Sram with elastic power supply for improved read and write margins

<u>CROSS-REFERENCE TO RELATED APPLICATIONS</u>

0001This application claims the benefit of <patcit id="pcit0001" dnum="US93296707A" dnum-type="L"><text>U.S. Nonprovisional Patent Application No. 11/932,967 filed on October 31, 2007</text></patcit> and entitled "Elastic Power for Read and Write Margins" and of <patcit id="pcit0002" dnum="US93819607A" dnum-type="L"><text>U.S. Nonprovisional Patent Application No. 11/938,196 filed on November 9, 2007</text></patcit> and entitled "Elastic Power for Read Margin." This application also claims the benefit of <patcit id="pcit0003" dnum="US88800607P" dnum-type="L"><text>U.S. Provisional Patent Application No. 60/888,006 filed on February 2, 2007</text></patcit> and entitled "Split Power Switch for Memory Cells" and of <patcit id="pcit0004" dnum="US93255507A" dnum-type="L"><text>U.S. Nonprovisional Patent Application No. 11/932,555 filed on October 31, 2007</text></patcit> and entitled "Memory Device With Split Power Switch" and of <patcit id="pcit0005" dnum="US93264307A" dnum-type="L"><text>U.S. Nonprovisional Patent Application No. 11/932,643 filed on October 31, 2007</text></patcit> and entitled "Method of Selectively Powering Memory Device. "

<u>BACKGROUND OF THE INVENTION</u>

0002Static random access memory (SRAM) cells can be implemented using cross- coupled logic gates which maintain logic states representing data values. Ideally, SRAM cells should hold their stored logic states despite possible changes in voltage, temperature, or other operating conditions. Unfortunately, existing SRAM cell designs often fail to provide high degrees of read stability.

0003As SRAM cell operating voltages are reduced, the internal nodes of the SRAM cell may be vulnerable to read disturbance. For example, during a read operation, the internal SRAM nodes may be inadvertently charged through the access transistors to rise above a trip voltage of the SRAM cell, thereby causing the SRAM cell to inadvertently switch logic states.

0004The Static Noise Margin (SNM) is a "figure of merit" which measures read stability and read margin. See <nplcit id="ncit0001" npl-type="s" url="http://www-mtl.mit.edu/researchgroups/icsystems/pubs/conferences/2005/bcalhoun_esscirc2005_pap er.pdf"><text>Benton H. Calhoun and Anantha Chandrakasan, Analyzing Static Noise Margin for Subthreshold SRAM in 65nm CMOS, http://www-mtl.mit.edu/researchgroups/icsystems/pubs/conferences/2005/bcalhoun_esscirc2005_pap er.pdf (September 2005</text></nplcit>). See also Evert Seevinck,<nplcit id="ncit0002" npl-type="s"><text> Frans J. List, Jan Lohstroh, Static- Noise Margin Analysis of MOS SRAM Cells, IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. SC-22, No. 5, pp. 748-754 (October 1987</text></nplcit>).

0005Essentially, designers aim for the most symmetrical SNM curve that is possible, with the widest "eye." The "eye" is the gap between the lowest logic high and the highest logic low voltages that form the SNM curve. A symmetrical SNM curve with a wide "eye" represents strong read stability, optimal read margin, and minimal read disturbance. Conversely, an asymmetrical graph with a reduced eye represents low read stability, minimal read margin, and high read disturbance. Therefore, to improve read margin, a designer's goal is to achieve a symmetrical SNM curve with the widest eye possible.

0006To minimize read disturbance, increase read stability, improve read margin, and improve the SNM, a designer can reduce the ratio between the sizes of the NMOS driver transistor and the NMOS pass transistor in a SRAM cell. However, when NMOS transistors are manufactured in a modern manufacturing process (e.g. a 65 nm process), the variation from the desired ratio between the driver and the pass transistor can be significant. For example, in NMOS transistors manufactured in a 65 nm process, the variation from the desired ratio can be as large as 10:1. These large variations lead to differences in resistance, channel length, threshold voltage, and other device characteristics. Large variations in size ratios and device characteristics are a major cause of low read stability.

0007In one approach to reduce read disturbance, improve read stability, and increase read margin, an additional pair of PMOS transistors is added to the SRAM cell so that the overall ratio of the PMOS transistors to the NMOS transistors in the SRAM cell is minimized. The additional pair of PMOS transistors also makes the SNM curve symmetrical, resulting in a wider "eye." This approach tries to minimize the variation in size ratio between the NMOS driver and pass transistors by counter-balancing voltages in the SRAM cell. Although adding a pair of PMOS transistors alleviates the problem slightly, such an approach has its own drawbacks. The PMOS transistors eventually get so strong that they pull nodes in the SRAM cell high when they should not be pulled high, leading to poor read stability.

