Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
MEMORY ARRAY WITH INDIVIDUALLY TRIMMABLE SENSE AMPLIFIERS
Document Type and Number:
WIPO Patent Application WO/2019/133328
Kind Code:
A1
Abstract:
A device includes an array of memory cells, input/output lines coupled to the memory cells, and sense amplifiers coupled to the input/output lines. Each sense amplifier is associated with a respective input/output line. The device also includes trim circuits. Each trim circuit is associated with and coupled to a respective sense amplifier. Each sense amplifier receives a respective reference voltage that allows the sense amplifier to sense a bit value of an addressed memory cell. Each trim circuit is operable for compensating for variations in the reference voltage used by the respective sense amplifier.

Inventors:
KARMAKAR, Susmita (45500 Northport Loop West, Fremont, CA, 94538, US)
BERGER, Neal (22623 Woodridge Court, Cupertino, CA, 95014, US)
EL BARAJI, Mourad (3158 Cardenas Terrace, Apt. 233Fremont, CA, 94536, US)
LOUIE, Benjamin (2010 Laurel Canyon Court, Fremont, CA, 94539, US)
Application Number:
US2018/066041
Publication Date:
July 04, 2019
Filing Date:
December 17, 2018
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
SPIN TRANSFER TECHNOLOGIES, INC. (45500 Northport Loop West, Fremont, CA, 94538, US)
International Classes:
G11C11/16; G11C7/06; G11C7/08
Foreign References:
US20170352395A12017-12-07
US20160189784A12016-06-30
US20170047107A12017-02-16
US20160093375A12016-03-31
US20150078081A12015-03-19
Attorney, Agent or Firm:
MURABITO, Anthony, C. (Murabito, Hao & Barnes LLP,2 North Market Street, 3rd Floo, San Jose CA, 95113, US)
Download PDF:
Claims:
CLAIMS

What is claimed is:

1. A device, comprising:

an array of memory cells that are individually addressable;

a plurality of input/output lines coupled to the memory cells;

a plurality of sense amplifiers coupled to the input/output lines, wherein each sense amplifier of the plurality of sense amplifiers is associated with and coupled to a respective input/output line of the plurality of input/output lines; and

a plurality of trim circuits, wherein each trim circuit of the plurality of trim circuits is associated with and coupled to a respective sense amplifier of the plurality of sense amplifiers and further coupled to receive a respective reference voltage that allows said each sense amplifier to sense a bit value of an addressed memory cell of the array of memory cells, wherein further each trim circuit of the plurality of trim circuits is operable for compensating for variations in the respective reference voltage.

2. The device of Claim 1 , wherein said each trim circuit comprises a plurality of transistors coupled to a ground and selectively coupled to a line coupled to the respective sense amplifier.

3. The device of Claim 2, further comprising a selection circuit coupled to the plurality of transistors and operable for selectively coupling one or more of the transistors to the line and for selectively decoupling one or more of the transistors from the line.

4. The device of Claim 2, wherein the plurality of transistors comprises transistors having sizes that differ from each other by a factor of two.

5. The device of Claim 2, wherein the line comprises a reference bit line coupled to the respective sense amplifier through a bit line bias transistor.

6. The device of Claim 2, wherein the line comprises a bit line bias line coupled to a bit line bias transistor coupled to a reference bit line coupled to the respective sense amplifier.

7. The device of Claim 1 , wherein each said trim circuit comprises a variable resistor coupled to a supply voltage and coupled to a reference bit line coupled to the respective sense amplifier.

8. The device of Claim 1 , wherein the memory cells comprise magnetoresistive random access memory (MRAM) cells, wherein the MRAM cells comprise magnetic tunnel junctions (MTJs).

9. A device, comprising:

an array of magnetoresistive random access memory (MRAM) cells, wherein the MRAM cells comprise magnetic tunnel junctions (MTJs);

a plurality of input/output lines coupled to the MRAM cells;

a plurality of sense amplifiers coupled to the input/output lines, wherein each sense amplifier of the plurality of sense amplifiers is associated with and coupled to a respective input/output line of the plurality of input/output lines to receive a voltage from the respective input/output line, the voltage having a first level if a bit value of an addressed memory cell of the MRAM cells has a first binary value and having a second level if the bit value of the addressed memory cell has a second binary value, said each sense amplifier further coupled to receive a respective reference voltage operable for allowing the sense amplifier to sense the bit value of the addressed memory cell; and a plurality of trim circuits coupled to the sense amplifiers, wherein the plurality of trim circuits are operable to individually correct variations in reference voltages respectively supplied to the sense amplifiers, wherein each trim circuit of the plurality of trim circuits is operable to maintain a reference voltage at the respective sense amplifier between the first level and the second level.

