(12) United States Patent
`Rimondi
`
`USOO6212094B1
`(10) Patent No.:
`US 6,212,094 B1
`(45) Date of Patent:
`Apr. 3, 2001
`
`(54) LOW POWER SRAM MEMORY CELL
`HAVING ASINGLE BIT LINE
`
`(75) Inventor: Danilo Rimondi, Mozzo (IT)
`(73) Assignee: STMicroelectronics S.r.l., Agrate
`Brianza (IT)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(*) Notice:
`
`(21) Appl. No.: 09/200,075
`(22) Filed:
`Nov. 25, 1998
`(30)
`Foreign Application Priority Data
`Nov. 28, 1997
`(EP) ................................................ 97 120944
`(51) Int. Cl." ..................................................... G11C 11/00
`(52) U.S. Cl. ..................
`... 365/156; 365/154
`(58) Field of Search ..................................... 365/156, 154,
`365/203, 69
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`1/1994 Anami .................................. 365/227
`5,276,652
`10/1994 Uratani et al.
`365/189.05
`5,353,251
`5,850,364 * 12/1998 Ueno .............
`... 365/203
`5,966,319 * 10/1999 Sato ..............
`... 36.5/154
`5,986,923 * 11/1999 Zhang et al...
`... 36.5/154
`6,005,795 * 12/1999 Hawkins et al...
`... 36.5/156
`6,011,711
`1/2000 Hodges et al. ....................... 365/154
`FOREIGN PATENT DOCUMENTS
`4128919
`3/1992 (DE).
`
`
`
`OTHER PUBLICATIONS
`Patent Abstract of Japanese publication No. 61026997,
`published Feb. 6, 1986.
`European Search Report dated May 13, 1998 with annex on
`European Application No. 97 120944.
`* cited by examiner
`Primary Examiner Richard Elms
`Assistant Examiner Anh Phung
`(74) Attorney, Agent, or Firm Theodore E. Galanthay;
`Stephen C. Bongini; Fleit, Kain, Gibbons, Gutman &
`Bongini P.L.
`ABSTRACT
`(57)
`A Semiconductor memory cell having a word line, a bit line,
`a precharge line, an acceSS transistor, and first and Second
`cross-coupled inverters. The first inverter includes a first
`P-channel transistor and a first N-channel transistor, and the
`Second inverter includes a Second P-channel transistor and a
`Second N-channel transistor. The access transistor Selec
`tively couples the bit line to an output of the first or second
`inverter, and one terminal of the first N-channel transistor is
`connected to the precharge line. In a preferred embodiment,
`a control circuit is provided that, during a writing operation,
`Supplies data to be written to the memory cell to the bit line,
`Supplies a pulse Signal to the precharge line, and activates
`the word line. A method of writing data to a Semiconductor
`memory cell that is coupled to a word line and Single bit line
`is also provided. According to the method, the level of the
`bit line is Set in accordance with data to be written, the
`memory cell is precharged So as to force the output of one
`of the inverters of the memory cell to a predetermined logic
`level, and the word line is activated to couple the bit line to
`the memory cell.
`19 Claims, 3 Drawing Sheets
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`U.S. Patent
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`Apr. 3, 2001
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`Sheet 1 of 3
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`US 6,212,094 B1
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`FIG. 1
`PRIOR ART
`
`FIG. 2
`PRIOR ART
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`U.S. Patent
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`Apr. 3, 2001
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`Sheet 2 of 3
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`US 6,212,094 B1
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`FIG. 3
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`
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`bI2
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`bl1
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`U.S. Patent
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`Apr. 3, 2001
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`Sheet 3 of 3
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`US 6,212,094 B1
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`bI2
`blin
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`X
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`n2 1
`Write 0 Cell
`nil 1 X
`n2 2
`write 1 cell 2
`n 2 X
`n1n
`write Oceln
`X
`n2 n
`initial state: Cell n1 l=0, Cel2nl2=0, Cellnnl n=1
`
`FIG. 5
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`US 6,212,094 B1
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`1
`LOW POWER SRAM MEMORY CELL
`HAVING ASINGLE BIT LINE
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`This application is based upon and claims priority from
`prior European Patent Application No. 97-120944.0, filed
`Nov. 28, 1997, the entire disclosure of which is herein
`incorporated by reference.