0008In another approach to reduce read disturbance and improve read stability, the variations in device characteristics are reduced by a "Dual Stress Layer" in selected transistors (e.g. a pass gate transistor) of a SRAM cell. See Shou-Gwo Wuu, Jin- Yuan Lee, Dun-Nian Yaung, Jeng-Han Lee, <patcit id="pcit0006" dnum="US6635936B"><text>U.S. Patent No. 6,635,936</text></patcit> ("SRAM Layout for Relaxing Mechanical Stress in Shallow Trench Isolation Technology"); Mark Craig, Karsten Wieczorek, Manfred Horstmann, <patcit id="pcit0007" dnum="WO2007018780A"><text>WO/2007/018780</text></patcit> ("SRAM Devices Utilizing Tensile-Stressed Strain Films"). However, this approach restricts the variations in device characteristics only from a device physics perspective. This approach does not address the larger problem of voltage and size ratio variance in a SRAM cell that leads to read instability, higher read disturbance, and a lower SNM.

0009As can be seen, both adding a pair of PMOS transistors and a solution aimed at altering the device characteristics of SRAM transistors are problematic. [009A] <patcit id="pcit0008" dnum="US7092280B"><text>US 7,092,280</text></patcit> discloses a CMOS static random access memory (SRAM) array with dynamically asymmetric cells, an integrated circuit (IC) chip including the SRAM and a method of accessing data in the SRAM. Each column of cells is connected to a pair of column supply lines supplying power to the column. During each SRAM access, a higher voltage is applied to one column supply line in each pair of the columns being accessed to unbalance cells in the columns being accessed. Unbalanced cells become asymmetric during accesses and the supply imbalance favors the data state being written/read.

<u>SUMMARY OF THE INVENTION</u>

0010[009B] The invention is defmed in the claims.

0011An embodiment of the invention enables an increase read stability, decrease read disturbance, improve the SNM, and improve read margin for an SRAM memory cell. An embodiment of this invention provides a memory device that includes: a reference voltage; a first and a second power supply line; a static random access memory (SRAM) cell with cross-coupled first and second logic gates, with the first power supply line and the second power supply line respectively supplying the first and the second logic gates; and an elastic power header device provides, during a write operation of the first SRAM cell, different power levels on the first power supply line and the second power supply line and that provides, during a read operation, a programmable resistance in the first power supply line and the second power supply line.

0012Further to meet this need, other embodiments of this invention provide an elastic power header device that provides a memory device with a first and second power supply lines. The elastic power device includes: (1) a first path having first and second power differences between a reference voltage and, respectively, the first and second power supply lines; (2) second and third paths that are coupled between the reference voltage and, respectively, the first and second power supply lines wherein, during a write operation, resistance in either the second path or the third path is varied to provide a power difference between the first and second power supply lines; and (3) fourth and fifth paths that are coupled between the reference voltage and, respectively, the first and second power supply lines wherein, during a read operation, a programmable resistance is provided in a selected one of the fourth path or the fifth path.

0013Various embodiments of this invention include methods of operating the above or similar memory devices, and computer-readable media that use hardware description languages (HDLs) to describe the above or similar memory devices.

<u>BRIEF DESCRIPTION OF THE FIGURES</u>

0014<figref idref="f0001">Figure 1</figref> is a conceptual block diagram showing circuit 100, in which a number of SRAM cells are connected to an elastic power header device, in accordance with one embodiment of the invention.

0015<figref idref="f0002">Figure 2</figref> shows circuit 200, which implements an SRAM cell suitable for use as any of SRAM cells 110 of <figref idref="f0001">Figure 1</figref>, in accordance with an embodiment of the invention.

0016<figref idref="f0003">Figure 3</figref> illustrates circuit 300, which implements an elastic power header device in accordance with an embodiment of the invention.

0017<figref idref="f0004">Figure 4</figref> illustrates circuit 400, which may be also be used to implement elastic power header device 180 of <figref idref="f0001">Figure 1</figref>, in accordance with another embodiment of the invention.

0018<figref idref="f0005">Figure 5</figref> illustrates circuit 500, which implements elastic power header device 180 of <figref idref="f0001">Figure 1</figref>, in accordance with another embodiment of the invention.

0019<figref idref="f0006">Figure 6</figref> shows circuit 600, which provides a low-going pulse, which may be used to provide write enable signal 425 during a read or write operation in elastic power header circuit 180 of <figref idref="f0001">Figure 1</figref>, in accordance with an embodiment of the invention.

0020<figref idref="f0006">Figure 7</figref> illustrates an example of a content-addressable memory (CAM) cell that may be implemented using the SRAM cell circuits disclosed above.

0021To allow cross-referencing among the figures, like elements in the figures are provided like reference numerals.

<u>DETAILED DESCRIPTION</u>

0022In accordance with the present invention, an elastic power header device improves the read margin and read stability of a memory cell, such as an SRAM cell. In particular, an elastic power header device described herein acts as a programmable resistance to allow the power supply lines to reach a voltage equal to a desired reference voltage, without experiencing a threshold drop. Therefore, an elastic power header device of the present invention provides additional flexibility in both read margin and read stability. The additional flexibility may also be varied with an adjustment voltage, which can be used to fine-tune and control the read margin.