10. The device of Claim 9, wherein each trim circuit of the plurality of trim circuits is associated with a respective sense amplifier of the plurality of sense amplifiers.

11. The device of Claim 9, wherein said each trim circuit comprises a plurality of transistors coupled to a ground and selectively coupled to a line coupled to the respective sense amplifier.

12. The device of Claim 1 1 , further comprising a selection circuit coupled to the plurality of transistors and operable for selectively coupling one or more of the transistors to the line and for selectively decoupling one or more of the transistors from the line.

13. The device of Claim 1 1 , wherein the plurality of transistors comprises transistors having sizes that differ from each other by a factor of two.

14. The device of Claim 1 1 , wherein the line comprises a reference bit line coupled to the respective sense amplifier through a bit line bias transistor.

15. The device of Claim 1 1 , wherein the line is coupled to a bit line bias transistor coupled to a reference bit line coupled to the respective sense amplifier.

16. The device of Claim 9, wherein each said trim circuit comprises a variable resistor coupled to a supply voltage and coupled to a reference bit line coupled to the respective sense amplifier.

17. A method, comprising:

for each sense amplifier of a plurality of sense amplifiers coupled to a memory array, wherein said each sense amplifier is associated with i) a respective input/output line of a plurality of input/output lines and ii) a respective reference bit line voltage of a plurality of reference bit line voltages, and wherein said each sense amplifier is coupled to a respective programmable trim circuit of a plurality of programmable trim circuits: detecting a variation of an associated reference bit line voltage that places the associated reference bit line voltage outside a range of voltage values;

determining a respective program value to configure the respective

programmable trim circuit to compensate for the variation; and

storing the respective program value, wherein the respective program value when applied to configure the respective programmable trim circuit causes the associated reference bit line voltage to be within the range of voltage values.

18. The method of Claim 17, wherein the respective programmable trim circuit comprises a plurality of transistors coupled to a ground and selectively coupled to a line coupled to the respective sense amplifier, wherein the method further comprises applying the respective program value to select a number of the transistors to be coupled to the line, wherein the line is selected from the group consisting of: a reference bit line coupled to the respective sense amplifier through a bit line bias transistor, and a bit line bias line coupled to a bit line bias transistor coupled between a reference bit line and the respective sense amplifier.

19. The method of Claim 17, wherein the respective programmable trim circuit comprises a variable resistor coupled to a supply voltage and coupled to a line coupled to the respective sense amplifier, wherein the method further comprises applying the respective program value to select an amount of resistance to be applied to the line, wherein the line is a reference bit line coupled to the respective sense amplifier.

20. The method of Claim 17, wherein the memory array comprises

magnetoresistive random access memory (MRAM) cells, wherein the MRAM cells comprise magnetic tunnel junctions (MTJs).

Description:
MEMORY ARRAY WITH INDIVIDUALLY TRIMMABLE SENSE AMPLIFIERS

BACKGROUND

[0001 ] Magnetoresistive random access memory (MRAM) is a non-volatile memory technology that stores data through magnetic storage elements. The elements are two ferromagnetic plates or electrodes that can hold a magnetic field and are separated by a non-magnetic material, such as a non-magnetic metal or insulator. This structure is known as a magnetic tunnel junction (MTJ). MRAM devices are considered to be a next generation structure for a wide range of memory applications.

[0002] Figure 1 illustrates an example of an MRAM cell 110 including an MTJ 120. In general, one of the plates (a reference layer or fixed layer 130) has its magnetization pinned, meaning that this layer has a higher coercivity than the other layer and requires a larger magnetic field or spin-polarized current to change the orientation of its magnetization. The second plate is typically referred to as the free layer 140 and its magnetization direction can be changed by a smaller magnetic field or spin-polarized current relative to that of the reference layer 130. The two plates can be sub-micron in lateral size, and the magnetization direction can still be stable with respect to thermal fluctuations.

[0003] MRAM devices can store information by changing the orientation of the magnetization of the free layer 140. In particular, based on whether the free layer 140 is in a parallel or anti-parallel alignment relative to the reference layer 130, either a binary value of“1” or a binary value of“0” can be stored in the MRAM cell 110 as represented in Figure 1.

