In-cell differential read-out circuitry for reading signed weight values in resistive processing unit architecture

• US 10,468,098 B2
• Filed: 03/14/2019
• Issued: 11/05/2019
• Est. Priority Date: 03/30/2018
• Status: Active Grant

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Kim, Seyoung, Lee, Hyung-Min, Gokmen, Tayfun, Han, Shu-Jen
• Primary Examiner(s): Ho, Hoai V
• Assistant Examiner(s): Radke, Jay W.
• Application Number:
  US16/353,111
• Publication Number:
  US 20190304539A1
• Time in Patent Office: 236 Days
• CPC Class Codes: G06N 3/065 Analogue means
• CPC Class Codes: G06N 3/084 Backpropagation, e.g. using gradient descent
• CPC Class Codes: G11C 13/0038 Power supply circuits
• CPC Class Codes: G11C 13/004 Reading or sensing circuits or methods
• CPC Class Codes: G11C 2013/0042 Read using differential sensing, e.g. bit line [BL] and bit line bar [BLB]
• CPC Class Codes: G11C 2013/0045 Read using current through the cell
• CPC Class Codes: G11C 2013/0054 Read is performed on a reference element, e.g. cell, and the reference sensed value is used to compare the sensed value of the selected cell
• CPC Class Codes: G11C 2213/77 Array wherein the memory element being directly connected to the bit lines and word lines without any access device being used

Abstract

A resistive processing unit (RPU) device includes a weight storage device to store a weight voltage which corresponds to a weight value of the RPU device, and a read transistor having a gate connected to the weight storage device, and first and second source/drain terminals connected to first and second control ports, respectively. A current source connected to the second source/drain terminal generates a fixed reference current. The read transistor generates a weight current in response to the weight voltage. A read current output from the second control port represents a signed weight value of the RPU device. A magnitude of the read current is equal to a difference between the weight current and the fixed reference current. The sign of the read current is positive when the weight current is greater than the fixed reference current, and negative when the weight current is less than the fixed reference current.

Claims

1. A method, comprising:

receiving, by a resistive processing unit (RPU) cell, a read control signal to read a weight of the RPU cell, wherein the weight of the RPU cell comprises a weight value and a weight sign;

in response to the read control signal, the RPU cell generating a read current based on the weight of the RPU cell, and outputting the read current to an output port of the RPU cell;

wherein the read current is generated as a difference between a first current and a second current;

wherein the read current comprises a magnitude and sign which represent the weight value and the weight sign, respectively, of the RPU cell;

wherein the sign of the read current is deemed positive when the first current is greater than the second current; and

wherein the sign of the read current is deemed negative when the first current is less than the second reference current.

2. The method of claim 1, wherein the sign of the read current is deemed zero when the first current is equal to the second current.

3. The method of claim 1, further comprising storing a weight voltage in a weight storage device of the RPU cell, wherein the weight voltage represents the weight of the RPU cell.

4. The method of claim 3, wherein the weight storage device comprises a capacitor.

5. The method of claim 3, comprising:

generating the first current by a read transistor of the RPU cell, in response to the stored weight voltage applied to a gate terminal of the read transistor; and

generating the second current by a constant current source of the RPU cell.

6. The method of claim 5, wherein the read transistor comprises a first source/drain terminal coupled to a first port of the RPU cell, and a second source/drain terminal coupled to a second port of the RPU cell, wherein the gate terminal of the read transistor is coupled to the weight storage device, wherein the constant current source is coupled to the second source/drain terminal of the read transistor, and wherein the read current is output from the second port of the RPU cell.

7. The method of claim 6, wherein the read transistor comprises an NMOS device and wherein the constant current source is coupled to a source terminal of the read transistor, wherein the constant current source serves as a current sink to sink a fixed reference current from the first current flowing from the source terminal of the read transistor.