0023<figref idref="f0001">Figure 1</figref> is a conceptual block diagram showing circuit 100, in which a number of SRAM cells 110 are connected to elastic power header device 180, in accordance with an embodiment of the invention. Several possible circuit implementations for elastic power header device 180 are illustrated in <figref idref="f0003 f0004 f0005">Figs. 3 through 5</figref>, as further described herein.

0024In <figref idref="f0001">Figure 1</figref>, SRAM cells 110 are each connected to power supply lines 125 and 135. In one embodiment, each of SRAM cells 110 may be implemented by cross-coupled logic gates (e.g., cross-coupled inverters). Elastic power header device 180 may also act as "split power switch" that provide voltages of power supply lines 125 and 135 from power supply voltage 120 ("V_dd") Split power switches are discussed, for example, in U.S. patent application, entitled "Split Power Switch for Memory Cells" ("Copending Application"), serial no. <patcit id="pcit0009" dnum="US60888006B"><text>60/888,006, filed on February 2, 2007</text></patcit>. In a split power switch, voltages of power supply lines 125 and 135 are slightly different, to facilitate writing into an SRAM cell. The disclosure of the Copending Application is hereby incorporated by reference to provide background for the split power technique.

0025Elastic power header device 180 selectively adjusts the voltages provided to power supply lines 125 and 135, in response to one or more control signals 150. In one embodiment, elastic power header device 180 may be configured to provide substantially reference voltage 120 during a read operation, while allowing the voltage of one of power supply lines 125 and 135 to fall during an appropriate write operation of SRAM cells 110.

0026Although SRAM cells 110 are shown in <figref idref="f0001">Figure 1</figref> as a single column of memory cells connected to a single split power switch 140, it is understood that <figref idref="f0001">Figure 1</figref> is merely illustrative. In a typical implementation, additional groups of SRAM cells 110 connect to power supply lines 125 and 135, and additional elastic power header devices 180 may be used to provide power to these additional groups of SRAM cells. Alternatively, the additional SRAM cells may share power supply lines 125 and 135 under control by a single elastic power header device 180. Furthermore, the single column of memory cells may be split into multiple sub-columns, each sub-column having its own power supply lines 125 and 135 but sharing the same bit line.

0027As also shown in <figref idref="f0001">Fig. 1</figref>, bit lines 270 and 280 are connected with each of SRAM cells 110-1 through 110-N. In this regard, SRAM cells 110-1 through 110-N may share bitlines 270 and 280. However, in other embodiments, bit lines may not be shared by all of SRAM cells 110-1 through 110-N. For example, in one embodiment, a first set of bit lines may be provided to SRAM cells 110-1 and 110-2, and a second set of bit lines may be provided to SRAM cell 110-N. In such an embodiment, SRAM cells 110-1 and 110-2 may use bit lines separate from those used by SRAM cell 110-N. In other embodiments, any desired combination of shared and/or unshared bit lines may be used.

0028<figref idref="f0002">Figure 2</figref> shows circuit 200, which implements an SRAM cell that is suitable for use as any of SRAM cells 110 of <figref idref="f0001">Figure 1</figref>, in accordance with an embodiment of the invention. As shown in <figref idref="f0002">Figure 2</figref>, circuit 200 includes cross-coupled inverters 225 and 245 implemented by PMOS transistors 210 and 230 and NMOS transistors 220 and 240, respectively. PMOS transistor 210 connects to power supply line 135, while PMOS transistor 230 connects to power supply line 125. NMOS transistors 220 and 240 each connect to reference voltage 295 ("V_ss") which may correspond, for example, to ground. <figref idref="f0002">Figure 2</figref> further shows that n-well and p-well voltages 205 and 215 (labeled V_nw and V_pw, respectively), each may differ from either reference voltage 120 or reference voltage 295. The n-well voltage 205 must be greater than or equal to the maximum of power supply lines 125 or 135. The n-well voltage is often the same line as reference voltage 120 ("V_dd"). The n-well voltage 205 is not the same line as either power supply lines 125 or 135, but it may happen to have the same voltage as power supply lines 125 or 135.

0029Access or pass transistors 250 and 260 connect to word line 290 and bit lines 270 and 280, and internal nodes 255 and 265, respectively. In circuit 200, to maintain a stored logic state, elastic power header device 180 provides that the voltage of each of power supply lines 125 and 135 is approximately reference voltage 120. During a read operation, elastic power header device 180 maintains each of power supply lines 125 and 135 at approximately reference voltage 120. During a read operation, bit lines 270 and 280 are precharged to an intermediate voltage, and word line 290 may then be set to a logic high voltage, so that access transistors 250 and 260 becomes conducting to allow inverters 225 and 245 to drive the stored logic values at nodes 255 and 265 to bit lines 270 and 280.