[0004] MRAM products based on spin transfer torque switching, or spin transfer switching, are already making their way into larger data storage devices. Spin transfer torque MRAM (STT-MRAM) devices, such as the one illustrated in Figure 1 , use spin- aligned (polarized) electrons to change the magnetization orientation of the free layer in the magnetic tunnel junction. In general, electrons possess a quantized number of angular momentum intrinsic to the electron referred to as spin. An electrical current is generally unpolarized; that is, it consists of 50% spin-up and 50% spin-down electrons. By passing a current though a magnetic layer, electrons are polarized with a spin orientation corresponding to the magnetization direction of the magnetic layer (e.g., polarizer), thereby producing a spin-polarized current. If the spin-polarized current is passed to the magnetic region of the free layer 140 in the MTJ device, the electrons will transfer a portion of their spin-angular momentum to the magnetization layer to produce a torque on the magnetization of the free layer. This spin transfer torque can switch the magnetization of the free layer 140, which in effect writes either a Ί” or a“0” based on whether the free layer is in the parallel or anti-parallel state relative to the reference layer 130.

[0005] Due to the spin-polarized electron tunneling effect, the electrical resistance of the cell changes due to the orientation of the magnetic fields of the two layers 130 and 140. The electrical resistance is typically referred to as tunnel magnetoresistance (TMR), which is a magnetoresistance effect that occurs in an MTJ. The cell’s resistance will be different for the parallel and anti-parallel states, and thus the cell’s resistance can be used to distinguish between a“1” and a“0.”

[0006] Generally speaking, to read a memory cell, a current is applied to the bit line that includes that cell (the“main bit line”) to detect the value of the cell’s resistance by monitoring the voltage across the cell. A sense amplifier is used to sense the voltage level on the main bit line and compare it to a reference bit line voltage. The reference bit line voltage is established at a level that is between the main bit line voltage that corresponds to a bit value of 1 and the main bit line voltage that

corresponds to a bit value of 0. If the main bit line voltage is greater than the reference bit line voltage, then a“1” is stored in the cell; and if the main bit line voltage is less than the reference bit line voltage, then a“0” is stored in the cell.

[0007] There is a sense amplifier for each bit line. Thus, for example, for a 32-bit read, 32 sense amplifiers are used. Ideally, the sense amplifiers would be identical, but often there is a mismatch between them due to variations in circuit parameters deriving from manufacturing process, layout, position in the memory array, and location of the ground. These mismatches can result in erroneous reads.

[0008] This problem is illustrated by the example of Figures 2A and 2B. A reference bit line bias transistor 21 1 is connected to the reference bit line 201 , and a main bit line bias transistor 212 is connected to the main bit line 202 of a memory cell being read. As illustrated in Figure 2B, during the clock period of the sense amplifier 220, the reference bit line voltage supplied to the sense amplifier (Vs) is less than both the voltage (V1 ) that corresponds to a bit value of 1 and the voltage (V0) that corresponds to a bit value of 0 due to circuit parameter mismatch between the main bit line and the reference bit line. Thus, regardless of the actual bit value (0 or 1 ) stored by the memory cell that is being read, the sense amplifier 220 will sense a bit value of 1. That is, even if the actual stored bit value is 0, the sense amplifier 220 may erroneously sense a bit value of 1 .

[0009] In an MTJ device, the difference between the main bit line voltage corresponding to a Ί” and that corresponding to a“0” (the sensing margin) is small because the bit line resistance in series reduces the resistance ratio of the MTJs as seen by the sense amplifier. Also, a bias across an MTJ reduces the MTJ’s resistance, especially the resistance of the anti-parallel state, and so TMR is reduced relative to the case of zero bias. Reduced TMR further decreases the sensing margin, which increases the possibility of an erroneous read. Thus, because of the relatively small sensing margin in MTJ devices, the problem of mismatches due to variations in circuit parameters and reference bit line voltage is particularly acute.

SUMMARY

[0010] Embodiments according to the present invention include a memory device that includes an array of memory cells, input/output lines (e.g., bit lines) coupled to the memory cells, and sense amplifiers coupled to the input/output lines. Each sense amplifier is associated with a respective input/output line. The device also includes trim circuits. Each trim circuit is associated with and coupled to a respective sense amplifier. Each sense amplifier receives a respective reference voltage that allows the sense amplifier to sense a bit value of an addressed memory cell. Each trim circuit is operable for independently compensating for variations in the reference voltage used by the respective sense amplifier.

