In-memory multiply-add computing device adapted to process input signals with improved accuracy

This application claims the priority benefit of U.S. Provisional Application No. 62/938,956, filed on Nov. 22, 2019. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

The invention relates to an in-memory computing device, and particularly relates to an in-memory computing device with improved accuracy.

Along with development of artificial intelligence, it is an important issue to provide chips with high-efficiency multiply-add computation capabilities in integrated circuits. In today's technology, it has become a mainstream to set up an in-memory computing device to perform multiply-add operations.

In a conventional in-memory computing device, a memory is often divided into a plurality of memory cell blocks, and results of multiply-add operations performed on the memory cell blocks are quantified and normalized, and then a plurality of digital shift circuits and accumulation circuits are used to sum up the multiply-add operation results to obtain a final result. According to such method, in case of a large number of input signals, the numbers of the digital shift circuits and the accumulation circuits may be greatly increased, resulting in increased circuit complexity. Moreover, the conventional in-memory computing device requires multiple quantization operations based on digitization processing operations. These quantization operations also cause errors in computation results and reduce computation accuracy.

The invention is directed to an in-memory computing device, which is adapted to process a large number of input signals and reduce a delay time generated in a computation process.

The invention provides an in-memory computing device including a plurality of memory cell arrays and a plurality of sensing amplifiers. The memory cell arrays respectively receive a plurality of input signals. The input signals are divided into a plurality of groups. The groups respectively have at least one partial input signal. The at least one partial input signal of each of the groups has a same value. Numbers of the at least one partial input signal in the groups sequentially form a geometric sequence with a common ratio of 2. The sensing amplifiers are respectively coupled to the memory cell arrays. The memory cell arrays respectively provide a plurality of weightings, and respectively perform multiply-add operations according to the received input signals and the weightings to generate a plurality of computation results. The sensing amplifiers respectively generate a plurality of sensing results according to the computation results.

Based on the above description, in the invention, the sensing result is generated through pure analog multiply-add operations and current sensing operations. In this way, in the application of a large number of input signals, the necessary digital shift circuit is omitted, and it is unnecessary to set the digital accumulation circuit, which effectively reduces the complexity of circuit.

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 is a schematic diagram of an in-memory computing device according to an embodiment of the invention.

FIG. 2 is a schematic diagram of an in-memory computing device according to another embodiment of the invention.

FIG. 3 is a schematic diagram of an in-memory computing device according to another embodiment of the invention.

FIG. 4 is a schematic diagram of an implementation of a memory cell array according to an embodiment of the invention.

FIG. 5 is an operation flowchart of an in-memory computing device according to an embodiment of the invention.

Referring to FIG. 1 , FIG. 1 is a schematic diagram of an in-memory computing device according to an embodiment of the invention. The in-memory computing device 100 includes a plurality of memory cell arrays 111 - 11 N and a plurality of sensing amplifiers 121 - 12 N. The memory cell arrays 111 - 11 N respectively receive a plurality of input signals X 1 -XN. The input signals X 1 -XN may be divided into a plurality of groups. Each of the groups has one or a plurality of partial input signals in the plurality of input signals X 1 -XN. The partial input signals of each of the groups are the same. According to an arranging order, numbers of the partial input signals in the groups sequentially form a geometric sequence with a common ratio of 2.

In detail, the input signals X 1 ˜XN may include a plurality of input signal sub-sets Z 1 ˜ZH. Taking a sum of the input signal subsets=a0+a1×2 2 +a 2 ×2 4 as an example, in which the input signal subsets Z 1 -ZH may be set as seven partial input signals of a 0 , a 1 , a 1 , a 2 , a 2 , a 2 and a 2 . The above seven partial input signals may be divided into three groups. The first group includes one partial input signal a 0 ; the second group includes two partial input signals a 1 ; and the third group includes four partial input signals a 2 . In the embodiment, the seven partial input signals a 0 -a 2 may be respectively input to seven memory cell arrays. The memory cell arrays respectively perform multiply-add operations according to the corresponding partial input signals a 0 -a 2 based on provided weightings, so as to generate a plurality of computation results.