0030During a write operation, to allow a logic state represented by a higher voltage ("logic high") to be establish in node 255 (relative to the voltage representing the opposite logic state ("logic low") to be established in node 265), elastic power header device 180 permits the voltage of power supply line 125 to fall below reference voltage 120, while maintaining the voltage of power supply line 135 at approximately reference voltage 120. In this example, nodes 255 and 265 are initially at logic low and logic high, respectively. In that logic state, NMOS transistor 220 and PMOS transistor 230 are conducting, while PMOS transistors 210 and NMOS transistor 240 are non-conducting. During the write operation, bit lines 270 and 280 are driven to logic high and logic low, respectively, by appropriate write circuitry (not shown), as word line 290 is driven to logic high to turn on access transistors 250 and 260. Accordingly, bit line 280 pulls node 265 down from logic high to logic low. Because PMOS transistor 230 is initially conducting, its current acts to pull up the voltage at node 265. However, with power supply line 125 falling below reference voltage 120, resulting in less current flowing in transistor 230 than would otherwise flow, bit line 280 can more rapidly turn on PMOS transistor 210 which, in turn, turns on NMOS transistor 240 to pull down node 265. By maintaining the voltage of power supply line 135 approximately at reference voltage 120, transistor 210 allows the voltage at node 255 to be more rapidly pulled up.

0031Selectively reducing the voltage at power supply line 125 during a write operation does not degrade the switch points of inverters 225 and 245. Even though PMOS transistor 230 on the side of the SRAM cell being pulled to logic low continues to operate in a linear mode (e.g., operating substantially as a resistor), the supply voltage at power supply line 135 is not reduced. As a result, a sufficient write current is provided to turn on NMOS transistor 240 to rapidly lower the voltage of node below the switch point of the inverter to properly regenerate a newly logic state in circuit 200.

0032As one would learn from the above description, writing the opposite logic state (i.e., to bring node 255 to logic low and node 265 to logic high) may be facilitated by allowing power supply line 135 to fall below reference voltage 120, while maintaining power supply line 125 at approximately reference voltage 120.

0033<figref idref="f0003">Figure 3</figref> illustrates circuit 300, which implements an elastic power header device (e.g., elastic power header device 180 of <figref idref="f0001">Figure 1</figref>), in accordance with an embodiment of the invention. Circuit 300 improves read margin. Circuit 300 includes resistive transistors 320A and 320B, which connect reference voltage 120 to power supply lines 125 (labeled "levdd") and 135 (labeled "rivdd"), respectively. Transistors 320A and 320B are shown in <figref idref="f0003">Figure 3</figref> as being implemented by PMOS transistors. Adjustment voltage 345 (labeled "vref"), provided at the gate electrodes of resistive transistors 320A and 302B, adjusts the effective resistance of transistors 320A and 320B. Because of this resistance, during a read operation, a node that should not be pulled up is less likely to be pulled up, thereby avoiding read disturbance. To improve read margin, adjustment voltage 345 (1) may be fixed at a predetermined voltage, (2) may be a dynamically adjusted voltage, or (3) may be controlled by a thermal-sensing self-calibration circuit to automatically adjust to a proper voltage.

0034When maintained in the linear region, resistive transistors 320A and 320B can each pull power supply lines 125 and 135 to reference voltage 120. Within this region, the adjustment voltage 345 flexibly fine-tunes main resistive transistors 320A and 320B to be as close to reference voltage 120 as desired, taking into consideration the current drawn by the switching SRAM cell, thereby achieving improved read margin and increased read stability. Adjustment voltage 345 may also be used effectively to improve write margin in a write operation, as illustrated by <figref idref="f0005">Figure 5</figref>.

0035<figref idref="f0004">Figure 4</figref> illustrates circuit 400, which may be also be used to implement elastic power header device 180 of <figref idref="f0001">Figure 1</figref>, in accordance with another embodiment of the invention. Circuit 400 improves write margin.

0036Circuit 400 receives write data signals 410A (labeled "blb") and 410B (labeled "bl"), and a write enable signal 425 (labeled "wyb"). Signals 410A, 410B and 425 selectively adjusts the voltages at power supply lines 125 and 135 through logic circuit 490, which includes NOR gates 430A and 430B. NOR gates 430A and 430B controls gate electrodes of transistors 470A and 470B.

0037Transistors 450A, 450B, 470A and 470B connect power supply lines 125 and 135, respectively, to reference voltage 120. Each of transistors 470A and 470B selectively provides reference voltage 120 to power supply lines 125 and 135, respectively, in response to signals 410A-B and 425.

0038In <figref idref="f0004">Figure 4</figref>, NOR gate 430A provides a logic low output value, except when write data signal 410A and write enable signal 425 are both at logic low. Similarly, NOR gate 430B provides a logic low output, except when write data signal 410B and write enable signal 425 are both at logic low. Thus, both transistors 470A and 470B remain turned on except during write operations, at which time one turns off to permit the voltage of one of power supply lines 125 or 135 to drop. Alternatively, other than in a write operation, write enable signal 425 is at logic high, so that transistors 470A and 470B are both conducting to maintain power supply lines 125 and 135 at reference voltage 120.