[001 1] In an embodiment, the memory cells include magnetoresistive random access memory (MRAM) cells, and the MRAM cells include magnetic tunnel junction (MTJ) memory cells.

[0012] In some embodiments, each trim circuit includes a number of transistors coupled to a ground and selectively coupled to a line that is coupled to a respective sense amplifier. In an embodiment, the trim circuit is coupled to a reference bit line that is coupled to the sense amplifier through a respective bit line bias transistor. In another embodiment, the trim circuit is coupled to a bit line bias line that is coupled to a respective bit line bias transistor that is coupled to a respective reference bit line that is coupled to the sense amplifier. In some of these embodiments, each trim circuit also includes a selection circuit that is coupled to the transistors and that is operable for selectively coupling one or more of the transistors to the line and for selectively decoupling one or more of the transistors from the line. In an embodiment, the transistors in the trim circuits have sizes that differ from each other by a factor of two.

[0013] In other embodiments, each trim circuit includes a variable resistor that is coupled to a supply voltage and that is also coupled to a reference bit line that is coupled to a respective sense amplifier.

[0014] Embodiments according to the present invention also include a method for configuring a trim circuit in a memory array. For each sense amplifier coupled to the memory array, a variation in an associated reference bit line voltage that places the associated reference bit line voltage outside a range of voltage values is detected; a respective program value to configure the respective programmable trim circuit to compensate for the variation is determined; and the respective program value is stored. When the program value is applied to configure a respective trim circuit, the associated reference bit line voltage is compensated so that it is within the range of voltage values. For example, the program value can be used to select (enable or disable) a number of transistors in a trim circuit or to select an amount of resistance in a trim circuit coupled to the sense amplifier. [0015] In summary, embodiments according to the present invention use a trim circuit per sense amplifier to compensate for variations in the respective reference bit line voltage supplied to the sense amplifier. As a result, the reference bit line voltage supplied to each sense amplifier is maintained between the voltage associated with a bit value of 1 and the voltage associated with a bit value of 0. Mismatches between sense amplifiers are thereby compensated for or precluded. Consequently, the accuracy and reliability of read operations are increased. Specifically, erroneous reads due to mismatches in circuit parameters and variations in the reference bit line voltages are reduced in number if not completely eliminated, even when the sensing margin is small as it is for MTJ devices in MRAM arrays.

[0016] These and other objects and advantages of the various embodiments of the present invention will be recognized by those of ordinary skill in the art after reading the following detailed description of the embodiments that are illustrated in the various drawing figures.

BRIEF DESCRIPTION OF DRAWINGS

[0017] The accompanying drawings, which are incorporated in and form a part of this specification and in which like numerals depict like elements, illustrate embodiments of the present disclosure and, together with the detailed description, serve to explain the principles of the disclosure.

[0018] Figure 1 illustrates an example of a conventional magnetoresistive random access memory cell.

[0019] Figure 2A illustrates an example of a conventional memory device including a sense amplifier coupled to a reference bit line and to a main bit line.

[0020] Figure 2B illustrates an example of voltages versus time for an erroneous read operation in a conventional memory device.

[0021 ] Figure 3A illustrates a memory array in a memory device in embodiments according to the present invention. [0022] Figure 3B illustrates an example of voltages versus time for a read operation in embodiments according to the present invention.

[0023] Figure 4 illustrates an embodiment of a trim circuit according to the present invention.

[0024] Figure 5 illustrates a trim circuit coupled to a reference bit line in embodiments according to the present invention.

[0025] Figure 6 illustrates a trim circuit coupled to a bit line bias line in embodiments according to the present invention.

[0026] Figure 7 illustrates another embodiment of a trim circuit according to the present invention.

[0027] Figure 8 is a flowchart of examples of operations in a method for configuring a trim circuit in embodiments according to the present invention.

[0028] Figure 9 is an example of a computing system upon which embodiments according to the present invention can be implemented.

DETAILED DESCRIPTION

[0029] Reference will now be made in detail to the various embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. While described in conjunction with these embodiments, it will be understood that they are not intended to limit the disclosure to these embodiments. On the contrary, the disclosure is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the disclosure as defined by the appended claims. Furthermore, in the following detailed description of the present disclosure, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. However, it will be understood that the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present disclosure.