`
`BACKGROUND OF THE INVENTION
`
`2
`fast wide memories” (Dig. Tech. Papers, October 1994,
`Symp. on Low Power Electronics, pages 92–93). This
`technique reduces power consumption by limiting the bit
`line Voltage Swing during a read by controlling the word line
`pulse length. Yet another power reduction technique is
`disclosed by T. Blalock and R. Jager in “A high-speed
`clamped bit line current-mode sense amplifier” (IEEE J.
`Solid State Circuits, Vol. 26, No. 4, April 1991, pages
`542–548). This solution also reduces power consumption by
`limiting the bit line Voltage Swing during a read, but does So
`using current-mode Sense amplifiers So as to reduce AV.
`Still another power reduction technique limits the bit line
`voltage Swing during a write to a predetermined value (i.e.,
`Vdd-Vt) using NMOS transistors during precharging.
`Further efforts at reducing power consumption have
`focused on reducing the current consumption by coupling
`each memory cell to a Single bit line instead of the conven
`tional bit line pair. In such devices, the lower bit line
`capacitance presented by the Single bit line cells decreases
`current consumption. For example, in “A Single-bit-line
`cross-point cell activation (SCPA) architecture for ultra-low
`power SRAMs” (IEEE J. Solid State Circuits, Vol. 28, No,
`11, November 1992, pages 1114-1118) M. Ukita et al.
`disclose a single bit line architecture that includes five
`transistor SRAM cells with a single bit line, as shown in
`FIG. 2. Moreover, in “A Source Sensing technique applied to
`SRAM cells” (IEEE J. Solid State Circuits, Vol. 30, No. 4,
`April 1995, pages 500-511), K. J. O’Connor addresses the
`problem of writing to Such a Single acceSS SRAM cell.
`With such single access (i.e., single bit line) SRAM
`memory cells, whether or not Switching occurs during a read
`operation is dependent upon the data Stored in the memory
`cell being read. Because Switching only occurs when one of
`the two possible logic values is Stored in the memory cell,
`power consumption (over time) is reduced by one half.
`Similarly, during a writing operation, the bit lines are only
`discharged when one of the two possible logic values (e.g.,
`“0”) is to be written. Thus, the power consumption during
`writing (over time) is also reduced by half. Although con
`ventional single access SRAM memory cells offer such
`Significant reductions in power consumption, a Serious
`drawback is presented in that it is difficult to write the other
`non-discharging logic value (e.g., “1”) to the memory cells.
`More specifically, when writing a high (1) logic level to
`a memory cell that is storing a low (O) logic level, the node
`N1 is low and a high level must be written into the memory
`cell. When the bit line BL is set high and the word line WL
`is activated, the transistors M5 and M1 fight one another. In
`order to make the memory cell Stable during Such an
`operation, the O'Connor reference teaches dimensioning the
`transistor M1 so that it is larger than the transistor M5.
`However, writing remains quite difficult and the proposed
`transistor dimension Solution requires complex techniques
`that increase the design complexity of the memory device.
`SUMMARY OF THE INVENTION
`In View of these drawbacks, it is an object of the present
`invention to remove the above-mentioned drawbacks and to
`provide an SRAM memory device with reduced power
`consumption. An SRAM memory device is formed with
`memory cells that each have cross-coupled inverters coupled
`to a single bit line. During operation, one terminal of a
`pull-down transistor of a memory cell is precharged. Thus,
`the memory cell is precharged to a logic State that can be
`easily changed during writing.