0039Circuit 400 also includes clamper transistors 450A and 450B, shown in <figref idref="f0004">Figure 4</figref> as being implemented by NMOS transistors. As shown in <figref idref="f0004">Figure 4</figref>, the gate and drain terminals of clamper transistors 450A and 450B connect to reference voltage 120, so that power supply lines 125 and 135 are maintained approximately at reference voltage 120 minus a threshold voltage. Accordingly, clamper transistors 450A and 450B each maintain a minimum voltage at the corresponding one of power supply lines 125 and 135, when transistors 470A or 470B is turned off in response to signals 410A, 410B and 425.

0040If write data signals 410A and 410B are implemented as complementary signals during a write operation (when write enable signal 425 is at logic low), only one of transistors 470A and 470B is conducting. For example, when write data signal 410A is at logic high (write data signal 410B is at logic low) value in a write operation, NOR gate 430A provides a logic low value to maintain transistor 470A conducting, while switching off transistor 470B. As a result, power supply line 125 remains at substantially reference voltage 120, while power supply line 135 falls to reference voltage 120 minus the threshold voltage of transistor 450B. As described above, the voltage difference in power supply lines 125 and 135 facilitates the write operation in the selected memory cell.

0041Circuit 400 also includes weak keeper transistor 460 having its gate electrode controlled by reference voltage 495, which may be the ground reference in this embodiment. As a result, weak keeper transistor 460 provides a weak current flowing between power supply lines 125 and 135 to limit the voltage difference between power supply lines 125 and 135, in response to leakage currents in SRAM circuit 200.

0042<figref idref="f0005">Figure 5</figref> illustrates circuit 500, which implements elastic power header device 180 of <figref idref="f0001">Figure 1</figref>, in accordance with another embodiment of the invention. Circuit 500 improves both read margin and write margin. As shown in <figref idref="f0005">Figure 5</figref>, circuit 500 includes, in addition to the elements of circuit 400 of <figref idref="f0004">Figure 4</figref>, additional resistive transistors 520A and 520B (shown implemented by PMOS transistors) are respectively provided across reference voltage 120 and a corresponding one of power supply lines 125 and 135. To simplify the discussion herein, like elements in circuits 400 and 500 are provided the same reference numerals.

0043When adjustment voltage 495 is provided such that resistive transistors 520A and 520B are turned off, circuit 500 behaves substantially the same as circuit 400. Such an adjustment voltage may be provided during a write operation so that, as explained above, circuit 500 may improve write margin in the same manner as circuit 400. However, if the adjustment voltage 545 is set so that resistive transistors 520A and 520B are turned on in the linear region, during a read operation, while at the same time signals 410A, 410B and 425 are kept at logic low, then circuit 500 operates similar to circuit 300, i.e., with the benefit of an improved read margin. Outside of the read and write operations, transistor 470A and 470B remain conducting, so that power supply lines 125 and 135 are at substantially reference voltage 120 to ensure that the stored value in the SRAM cells maintain a high immunity to noise. In this manner, circuit 500 provides both improved read margin and improved write margin.

0044<figref idref="f0006">Figure 6</figref> shows circuit 600, which provides a low-going pulse, which may be used to provide write enable signal 425 a logic low state of sufficient duration during a read or write operation in elastic power header circuit 180 of <figref idref="f0001">Figure 1</figref>, in accordance with an embodiment of the invention. Circuit 600 includes a delay element block 620 implemented by appropriate circuitry to delay the input signal, so that, for a brief time period, because of inverter 640, a high-going signal transition at input terminal 610 causes output terminal 660 of NAND gate 650 to go to logic low. At other times, inverter 640 ensures that output terminal 660 of NAND gate 650 is at logic high.

0045Embodiments incorporating various features disclosed herein may be implemented in embedded or standalone SRAM memory devices, caches, register files, multi-port memories, translation lookaside buffers (TLBS), content-addressable memories (CAMS), ternary CAMS (TCAMS), or other appropriate devices to operate at lower voltages as compared to traditional six transistor SRAM cells. Such features can be particularly advantageous for mobile, portable, or ultra-low voltage devices in which lower supply voltages may advantageously permit longer battery life and/or use time.

0046<figref idref="f0006">Figure 7</figref> illustrates an example of a content-addressable memory (CAM) cell that may be implemented using the SRAM cell circuits disclosed above. In <figref idref="f0006">Figure 7</figref>, CAM memory device 700 includes SRAM cell 710 and match comparator 760. SRAM cell 710 may be implemented, for example, by circuit 200 of <figref idref="f0002">Figure 2</figref>. As shown in <figref idref="f0006">Figure 7</figref>, SRAM cell 710 includes a read/write port 720 implemented, for example, by access transistors 250 and 260 of circuit 200. A logic state and its complement stored in SRAM cell 710 are provided to match comparator 760 (e.g., as bit lines 270 and 280of circuit 200). Match comparator 760 compares a data value received at match-data port 780 against the value stored in SRAM cell 710.to provide an output value for match signal 770.