[0030] Some portions of the detailed descriptions that follow are presented in terms of procedures, logic blocks, processing, and other symbolic representations of operations on data bits within a computer memory. These descriptions and

representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. In the present application, a procedure, logic block, process, or the like, is conceived to be a self- consistent sequence of steps or instructions leading to a desired result. The steps are those utilizing physical manipulations of physical quantities. Usually, although not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated in a computing system. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as transactions, bits, values, elements, symbols, characters, samples, pixels, or the like.

[0031] It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the present disclosure, discussions utilizing terms such as“reading,”“programming,”“detecting,”“determi ning,” “storing,”“applying,” or the like, refer to actions and processes (e.g., the flowchart 800 of Figure 8) of a computing system or similar electronic computing device or hardware processor (e.g., the system 900 of Figure 9). The computing system or similar electronic computing device manipulates and transforms data represented as physical (electronic) quantities within the computing system memories, registers or other such information storage, transmission or display devices.

Memory Array with Individually Trimmable Sense Amplifiers

[0032] Figure 3A illustrates a memory array 301 in a memory device 300 in embodiments according to the present invention. In embodiments, the memory device 300 is a magnetoresistive random access memory (MRAM) device. In an embodiment, the memory device 300 is a spin transfer torque (STT-MRAM) device. [0033] The memory array 301 includes a number of memory cells or storage elements exemplified by the memory cell 302. As just noted, the memory cell 302 may be an MRAM cell. In embodiments, an MRAM cell includes a magnetic tunnel junction (MTJ).

[0034] The memory device 300 includes a number of word lines exemplified by the word lines WL(0), WL(1 ), ... , WL(n), a number of bit lines exemplified by BL(0),

BL(1 ), BL(m), and a number of source lines exemplified by SL(0), SL1 (1 ), SL(m). Not all memory cells, word lines, bit lines, and source lines are shown in the figure. The bit lines may also be referred to herein as input/output lines.

[0035] The memory cell 302 is at the intersection of a particular word line, bit line, and source line. The memory cell 302 is individually addressable using a row address, which specifies the particular word line, and a column address, which specifies the particular bit line. The memory device 300 includes an x-decoder block 304 that decodes a row address signal and supplies a signal to the corresponding word line.

The memory device 300 also includes a number of y-multiplexers (y-muxes) exemplified by the y-mux 306. The y-muxes provide column select logic for selecting a bit line and source line corresponding to a column address signal. A number of memory cells are typically addressed together to form a data word.

[0036] The memory device 300 also includes a number of sense amplifiers 310- 0, 310-1 , ... , 310-N, collectively referred to herein as the sense amplifiers 310. Each of the sense amplifiers 310 is coupled to a respective set of the input/output lines (set of bit lines) through a respective y-mux as shown in Figure 3A. Each sense amplifier can receive a voltage from a respective input/output line (bit line), specifically an input/output line (bit line) corresponding to the memory cell being read (the addressed memory cell). The voltage has a first level if a bit value of the addressed memory cell has a first binary value, and has a second level if the bit value of the addressed memory cell has a second binary value. Each sense amplifier also can receive a respective reference bit line voltage that allows the sense amplifier to sense the bit value of the addressed memory cell. Each sense amplifier 0, 1 , ... , N outputs a respective output (I/O) value (bit value) I/O 0, I/O 1 , ... , I/O N. [0037] Significantly, in embodiments according to the present invention, the memory device 300 also includes a number of circuits 320-0, 320-1 , ... , 320-N collectively referred to herein as trim circuits 320. Each of the trim circuits 320 is associated with and coupled to a respective one of the sense amplifier 310. That is, there is a different trim circuit for each sense amplifier.

[0038] As described further below, each of the trim circuits 320 compensates for variations in the respective bit line reference voltage received by the corresponding one of the sense amplifiers 310. That is, the trim circuits 320 are operable to individually correct variations in the reference bit line voltages respectively supplied to the sense amplifiers 310, and each trim circuit is operable to maintain the respective reference bit line voltage between a first voltage level (the voltage level associated with a bit value of 1 ) and a second voltage level (the voltage level associated with a bit value of 0), as shown in Figure 3B.

[0039] Figure 3B illustrates an example of voltages versus time for a read operation in embodiments according to the present invention. For example, the trim circuit 320-N compensates for variations in the reference voltage received by the sense amplifier 310-N. As a result, during the clock period of the sense amplifier 310-N, there is no mismatch between the main and reference bit line voltages and the reference bit line voltage supplied to the sense amplifier (Vs) is between the voltage (V1 ) that corresponds to a bit value of 1 and the voltage (V0) that corresponds to a bit value of 0. Thus, the sense amplifier 310-N will sense the proper bit value.