`In other words, the memory cell is “reset' before a writing
`operation. In one illustrative embodiment in which five
`
`25
`
`1. Field of the Invention
`The present invention relates to Semiconductor memory
`devices, and more specifically to a low power SRAM
`15
`memory cell with cross-coupled CMOS inverters coupled to
`a single bit line.
`2. Description of the Related Art
`A conventional Semiconductor Static random access
`memory (SRAM) device is formed with static memory cells
`that each have six transistors. FIG. 1 shows a conventional
`CMOS six transistor SRAM memory cell. The memory cell
`1 includes a pair of cross-coupled CMOS inverters 2 and 3,
`each of which is coupled to a bit line 4 and 5. In particular,
`the first inverter 2 is coupled to a first bit line 4 through a
`bi-directional access device 6, and the Second inverter 3 is
`coupled to an adjacent Second bit line 5 through a Second
`access device 7. During reading and writing operations,
`different voltages must be applied to the two bit lines 4 and
`5. Thus, this type of access to the Storage node of the
`memory cell can be termed “differential.”
`More specifically, during reading from the memory cell of
`FIG. 1, the bit line Voltage Swing amplitude is dependent
`upon the length of time the memory cell has been activated.
`The Voltage difference caused by the Swing can be kept quite
`Small and Sensed by the Sense amplifier of the memory
`device in order to reduce power consumption. Further,
`during writing to the memory cell, the bit line Voltage Swing
`is made as large as possible (e.g., the full CMOS logic
`voltage level) in order to toggle (i.e., write to) the memory
`cell. Thus, in an SRAM six transistor memory cell array with
`m rows and n columns, the current consumption during
`reading and writing can be estimated using the following
`formulas:
`
`35
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`40
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`45
`
`where n is the number of bits in the word being read or
`written, Cb is the bit line capacitance associated with a given
`cell, AV, is the bit line Voltage Swing during a read
`operation, and AV is the bit line Voltage Swing during a
`write operation. Typically, AV corresponds to the Supply
`voltage level Vdd.
`Previous efforts to reduce the power consumed by Such a
`memory matrix focus on changing one or more of the
`parameters in the above formulas. One Such technique is
`disclosed by N. Kushiyama et al. in “A295 MHz CMOS 1M
`(x256) embedded SRAM using I-directional read/write
`shared Sense amplifiers and Self-timed pulsed word-line
`drivers” (ISSCC Dig. Tech. Papers, February 1995, pages
`182-183). According to this technique, power consumption
`is reduced by reducing the number of cells on the bit line
`through a hierarchical bit line Scheme.
`Another power reduction technique is disclosed by B.
`Amrutur and H. Horowitz in “Technique to reduce power in
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`US 6,212,094 B1
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`3
`transistor SRAM memory cells are used, the precharging
`logic State is logic “1” and the “reset' operation is performed
`by applying a pulse signal to the Source terminal of one of
`the driver transistors.
`Another object of the present invention is to provide an
`SRAM memory device that reduces the probability of bit
`line Switching by coupling a Single bit line to each memory
`cell.
`A further object of the present invention is to provide an
`SRAM memory device having a simple write operation.
`Yet another object of the present invention is to provide an
`SRAM memory device that has relatively long word length
`but reduced power consumption.
`One embodiment of the present invention provides a
`Semiconductor memory cell having a word line, a bit line, a
`precharge line, an access transistor, and first and Second
`cross-coupled inverters. The first inverter includes a first
`P-channel transistor and a first N-channel transistor, and the
`Second inverter includes a Second P-channel transistor and a
`Second N-channel transistor. The access transistor Selec
`tively couples the bit line to an output of the first or second
`inverter, and one terminal of the first N-channel transistor is
`connected to the precharge line. In a preferred embodiment,
`a control circuit is provided that, during a writing operation,
`Supplies data to be written to the memory cell to the bit line,
`Supplies a pulse Signal to the precharge line, and activates
`the word line.