0047In one embodiment of the invention, the elastic power header device includes transistors and an adjustable bias to mimic a programmable resistor. Adjustment can be continuously adjustable or step-wise adjustable. The adjustment bias can also be either an adjustable current or an adjustable voltage, even though it will be referred to as an adjustment voltage hereafter. This transistor and adjustment voltage combination is utilized in an SRAM cell for read operations. A transistor can be held to operate at an adjustable strength such as in linear or saturation region by an adjustment voltage. This in turn varies the resistance in the transistor and allows the voltage on a power supply line to approximate a reference voltage, for instance, V_dd. The resistance of the elastic power header device can also be made programmable. Properties of the elastic power header device may be programmed before processing, or set "on-the-fly." This implementation improves the read margin of the SRAM cell.

0048In another embodiment, the elastic power header device includes actual programmable resistors, instead of transistors, which have resistance values that may be varied in a SRAM cell during a read. The read margin of the SRAM cell is thereby improved.

0049In another embodiment, the elastic power header device includes transistors and a logic circuit used during a write operation. The transistors and the logic circuit act like a switch to affect the relative resistances in split-power lines. In this manner, the write operation is optimally performed, thereby improving the write margin of the SRAM cell.

0050In another embodiment of the present invention, the elastic power header device includes transistors, an adjustment voltage, and a logic circuit to be used during both a read operation and a write operation. In this manner, the elastic power header device improves both the read margin and the write margin of the SRAM cell.

0051According to another embodiment of the invention, the elastic power header device splits the power supply into two power supply lines using a programmable resistor to control the effective resistances in the split power lines. One advantage of such an elastic power header device is improved read margin achieved by improving read stability, reducing read disturbance and improving SNM. The elastic power header device also improves both the read and write margin of the SRAM cell simultaneously.

0052Additional embodiments of the invention provide elastic power header devices in other devices, such as flash memories, to the extent that this invention helps with problems seen in practice. These and other features and advantages of the invention will be more readily apparent from the detailed description of the embodiments set forth below taken in conjunction with the accompanying drawings.

0053The elastic power header device in accordance with the various embodiments described herein may be used with SRAM cells providing multiple bidirectional or unidirectional read or write ports. In addition, although a positive reference voltage 120 has been described herein, the techniques of the invention are equally applicable when applied to a negative reference voltage (e.g., ground). Advantageously, the embodiments of the invention, including the various circuits for implementing elastic power header device 180 described herein can provide reliable voltage operation ranges for the connected SRAM cells, despite possible variations in individual circuit components.

0054As known by one of ordinary skill in the art, this invention, including any logic circuit or transistor circuit, may be modeled, generated, or both by computer based on a description of the hardware expressed in the syntax and the semantics of a hardware description language (HDL). Such HDL descriptions are often stored on a computer readable medium. Applicable HDLs include those at the layout, circuit netlist, register transfer, and/or schematic capture levels. Examples of HDLs include, but are not limited to: GDS II and OASIS (layout level); various SPICE languages, and IBIS (circuit netlist level); Verilog and VHDL (register transfer level); and Virtuoso custom design language and Design Architecture-IC custom design language (schematic capture level). HDL descriptions may also be used for a variety of purposes, including but not limited to layout, behavior, logic and circuit design verification, modeling and/or simulation.