[0040] Co-pending U.S. Patent Application No. _ , entitled“Multi-Port Random

Access Memory,” by Mourad El Baraji et al., Attorney Docket No. SPIN-0012-01 .01 US, is hereby incorporated by reference in its entirety. That application describes a multi-port memory device that can access two or more memory cells at the same time.

Embodiments according to the present disclosure can be utilized with and/or in a multi- port memory device as disclosed in the referenced application.

[0041] Figure 4 illustrates a trim circuit 400 in embodiments according to the present invention. In some embodiments, each of the trim circuits 320 of Figure 3A is implemented using the trim circuit 400. In other embodiments, each of the trim circuits 320 is implemented using the trim circuit 700 of Figure 7, described further below.

[0042] The transistor tree 404 of Figure 4 includes a number of transistors 401 -0, 401-1 , ... , 401 -k, collectively referred to herein as the transistors 401 . In an embodiment, the sizes of the transistors differ from each other by a factor of two. Thus, for example, the transistor 401-1 is twice the size of the transistor 401-0, the transistor 401-2 (not shown) is twice the size of the transistor 401-1 , and so on.

[0043] The transistors 401 receive a reference voltage from a voltage bias generator 405. The transistors 401 are coupled to a selection circuit 402. In general, as described further in conjunction with Figures 5 and 6 below, the selection circuit 402 is operable for selectively coupling and decoupling one or more of the transistors 401 to and from a respective reference bit line.

[0044] In an embodiment, the selection circuit 402 of Figure 4 includes a number of switches 403-0, 403-1 , ... , 403-k, collectively referred to herein as the switches 403. The switches 403 can be implemented as transistors. Each of the transistors 401 is coupled to a respective one of the switches 403. Each of the switches 403 is controlled by a respective program value or configuration bit. If the configuration bit has a first binary value, then the switch is closed (enabled), and the switch is opened (disabled) if the configuration bit has a second binary value. Each of the transistors 401 is also coupled to a respective ground.

[0045] When a transistor in the transistor tree 404 is coupled to a respective reference bit line or respective bit line bias line (see Figures 5 and 6) by closing the associated one of the switches 403, a corresponding amount of current is trimmed (drained) from that line through the transistor to ground. As noted above, one or more of the transistors may be coupled to the line at a time. As described above, in an

embodiment, the transistors 401 have different sizes. Thus, the amount of the trimmed current can be finely controlled by selecting a single one of the transistors (a single desired size) or any combination of the transistors (any desired combination of sizes). [0046] In general, the program values used to configure the trim circuit 400 (Figure 4) are determined by measuring the reference bit line voltage respectively supplied to each of the sense amplifiers 320 (Figure 3A) and then determining the program values that will cause each respective reference bit line voltage to be in the range between the voltage for a binary value of 1 and the voltage for a binary value of 0. Ideally, each reference bit line voltage supplied to a sense amplifier is at about the midpoint of that range.

[0047] The program values used to control the switches 403 (Figure 4) can be determined, for example, when the memory device 300 (Figure 3A) is manufactured, and/or when the memory device is installed in a computing system, and/or each time the memory device is powered up. Alternatively, after installation, testing of the memory device 300 can be periodically performed and the program values can be determined accordingly.

[0048] Because a respective trim circuit is coupled to each sense amplifier, each trim circuit can be individually tuned. That is, the program values can be different for each of the trim circuits 400 of Figure 4. Thus, the reference bit line voltage for each of the sense amplifiers 320 (Figure 3A) can be individually trimmed, as opposed to global trimming of all sense amplifiers.

[0049] Figure 5 illustrates a trim circuit 400 coupled to a reference bit line 502 in an embodiment according to the present invention. The sense amplifier 310-N is coupled to receive a reference bit line voltage from the reference bit line 502, which is connected to the sense amplifier through a reference bit line bias transistor 506. A reference bit line bias line 504 is connected to the gate of the reference bit line bias transistor 506. As described above, the trim circuit 400 is configured to trim a selected amount of current on the reference bit line 502 to compensate for variations in the reference bit line, so that a trimmed reference bit line voltage is supplied to the sense amplifier 310-N.

[0050] The sense amplifier 310-N is also coupled to receive a main bit line voltage from the main bit line 508, which is connected to the sense amplifier through a main bit line bias transistor 510. A main bit line bias line 512 is connected to the gate of the main bit line bias transistor 510. The main bit line voltage is the voltage on the bit line of the memory cell being read (the addressed memory cell). The sense amplifier 310-N can compare the main bit line voltage and the trimmed reference bit line voltage to determine the bit value of the memory cell being read.