8. The method of claim 3, comprising:

generating the first current by a first read transistor of the RPU cell, in response to the stored weight voltage applied to a gate terminal of the first read transistor;

generating the second current by a second read transistor of the RPU cell, in response to the stored weight voltage applied to a gate terminal of the second read transistor;

subtracting the second current from the first current to generate the read current as the difference between the first current and the second current.

9. The method of claim 8, wherein the first and second currents are complementary currents.

10. The method of claim 8, wherein the first read transistor comprises an NMOS device, wherein the second read transistor comprises a PMOS device, wherein the gate terminals of the first and second read transistors are commonly connected.

11. The method of claim 8, wherein subtracting the second current from the first current to generate the read current comprises:

inputting the second current as a reference current to a current mirror circuit;

generating a mirrored current based on the second current; and

sinking the mirrored current from the first current to thereby generate the read current.

12. The method of claim 1, comprising:

setting a variable resistor device of the RPU cell to have a resistance value which corresponds to the weight of the RPU cell;

applying a control voltage to a fixed resistor device of the RPU cell having a fixed resistance value to thereby generate the second current based on the fixed resistance value;

applying the control voltage to the variable resistor device to thereby generate the first current based on the resistance value of the variable resistor device;

inputting the second current as a reference current to a current mirror circuit;

generating a mirrored current based on the second current; and

sinking the mirrored current from the first current to thereby generate the read current.

13. The method of claim 12, further comprising equalizing voltage levels at nodes of the current mirror circuit which receive the second current and which sink the mirrored current.

14. The method of claim 12, further comprising adjusting a resistance value of the variable resistor device to update the weight of the RPU cell.

15. A method comprising:

configuring a resistive processing unit (RPU) system comprising an array of RPU cells to perform deep neural network computations, wherein each RPU cell within the array of RPU cells comprises a weight, wherein the weight of a given RPU cell comprises a weight value and a weight sign;

applying a read control signal to a selected RPU cell within an array of RPU cells to read the weight of the selected RPU cell;

in response to the read control signal, the selected RPU cell generating a read current based on the weight of the RPU cell, and outputting the read current to an output port of the RPU cell;

wherein the read current is generated as a difference between a first current and a second current;

wherein the read current comprises a magnitude and sign which represent the weight value and the weight sign, respectively, of the RPU cell;

wherein the sign of the read current is deemed positive when the first current is greater than the second current; and

wherein the sign of the read current is deemed negative when the first current is less than the second reference current.

16. The method of claim 15, wherein the sign of the read current is deemed zero when the first current is equal to the second current.

17. The method of claim 15, further comprising storing a weight voltage in a weight storage device of the RPU cells, wherein the weight voltage of a given RPU cell represents the weight of the given RPU cell.

18. The method of claim 17, comprising:

generating the first current by a read transistor of the selected RPU cell, in response to the stored weight voltage applied to a gate terminal of the read transistor; and

generating the second current by a constant current source of the selected RPU cell.

19. The method of claim 17, comprising:

generating the first current by a first read transistor of the selected RPU cell, in response to the stored weight voltage applied to a gate terminal of the first read transistor;

generating the second current by a second read transistor of the selected RPU cell, in response to the stored weight voltage applied to a gate terminal of the second read transistor;

subtracting the second current from the first current to generate the read current as the difference between the first current and the second current, wherein subtracting the second current from the first current comprises;

inputting the second current as a reference current to a current mirror circuit;

generating a mirrored current based on the second current; and

sinking the mirrored current from the first current to thereby generate the read current.

20. The method of claim 15, comprising:

setting a variable resistor device of the selected RPU cell to have a resistance value which corresponds to the weight of the selected RPU cell;

applying a control voltage to a fixed resistor device of the selected RPU cell having a fixed resistance value, to thereby generate the second current based on the fixed resistance value;

applying the control voltage to the variable resistor device to generate the first current based on the resistance value of the variable resistor device;

inputting the second current as a reference current to a current mirror circuit;

generating a mirrored current based on the second current; and

sinking the mirrored current from the first current to thereby generate the read current.