0055Aspects of the subject matter described herein are set out in the following numbered clauses: <ol id="ol0001" compact="compact"><li>1. A memory device comprising: <ul id="ul0001" list-style="none" compact="compact"><li>a first power supply line;</li><li>a second power supply line; a static random access memory (SRAM) cell comprising a first logic gate and a second logic gate that are cross-coupled, wherein the first power supply line supplies the first logic gate, and the second power supply line supplies the second logic gate; and</li><li>an elastic power header device connected between a reference voltage and each one of the first power supply line and the second power supply line wherein the elastic power header device is configured to provide, during a write operation of the first SRAM cell, different power supply levels on the first power supply line and the second power supply line and is configured to provide, during a read operation, a first resistance in the first power supply line and a second resistance in the second power supply line.</li></ul></li><li>2. The memory device of clause 1, wherein at least one of the first and second resistances comprise a programmable resistance.</li><li>3. The memory device of clause 1 wherein, during the write operation, the first power supply line has a different voltage level than the second power supply line.</li><li>4. The memory device of clause 1 wherein, during the write operation, the first power supply line has a different current flow than the second power supply line.</li><li>5. The memory device of clause 1 wherein, during the write operation, which of the first and the second power supply lines has a higher power supply level is determined by the value of the data bit being written.</li><li>6. The memory device of clause 1 wherein, during the read operation, the first resistance and the second resistance are substantially equal, whereby a difference in power supply levels between the first power supply line and the second power supply line depends on the current drawn from each power supply line.</li><li>7. The memory device of clause 1 wherein, during the write operation, the different power supply levels are provided by a variable resistance in a selected one of the power supply lines to facilitate writing into the first SRAM cell.</li><li>8. The memory device of clause 1, wherein the different power supply levels between the first power supply line and the second power supply line during the write operation corresponds to a threshold voltage drop across a transistor.</li><li>9. The memory device of clause 1, further comprising a keeper circuit for limiting the difference between the first power supply line and the second power supply line.</li><li>10. The memory device of clause 1, wherein the memory device is configured for use in at least one of: an embedded SRAM memory device, a standalone SRAM memory device, a cache, a register file, a multi-port memory, a translation lookaside buffer, a content-addressable memory, or a ternary content-addressable memory.</li><li>11. A memory device comprising: <ul id="ul0002" list-style="none" compact="compact"><li>a first power supply line;</li><li>a second power supply line;</li><li>a static random access memory (SRAM) cell comprising a first logic gate and a second logic gate that are cross-coupled, wherein the first power supply line supplies the first logic gate, and the second power supply line supplies the second logic gate; and</li><li>an elastic power header device connected between a reference voltage and each one of the first power supply line and the second power supply line wherein the elastic power header device is configured to provide, during a read operation, a first resistance in the first power supply line and a second resistance in the second power supply line.</li></ul></li><li>12. The memory device of clause 11, wherein at least one of the first and second resistances comprise a transistor.</li><li>13. A method for achieving improved read and write margins in a memory device comprising a first power supply line, a second power supply line, and a first logic gate and a second logic gate that are cross-coupled, the method comprising: <ul id="ul0003" list-style="none" compact="compact"><li>maintaining, during a read operation of the memory device, a resistance in each one of the first power supply line and the second power supply line to provide a power supply level greater than a predetermined value; and</li><li>varying, during a write operation of the memory device, a resistance in a selected one of the first power supply line and the second power supply line to provide a power supply level difference between the first power supply line and the second power supply line.</li></ul></li><li>14. The method of clause 13, wherein the resistance comprises a programmable resistance.</li><li>15. The method of clause 13, wherein the power supply level comprises a voltage.</li><li>16. The method of clause 13, wherein the power supply level comprises a current,</li><li>17. The method of clause 13, further comprising varying a conductance of a transistor connected between a reference voltage and the selected one of the first power supply line and the second power supply line.</li><li>18. The method of clause 13, further comprising equalizing the first power supply line and the second power supply line at a time when neither of the read or the write operations is occurring.</li><li>19. A method for achieving improved read margin in a memory device comprising a first power supply line, a second power supply line, and a first logic gate and a second logic gate that are cross-coupled, the method comprising: <ul id="ul0004" list-style="none" compact="compact"><li>maintaining, during a read operation of the memory device, a resistance in each one of the first power supply line and the second power supply line; and</li><li>providing a power supply level to a selected one of the first power supply line and the second power supply line greater than a predetermined value.</li></ul></li><li>20. The method of clause 19, further comprising controlling a device configured to achieve a resistance.</li><li>21. An elastic power header device for providing a first power supply line and a second power supply line in a split power fashion to a memory device, comprising: <ul id="ul0005" list-style="none" compact="compact"><li>a first conductive path, coupled between a reference voltage and both the first power supply line and the second power supply line, configured to provide a first power supply level difference between the reference voltage and each one of the first power supply line and the second power supply line;</li><li>a second conductive path, coupled between the reference voltage and the first power supply line;</li><li>a third conductive path, coupled between the reference voltage and the second power supply line, wherein, in response to a read operation, a resistance in a selected one of the second conductive path or the third conductive path is kept within a predetermined range of values during a write operation to provide a power supply level difference between the first power supply line and the second power supply line;</li><li>a fourth conductive path, coupled between the reference voltage and the first power supply line; and</li><li>a fifth conductive path, coupled between the reference voltage and the second power supply line wherein, in response to a write operation, the resistance in a selected one of the fourth and the fifth conductive paths is varied to provide a resistance.</li></ul></li><li>22. The elastic power header device of clause 21, wherein the resistance comprises a programmable resistance.</li><li>23. The elastic power header device of clause 21, wherein the first to fifth conductive paths each comprise a transistor.</li><li>24. The elastic power header device of clause 21, wherein the fourth and fifth conductive paths are attenuated during a write operation.