[0051] Figure 6 illustrates a trim circuit 400 coupled to a bit line bias line 604 in an embodiment according to the present invention. A reference bit line bias line 604 is connected to the gate of the reference bit line bias transistor 606 that is coupled to the reference bit line 602, which is coupled to the sense amplifier 310-N. As described above, the trim circuit 400 is configured to trim a selected amount of the current on the reference bit line bias line 604 to compensate for variations in the reference bit line 602, so that a trimmed reference bit line voltage is supplied to the sense amplifier 310-N.

More specifically, the float on the reference bit line bias transistor 606 is adjusted to provide a different bias for the reference bit line 602 and trim the reference bit line voltage supplied to the sense amplifier 310-N.

[0052] The sense amplifier 310-N is also coupled to receive a main bit line voltage from the main bit line 608, which is connected to the sense amplifier through a main bit line bias transistor 610. A main bit line bias line 612 is connected to the gate of the main bit line bias transistor 610. The main bit line voltage is the voltage on the bit line of the memory cell being read (the addressed memory cell). The sense amplifier 310-N can compare the main bit line voltage and the trimmed reference bit line voltage to determine the bit value of the memory cell being read.

[0053] Figure 7 illustrates a trim circuit 700 in another embodiment according to the present invention. As mentioned above, in some embodiments, each of the trim circuits 320 of Figure 3A is implemented using the trim circuit 700. The trim circuit 700 includes a variable resistor 702 coupled to the reference bit line 704 and to supply voltage Vdd. The trim circuit 700 also includes a resistor 703 coupled between the supply voltage Vdd and the main bit line 710. The current on the reference bit line 702 can be trimmed using the trim circuit 700, by varying the resistances of the variable resistor 702, so that a trimmed reference bit line voltage is supplied to the sense amplifier 310-N. [0054] The reference bit line 704 is coupled to the trim circuit 700 and to the sense amplifier 310-N through a reference bit line bias transistor 706. A reference bit line bias line 708 is connected to the gate of the reference bit line bias transistor 706.

[0055] The variable resistor 702 can be implemented using a digital

potentiometer, or digitally controlled variable resistor, that is controlled by program values or configuration bits in a manner similar to that described above for the trim circuit 400. The program values used to control the variable resistor 702 can be determined, for example, when the memory device 300 (Figure 3A) is manufactured, and/or when the memory device is installed in a computing system, and/or each time the memory device is powered up. Alternatively, after installation, testing of the memory device 300 can be periodically performed and the program values can be determined accordingly.

[0056] The sense amplifier 310-N is also coupled to receive a main bit line voltage from the main bit line 710, which is connected to the sense amplifier through a main bit line bias transistor 712. A main bit line bias line 714 is connected to the gate of the main bit line bias transistor 712. The main bit line 710 is also coupled to the trim circuit 700. The main bit line voltage is the voltage on the bit line of the memory cell being read (the addressed memory cell). The sense amplifier 310-N can compare the main bit line voltage and the trimmed reference bit line voltage to determine the bit value of the memory cell being read.

[0057] Figure 8 is a flowchart 800 of examples of operations in a method for configuring a trim circuit in embodiments according to the present invention. The operations can be performed in and by the computing system 900 of Figure 9. The computing system 900 can include the memory device 300 of Figure 3A or can be coupled to the memory device. The operations in the flowchart 800 can be performed, for example, by a tester system when the memory device 300 is manufactured, and/or when the memory device is installed, and/or each time the memory device is powered up, and/or each time the memory device is tested. The operations in the flowchart 800 can be performed for each of the sense amplifiers 310 of Figure 3A. [0058] In block 802 of Figure 8, a variation of a reference bit line voltage associated with a sense amplifier, that places the associated reference bit line voltage outside a range of voltage values, is detected.

[0059] In block 804, a respective program value or configuration bit to configure a respective programmable trim circuit (e.g., the trim circuit 400 or the trim circuit 700 of Figures 4 and 7, respectively) to compensate for the variation is determined.

[0060] In block 806, the respective program value is non-volatilely stored. The respective program value, when applied to configure the respective programmable trim circuit, causes the associated reference bit line voltage to be within the range of voltage values.

