, 8/10,12, 16,20b¹

Table 1. Static random-access memory (SRAM) bitcells in CIM.

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Parameter		Ref. [19]	Ref. [28]	Ref. [20]	Ref. [29]	Ref. [5]	Ref. [68]	Ref. [11]
¹Row-wise search. ²Column-wise search. DDC, deeply depleted channel; FDSOI, full depleted silicon on insulator.
Technology		28-nm FDSOI	180 nm	55-nm DDC	28-nm FDSOI	65 nm	28 nm	28 nm
Cell type		6T	8T	4+2T	6T	8T	14T	10T
Array size		64×64	8×8	128×128	128×64	128×128	1024×320	64×64
Supply voltage (V)		1	1.2	0.8	0.9	1.2	0.9	0.9
CAM	Freq. (MHz)	370 (1 V)	NA	270 (0.8 V)	1560 (0.9 V) 8.90 (0.38 V)	813 (1.2 V)	1330 (0.9 V)	262 (0.9 V)	256 (0.9 V)
	Energy (fJ/bit)	0.6 (1 V)	NA	0.45 (0.8 V)	0.13 (0.9 V)	0.85 (1.2 V)	0.422 (0.9 V)	1.025 (0.9 V)	1.02 (0.9 V)
	Energy (fJ/bit)	0.6 (1 V)	NA	0.45 (0.8 V)	0.13 (0.9 V)	0.85 (1.2 V)	0.422 (0.9 V)	0.635 (0.7 V)	0.632 (0.7 V)
Logic	Freq. (MHz)	NA	NA	230 (0.8 V)	NA	793 (1.2 V)	NA	~300(0.9 V)
	Energy (fJ/bit)	NA	NA	24.1 (0.8 V)	NA	~31 (1.2 V)	NA	~15 (0.9 V)
						~22.5 (1 V)		~12.5 (0.8 V)
						16.6 (0.8 V)		~10.5 (0.7 V)
Search mode		1	1	2	2	1	2	1	2
Function		SRAM/ CAM/Logic	SRAM/ TCAM/ Left Shift/ Right/Shift	SRAM/ CAM/Logic	BCAM/SRAM/ Pseudo-TCAM	SRAM/ CAM/ Logic	SRAM/ TCAM	SRAM/CAM/ Logic/Matrix transpose

Table 2. Summary of chip parameters and performance of in-memory Boolean logic and CAM

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Parameter		Refs. [1, 2]	Ref. [10]	Refs. [15, 16]	Ref. [32]	Ref. [31]	Ref. [44]	Ref. [33]	Refs. [24, 37]	Ref. [34]
Tchnology		65-nm CMOS	65-nm CMOS	130-nm CMOS	55-nm CMOS	65-nm CMOS	65-nm TSMC	55-nm CMOS	7-nm FinFET	65nm
Cell structure		DCS 6T	10T	6T	Twin-8T	8T1C	6T	6T	8T	6T
Array size		4 Kb	16 Kb	16 Kb	64×60 b	2 KB	64 Kb	4 Kb	4 Kb	64 Kb
Chip area (μm²)		NA	6.3×10⁴	2.67×10⁵	4.69×10⁴	8.1×10⁴	NA	5.94×10⁶	3.2×10³	1.75×10⁵
Input precision (bit)		1	6	5	1, 2, 4	1	5	1, 2, 7, 8	4	4
Weight precision (bit)		1	1	1	2, 5	1	5	1, 2, 8	4	1, 2, 3 4, 5, 8
Output precision (bit)		1	6	NA	3, 5, 7	5	NA	3, 7, 10, 19	4	NA
Computing mechanism		Analog	Digital+ Analog	Digital+ Analog	Analog	Analog	Analog	Digital+ Analog	Analog	Analog
Model		XNORNN/MBNN	CNN	Classify	CNN	CNN	VGG LeNet-5	CNN	VGG-9 NN	CNN
Energy efficiency (TOPS/W)		30.49–55.8	40.3 (1 V) 51.3 (0.8 V)	NA	18.37– 72.03	671.5	NA	0.6–40.2	351 (0.8 V)	49.4 (Input: 4b Weight:1b)
Throughput (GOPS)		278.2	8 (1 V) 1 (0.4 V)	NA	21.2~ 67.5	1638	NA	5.14-329.14	372.4(0.8 V)	573.4 (Input: 4b Weight:2b)
Accu-racy	MNIST	96.5% (XNORNN) 95.1% (MBNN)	98%(0.8 V) 98.3%(1 V)	90%	90.02%– 99.52%	98.30%	99%	98.56%– 99.59%	98.51%– 99.99%	98.80%
Accu-racy	CIFAR 10	NA	NA	NA	85.56%~ 90.42%	85.50%	88.83%	85.97%-91.93%	22.89%-96.76%	89.00%

Table 3. Summary of chip parameters and performance of single- and multibit operations.

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Zhiting Lin, Zhongzhen Tong, Jin Zhang, Fangming Wang, Tian Xu, Yue Zhao, Xiulong Wu, Chunyu Peng, Wenjuan Lu, Qiang Zhao, Junning Chen. A review on SRAM-based computing in-memory: Circuits, functions, and applications[J]. Journal of Semiconductors, 2022, 43(3): 031401

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