`Another embodiment of the present invention provides a
`method of writing data to a Semiconductor memory cell that
`is coupled to a word line and Single bit line. According to the
`method, the level of the bit line is set in accordance with data
`to be written, the memory cell is precharged So as to force
`the output of one of the inverters of the memory cell to a
`predetermined logic level, and the word line is activated to
`couple the bit line to the memory cell. In one preferred
`method, the predetermined logic level is a logic level that
`can be easily changed during writing.
`Other objects, features, and advantages of the present
`invention will become apparent from the following detailed
`description. It should be understood, however, that the
`detailed description and Specific examples, while indicating
`preferred embodiments of the present invention, are given
`by way of illustration only and various modifications may
`naturally be performed without deviating from the present
`invention.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a schematic diagram of a conventional double bit
`line CMOS SRAM memory cell;
`FIG. 2 is a Schematic diagram of a conventional Single bit
`line CMOS SRAM memory cell;
`FIG. 3 is a Schematic diagram of a memory cell according
`to a preferred embodiment of the present invention;
`FIG. 4 is a Schematic diagram of a portion of a memory
`array that includes the memory cells of FIG. 3; and
`FIG. 5 is a timing diagram for a write operation in the
`memory cell of FIG. 3.
`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENTS
`Preferred embodiments of the present invention will be
`described in detail hereinbelow with reference to the
`attached drawings.
`FIG. 3 shows a static random access memory (SRAM)
`cell according to a preferred embodiment of the present
`
`4
`invention. The memory cell 10, which is formed using
`CMOS technology, is designed for a semiconductor SRAM
`memory device of the type in which each memory cell is
`coupled to a single bit line BL. In particular, the memory cell
`10 of FIG. 3 is integrated into a memory array or matrix that
`includes m rows and n columns, as partially shown in FIG.
`4. Each row is identified with a word line WL and each
`column is identified with a bit line BL of the memory device.
`One of the memory cells 10 is located at each intersection of
`a word line WL and a single bit line BL.
`As shown in FIG. 3, the memory cell 10 includes two
`CMOS inverters 12 and 13 that each have a pull-up PMOS
`transistor and a pull-down NMOS transistor. The two invert
`erS 12 and 13 are cross-coupled (i.e., for each inverter, the
`interconnection node for the two transistors of the inverter is
`connected to the gate terminals of the two transistors of the
`other inverter). More specifically, the first inverter 12 has a
`PMOS transistor M3 and an NMOS transistor M1 connected
`in series, and the Second inverter 13 has a PMOS transistor
`M4 and an NMOS transistor M2 connected in series. The
`gate terminals of the transistors M1 and M3 of the first
`inverter 12 are connected to a first node N1, which is the
`Series connection point of the drain terminals of the Second
`inverter 13. Likewise, the gate terminals of the transistors
`M2 and M4 of the second inverter 13 are connected to a
`Second node N2, which is the Series connection point of the
`drain terminals of the first inverter 12.
`The second node N2 is coupled to a bit line BL through
`an access transistor (e.g., PMOS transistor) M5, and the gate
`terminal of the acceSS transistor is connected to the word line
`WL. The Source terminals of the two PMOS transistors M3
`and M4 are connected to a voltage supply line Vdd. Further,
`the Source terminal of the NMOS transistor M2 of the
`second inverter 13 is connected to ground GND, and the
`Source terminal of the NMOS transistor M1 of the first
`inverter 12 is connected to a precharge line PL. The pre
`charge line PL is an added line that is common to all the
`memory cells 10 in a row. FIG. 4 shows a portion of the
`Structure of a memory matrix that includes Such memory
`cells 10.
`During operation of the SRAM memory device of FIG. 4,
`when a memory cell is not accessed or read from, the
`corresponding precharge line PL is coupled to ground. This
`provides the ground potential to the Source terminal of the
`NMOS transistor M1 So that both data retention and read
`operations are performed in the same manner as in a
`conventional SRAM memory cell. On the other hand, a
`memory cell is written to using a three Step process.