[0061 ] For implementations using the trim circuit 400 of Figure 4 implemented as shown in Figure 5, the respective program value is applied to select a number of transistors to be coupled to a reference bit line coupled to the respective sense amplifier through a respective bit line bias transistor. For implementations using the trim circuit 400 of Figure 4 implemented as shown in Figure 6, the respective program value is applied to select a number of transistors to be coupled to a bit line bias line coupled to a respective bit line bias transistor coupled to a respective reference bit line that is coupled to the respective sense amplifier.

[0062] For implementations using the trim circuit 700 of Figure 7, the respective program value is applied to select an amount of resistance to be applied to a reference bit line coupled to the respective sense amplifier.

[0063] Figure 9 is an example of a computing system 900 upon which

embodiments according to the present invention can be implemented. In its most basic configuration, the system 900 includes at least one processing unit 902 and memory 904. This most basic configuration is illustrated in Figure 9 by dashed line 906. The system 900 may also have additional features and/or functionality. For example, the system 900 may also include additional storage (removable and/or non-removable) including, but not limited to, magnetic or optical disks or tape. Such additional storage is illustrated in Figure 9 by removable storage 908 and non-removable storage 920. The system 900 may also contain communications connection(s) 922 that allow the device to communicate with other devices, e.g., in a networked environment using logical connections to one or more remote computers.

[0064] The system 900 can also include input device(s) 924 such as keyboard, mouse, pen, voice input device, touch input device, etc. Output device(s) 926 such as a display device, speakers, printer, etc., are also included.

[0065] In the example of Figure 9, the memory 904 includes computer-readable instructions, data structures, program modules, and the like associated with an application for executing a method for configuring a trim circuit in embodiments according to the present invention (e.g., the method of Figure 8). For example, in an embodiment, the system 900 can include a tester system that can be connected to a memory device that includes a trim circuit (e.g., the memory device 300 of Figure 3).

[0066] Thus, in embodiments according to the present invention, a trim circuit per sense amplifier is used to compensate for variations in the respective reference bit line voltage supplied to the sense amplifier. As a result, the reference bit line voltage supplied to each sense amplifier is maintained between the voltage associated with a bit value of 1 and the voltage associated with a bit value of 0. Mismatches between sense amplifiers are thereby compensated for or precluded. Consequently, the accuracy and reliability of read operations are increased. Specifically, erroneous reads due to mismatches in circuit parameters and variations in the reference bit line voltages are reduced in number if not completely eliminated, even when the sensing margin is small as it is for MTJ devices in MRAM arrays.

[0067] While the foregoing disclosure sets forth various embodiments using specific block diagrams, flowcharts, and examples, each block diagram component, flowchart step, operation, and/or component described and/or illustrated herein may be implemented, individually and/or collectively, using a wide range of hardware, software, or firmware (or any combination thereof) configurations. In addition, any disclosure of components contained within other components should be considered as examples because many other architectures can be implemented to achieve the same

functionality. [0068] The process parameters and sequence of steps described and/or illustrated herein are given by way of example only and can be varied as desired. For example, while the steps illustrated and/or described herein may be shown or discussed in a particular order, these steps do not necessarily need to be performed in the order illustrated or discussed. The example methods described and/or illustrated herein may also omit one or more of the steps described or illustrated herein or include additional steps in addition to those disclosed.

[0069] Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the disclosure is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the disclosure.

[0070] Embodiments described herein may be discussed in the general context of computer-executable instructions residing on some form of computer-readable storage medium, such as program modules, executed by one or more computers or other devices. By way of example, and not limitation, computer-readable storage media may comprise non-transitory computer-readable storage media and communication media; non-transitory computer-readable media include all computer-readable media except for a transitory, propagating signal. Generally, program modules include routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or distributed as desired in various embodiments.

[0071] Computer storage media includes volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules or other data. Computer storage media includes, but is not limited to, random access memory (RAM), magnetoresistive random access memory (MRAM), read only memory (ROM), electrically erasable programmable ROM (EEPROM), flash memory or other memory technology, compact disk ROM (CD-ROM), digital versatile disks (DVDs) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to store the desired information and that can be accessed to retrieve that information.

[0072] Communication media can embody computer-executable instructions, data structures, and program modules, and includes any information delivery media. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, radio frequency (RF), infrared, and other wireless media. Combinations of any of the above can also be included within the scope of computer-readable media.

[0073] Embodiments according to the present invention are thus described.

While the present invention has been described in particular embodiments, it should be appreciated that the present invention should not be construed as limited by such embodiments, but rather construed according to the following claims.