In the embodiment, the memory cell arrays 111 - 11 N perform the multiply-add operations to respectively generate a plurality of computation results COUT 1 -COUTN. The sensing amplifiers 121 - 12 N receive a reference current IREF, and perform sensing operations on the computation results COUT 1 -COUTN based on the reference current IREF, and accordingly generate a plurality of sensing results SOUT 1 -SOUTN.

It should be noted that, in the embodiment of the invention, the multiply-add operations of the memory cell arrays 111 - 11 N are performed based on signals in a full analog format. In other words, the computation results COUT 1 -COUTN are all analog format signals. Under such condition, the embodiment of the invention may reduce the number of quantization operations that cannot be avoided in a digitization processing mode, so as to effectively improve computation accuracy.

Through the computations of the full analog format, in the embodiment, it is unnecessary to set up digital shift register circuits and accumulation circuits in the in-memory computing device 100 , which may effectively reduce the complexity of the circuit.

Taking an input signal of n bits and providing a weighting of m bits as an example, the embodiment of the invention may be implemented by (2 n −1)×(2 m −1) memory cells.

In the embodiment, the memory cell arrays 111 - 11 N may be flash memory cell arrays composed of flash memory cells. The flash memory cells may provide different degrees of transconductances to serve as weightings through erasing operations or programming operations. The sensing amplifiers 121 - 12 N may be implemented by any sensing amplifier circuit well known to those with ordinary knowledge in the art, without certain restrictions.

Referring to FIG. 2 , FIG. 2 is a schematic diagram of an in-memory computing device according to another embodiment of the invention. An in-memory computing device 200 includes memory cell arrays 211 - 21 N, sensing amplifiers 221 - 22 N, a reference current generator 230 , and a normalization circuit 240 . The memory cell arrays 211 - 21 N respectively receive a plurality of input signals X 1 -XN, and perform multiply-add operations based on a plurality of provided weightings, and respectively generate a plurality of computation results COUT 1 -COUTN. The computation results COUT 1 -COUTN may be analog current signals.

A current IA generated by the reference current generator 230 is modulated by a reference current regulator 244 and a reference current multiplier 245 to generate a modulated current, and the modulated current can be transported to the sensing amplifiers 221 - 22 N. In the embodiment, the reference current generator 230 includes a current source 231 and a current multiplier 232 . The current source 231 is used to provide the current IA.

The normalization circuit 240 is coupled between the memory cell arrays 211 - 21 N and the sensing amplifiers 221 - 22 N. The normalization circuit 240 includes a plurality of current adders 2431 - 243 N, the reference current multiplier 245 , the reference current regulator 244 , and current sources 241 and 242 . First input terminals of the current adders 2431 - 243 N are respectively coupled to the memory cell arrays 211 - 21 N to receive the computation results COUT 1 -COUTN. Second input terminals of the current adders 2431 - 243 N commonly receive a first current IB+. The first current IB+ is provided by the current source 241 . The current adders 2431 - 243 N respectively add the computation results COUT 1 -COUTN and the first current IB+ to respectively generate a plurality of adjusted computation results COUT 1 ′-COUTN′.

The reference current multiplier 245 receives the current IA, multiplies the current IA by a scalar, and generates the reference current IREF. A first input terminal of the reference current regulator 244 receives the reference current IREF, and a second input terminal of the reference current regulator 244 receives a second current IB− provided by the current source 242 . The reference current regulator 244 adds the reference current IREF and the second current IB− to generate an adjusted reference current IREF′. In the embodiment, the reference current regulator 244 is a current adder.

Moreover, in the embodiment, the adjusted reference current IREF′ is provided to the sensing amplifiers 221 - 22 N, and the adjusted computation results COUT 1 ′-COUTN′ are respectively provided to the sensing amplifiers 221 - 22 N. The sensing amplifiers 221 - 22 N may respectively sense the adjusted computation results COUT 1 ′-COUTN′ according to the adjusted reference current IREF′ to generate a plurality of sensing results SOUT 1 -SOUTN.