`First, the data to be written to the memory cell is placed
`on the corresponding bit line (i.e., the levels of the bit lines
`are set according to the memory word to be written). Next,
`a pulse is Supplied to the corresponding precharge line PLSO
`as to “reset' the first node N1 of each memory cell con
`nected to the precharge line to the high (1) logic level. More
`specifically, if the first node N1 of a selected memory cell
`had been at a low (O) logic level, that node is raised to the
`voltage level Vdd-Vtn by the NMOS transistor M1, and is
`then raised all the way to the Supply voltage level Vdd by the
`PMOS transistor M3. Then, the word line WL is activated
`and connected memory cells having a low level bit line are
`“flipped” to the low logic level. The high level logic states
`of the remaining memory cells (i.e., those having a high
`level bit line) are not changed. Thus, a high (1) logic level
`can be written to a memory cell that is storing a low (O) logic
`level in a Simple and efficient manner.
`FIG. 5 is a timing diagrams for Such a write operation in
`the SRAM memory device of FIG. 4. The illustrated write
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`S
`operation relates to the writing of the first, second, and n”
`memory cells of a memory word.
`In another embodiment of the present invention, a Six
`transistor single bit line SRAM memory cell is provided.
`The sixth transistor M6 is connected between the first node
`N1 and ground GND, and the gate terminal of the sixth
`transistor M6 is connected to the precharge line PL. In this
`embodiment, the Sixth transistor operates as an additional
`access transistor for resetting the first node N1 to the high (1)
`logic level. This provides the memory cell with greater noise
`immunity because the “cell ratio” is not altered by the
`presence of the sixth transistor. Where W and W are the
`widths and L and L are the lengths of the transistorS M1
`and M6, the cell ratio (r) is defined by the following formula.
`
`AS previously explained, the present invention provides
`an SRAM memory cell formed by cross-coupled inverters
`coupled to a single bit line. During a write operation, the
`Source terminal of one of the pull-down transistors of the
`memory cell is precharged in order to reset the memory cell
`to a predetermined logic State that can be easily changed
`during writing. Thus, the SRAM memory cell of the present
`invention allows power consumption to be reduced without
`introducing Significant complexities into the writing process.
`The present invention is particularly useful for memory
`devices with very long word lengths.
`While there has been illustrated and described what are
`presently considered to be the preferred embodiments of the
`present invention, it will be understood by those skilled in
`the art that various other modifications may be made, and
`equivalents may be Substituted, without departing from the
`true Scope of the invention. Additionally, many modifica
`tions may be made to adapt a particular situation to the
`teachings of the present invention without departing from
`the central inventive concept described herein. Furthermore,
`embodiments of the present invention may not include all of
`the features described above. Therefore, it is intended that
`the present invention not be limited to the particular embodi
`ments disclosed, but that the invention include all embodi
`ments falling within the Scope of the appended claims.
`What is claimed is:
`1. A Semiconductor memory cell comprising:
`a word line;
`a bit line;
`a precharge line;
`first and Second cross-coupled inverters, the first inverter
`including a first P-channel transistor and a first
`N-channel transistor, and the Second inverter including
`a Second P-channel transistor and a Second N-channel
`transistor, and
`an access transistor that Selectively couples the bit line to
`an output of one of the first inverter and the Second
`inverter,
`wherein one terminal of the first N-channel transistor is
`connected to the precharge line.
`2. The Semiconductor memory cell as defined in claim 1,
`wherein the acceSS transistor Selectively couples the bit
`line to the output of the first inverter, and
`the Source terminal of the first N-channel transistor of the
`first inverter is connected to the precharge line.
`3. The Semiconductor memory cell as defined in claim 1,
`wherein the gate terminal of the access transistor is con
`nected to the word line.