</li><li>25. The elastic power header device of clause 21, wherein the second and the third conductive paths are attenuated during a read operation.</li><li>26. An elastic power header device for providing a first power supply line and a second power supply line in a split power fashion to a memory device, comprising: <ul id="ul0006" list-style="none" compact="compact"><li>a first conductive path coupled between the reference voltage and the first power supply line; and</li><li>a second conductive path coupled between the reference voltage and the second power supply line; wherein, in response to a read operation, the relative resistance between the first conductive path and the second conductive path is kept within a predetermined range of values.</li></ul></li><li>27. A computer-readable medium that includes a description of a circuit that implements an elastic power header device which includes a first power supply line and a second power supply line organized in a split power fashion in a memory device, where the circuit is adapted for use with a processor that executes operations that represent instructions, the instructions comprising: <ul id="ul0007" list-style="none" compact="compact"><li>configuring a first conductive path coupled between a reference voltage and both the first power supply line and the second power supply line, configured to provide a first voltage between the reference voltage and each one of the first power supply line and the second power supply line;</li><li>configuring a second conductive path, coupled between the reference voltage and the first power supply line;</li><li>configuring a third conductive path, coupled between the reference voltage and the second power supply line, wherein, in response to a read operation, a resistance in a selected one of the second conductive path or the third conductive path is kept within a predetermined range of values during a write operation to provide a power supply level difference between the first power supply line and the second power supply line;</li><li>configuring a fourth conductive path, coupled between the reference voltage and the first power supply line to provide a resistance in the first power supply line in response to a write operation; and</li><li>configuring a fifth conductive path, coupled between the reference voltage and the second power supply line to provide the resistance in the second power supply line in response to a write operation.</li></ul></li><li>28. The computer-readable medium as in clause 27, wherein the first to fifth conductive paths each comprise a device controlled to operate at an adjustable strength during a read operation.</li><li>29. A computer-readable medium that includes a description of a circuit that implements an elastic power header device which includes a first power supply line and a second power supply line organized in a split power fashion in a memory device, where the circuit is adapted for use with a processor that executes operations that represent instructions, the instructions comprising: <ul id="ul0008" list-style="none" compact="compact"><li>a first conductive path coupled between the reference voltage and the first power supply line; and</li><li>a second conductive path coupled between the reference voltage and the second power supply line;</li><li>wherein, in response to a read operation, the relative resistance between the first conductive path and the second conductive path is kept within a predetermined range of values.</li></ul></li><li>30. The computer-readable medium as in clause 29, wherein the first to fifth conductive paths each comprise a device controlled to operate at an adjustable strength during a read operation.</li><li>31. A computer readable medium that includes a description of a memory device, where the memory device comprises: <ul id="ul0009" list-style="none" compact="compact"><li>a first power supply line;</li><li>a second power supply line;</li><li>a static random access memory (SRAM) cell comprising a first logic gate and a second logic gate that are cross-coupled, wherein the first power supply line supplies the first logic gate, and the second power supply line supplies the second logic gate; and</li><li>an elastic power header device connected between the reference voltage and each one of the first power supply line and the second power supply line wherein the elastic power header device is configured to provide, during a write operation of the first SRAM cell, different power supply levels on the first power supply line and the second power supply line and is configured to provide, during a read operation, a first resistance in the first power supply line and a second resistance in the second power supply line.</li></ul></li><li>32. The computer-readable medium as in clause 31, wherein at least one of the first and second resistances comprise a programmable resistance.</li><li>33. A computer-readable medium that includes a description of a memory device, where the memory device comprises: <ul id="ul0010" list-style="none" compact="compact"><li>a first power supply line;</li><li>a second power supply line;</li><li>a static random access memory (SRAM) cell comprising a first logic gate and a second logic gate that are cross-coupled, wherein the first power supply line supplies the first logic gate, and the second power supply line supplies the second logic gate; and</li><li>an elastic power header device connected between the reference voltage and each one of the first power supply line and the second power supply line wherein the elastic power header device is configured to provide, during a read operation, a first resistance in the first power supply line and a second resistance in the second power supply line.</li></ul></li><li>34. The computer-readable medium as in clause 33, wherein at least one of the first and second resistances comprise a programmable resistance.</li><li>35. A memory device comprising: <ul id="ul0011" list-style="none" compact="compact"><li>means for providing a reference voltage;</li><li>means for configuring a first power supply line and a second power supply line, and means for connecting a static random access memory (SRAM) cell comprising a first logic gate and a second logic gate that are cross-coupled, wherein the first power supply line supplies the first logic gate, and the second power supply line supplies the second logic gates; and</li><li>means for providing elastic power connected between the reference voltage and each one of the first power supply line and the second power supply line providing, during a write operation of the first SRAM cell, different power supply levels on the first power supply line and the second power supply line and, during a read operation, providing a first resistance in the first power supply line and a second resistance in the second power supply line.</li></ul></li><li>36. A memory device comprising: <ul id="ul0012" list-style="none" compact="compact"><li>means for providing a reference voltage;</li><li>means for configuring a first power supply line and a second power supply line, and means for connecting a static random access memory (SRAM) cell comprising a first logic gate and a second logic gate that are cross-coupled, wherein the first power supply line supplies the first logic gate, and the second power supply line supplies the second logic gate; and</li><li>means for providing elastic power connected between the reference voltage and each one of the first power supply line and the second power supply line providing, during a read operation, a first resistance in the first power supply line and a second resistance in the second power supply line.</li></ul></li></ol>

0056The foregoing disclosure is not intended to limit the invention to the precise forms or particular fields of use disclosed. Various alternate embodiments, variations or modifications to the invention are possible within the scope of the invention in light of the disclosure. The invention is set forth in the following claims.