It should be noted that taking the reference current IREF as 1/K times of the current IA as an example, the normalization circuit 240 may perform a normalization operation based on a value K(y−B), where y is a value of the sensing result SOUT 1 -SOUTN in the condition of that the K is 1 and B is 0, B is a difference between the first current B+ and the second current B−. In the embodiment of the invention, the first current B+ may be greater than the second current B−. Wherein the normalization circuit 240 performs an inverse normalization operation on the reference current IREF and the adjusted reference current IREF′. The inverse normalization operation corresponding the sensing results SOUT 1 -SOUTN is a normalization operation.

The above-mentioned normalization operation may implement layer-wise normalization of a neural network when being applied to the computation of the neural network.

The current adders 2431 - 243 N, the reference current regulator 244 , and the current multiplier 245 in the embodiment may all be implemented by operational circuits related to current addition and multiplication that are well known to those with ordinary knowledge in the art without specific restrictions. In addition, the current sources 241 , 242 , and 231 in the embodiment may be implemented by current source circuits well known to those with ordinary knowledge in the art without specific restrictions.

Referring to FIG. 3 , FIG. 3 is a schematic diagram of an in-memory computing device according to another embodiment of the invention. An in-memory computing device 300 includes a plurality of memory cell arrays 311 - 31 N, a plurality of sensing amplifiers (SA) 321 - 32 N, and a controller 330 . The controller 330 is coupled to the memory cell arrays 311 - 31 N and the sensing amplifiers (SA) 321 - 32 N. In this embodiment, the sensing amplifiers (SA) 321 - 32 N may perform sensing operation based on the current IA.

The controller 330 is used to control a work flow of computation operations performed by the in-memory computing device 300 .

It should be noted that in the embodiment of the invention, the circuit in a dotted line box may also be implemented by the circuit of the embodiment of FIG. 2 .

Referring to FIG. 3 and FIG. 4 , FIG. 4 is a schematic diagram of an implementation of a memory cell array according to an embodiment of the invention. A memory cell array 400 includes a plurality of flash memory cells MC 1 -MC 8 coupled in parallel. The memory cells MC 1 -MC 8 are commonly coupled to a word line WL, respectively coupled to bit lines BL 1 -BL 8 , and commonly coupled to a source line SL. The memory cells MC 1 -MC 8 receive input signals through the bit lines BL 1 -BL 8 , and respectively provide a plurality of weightings according to a plurality of transconductances. The memory cells MC 1 -MC 8 generate the computation result COUT 1 on the source line SL after the word line WL completes a setting operation (a word line voltage on the word line WL is pulled up).

Referring to FIG. 3 and FIG. 5 , FIG. 5 is an operation flowchart of an in-memory computing device according to an embodiment of the invention. The controller 300 executes setting operations of multiple word lines of the memory cell arrays 311 - 31 N in a first time interval T 1 , and controls the memory cell arrays 311 - 31 N to consecutively execute multiple computation operations in a plurality of subsequent second time intervals T 21 and T 22 . In detail, in the computation operations, taking the second time interval T 21 as an example, in a sub-time interval t 1 , the controller 330 may perform voltage setting operations of the bit lines according to the input signals, such that the memory cell arrays 311 - 31 N may perform multiply-add operations. Then, in a sub-time interval t 2 , the sensing amplifiers 221 - 22 N may perform current sensing operations on the computation results COUT 1 -COUTN generated by the memory cell arrays 311 - 31 N.

It should be noted that in the operation of the flash memory, the setting operations of the word lines take a relatively long time. In the embodiment of the invention, through the setting operation of a single word line, multiple computation operations may be performed consecutively. In this way, the time delay generated in the computation process may be effectively reduced to increase a speed of executing the computation operation.

In summary, in the invention, the memory cell arrays in the in-memory computing device perform the multiply-add operations under a full analog condition. In this way, there is no need to set up the digital shift circuits and the accumulation circuits, which effectively reduces the complexity of the circuit. In addition, the multiply-add operations performed under the full analog condition do not require the quantization operation, which effectively reduces errors that may be caused by the quantization operation and improves the computation accuracy.