`4. The Semiconductor memory cell as defined in claim 1,
`further comprising a control circuit that Supplies a pulse
`Signal to the precharge line during a writing operation for the
`memory cell.
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`5. The semiconductor memory cell as defined in claim 1,
`further comprising a control circuit that, during a writing
`operation, Supplies data to be written to the memory cell to
`the bit line, Supplies a pulse signal to the precharge line, and
`activates the word line.
`6. The Semiconductor memory cell as defined in claim 1,
`wherein the memory cell is a static RAM memory cell.
`7. A Semiconductor memory cell comprising:
`a word line;
`a bit line;
`a precharge line;
`first and Second cross-coupled inverters, the first inverter
`including a first P-channel transistor and a first
`N-channel transistor, and the Second inverter including
`a Second P-channel transistor and a Second N-channel
`transistor;
`an access transistor that Selectively couples the bit line to
`an output of the first inverter; and
`a precharge transistor connected between the output of the
`first inverter and a predetermined Voltage,
`wherein one terminal of the first N-channel transistor and
`the gate terminal of the precharge transistor are con
`nected to the precharge line.
`8. The semiconductor memory cell as defined in claim 7,
`wherein the Source terminal of the first N-channel transistor
`of the first inverter is connected to the precharge line.
`9. The semiconductor memory cell as defined in claim 7,
`wherein the gate terminal of the access transistor is con
`nected to the word line.
`10. The semiconductor memory cell as defined in claim 7,
`further comprising a control circuit that Supplies a pulse
`Signal to the precharge line during a writing operation for the
`memory cell.
`11. The semiconductor memory cell as defined in claim 7,
`further comprising a control circuit that, during a writing
`operation, Supplies data to be written to the memory cell to
`the bit line, Supplies a pulse signal to the precharge line, and
`activates the word line.
`12. A Semiconductor memory device comprising:
`a plurality of word lines,
`a plurality of bit lines;
`a plurality of precharge lines, and
`a plurality of memory cells coupled to the word lines, bit
`lines, and precharge lines, each of the memory cells
`including:
`first and Second cross-coupled inverters, the first
`inverter including a first P-channel transistor and a
`first N-channel transistor, and the Second inverter
`including a Second P-channel transistor and a Second
`N-channel transistor, one terminal of the first
`N-channel transistor being connected to one of the
`precharge lines, and
`an acceSS transistor that Selectively couples an output of
`one of the first inverter and the second inverter to one
`of the bit lines.
`13. The semiconductor memory device as defined in claim
`12, wherein for each of the memory cells:
`the access transistor Selectively couples the one bit line to
`the output of the first inverter, and
`the Source terminal of the first N-channel transistor of the
`first inverter is connected to the one precharge line.
`14. The Semiconductor memory device as defined in claim
`12, wherein for each of the memory cells, the gate terminal
`of the access transistor is connected to one of the word lines.
`15. The semiconductor memory device as defined in claim
`12, further comprising a control circuit that Supplies a pulse
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`Signal to at least one of the precharge lines during a writing
`operation for the memory device.
`16. The semiconductor memory device as defined in claim
`12, further comprising a control circuit that, during a writing
`operation, Supplies data to be written to one of the memory 5
`cells to one of the bit lines, Supplies a pulse signal to at least
`one of the precharge lines, and activates at least one of the
`word lines.
`17. The semiconductor memory device as defined in claim
`12, wherein the precharge lines run parallel to the word 10
`lines.
`
`8
`18. The semiconductor memory device as defined in claim
`12, wherein each of the precharge lines is commonly
`coupled to all of the memory cells that are coupled to one of
`the word lines.
`19. The semiconductor memory device as defined in claim
`12, wherein each of the memory cells further includes a
`precharge transistor connected between the output of the
`first inverter and a predetermined Voltage, the gate terminal
`of the precharge transistor being connected to one of the
`precharge lines.
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