Abstract:
Reduced manufacturing costs and wafer size, lower power consumption, and increased operating speed are achieved in memory circuits by providing a novel sense amplifier design that is most sensitive to voltages variations around the source voltage (V dd ). The sense amplifier includes two inverters that are regeneratively cross-coupled through a circuit that is controlled by a system clock. The inverters are powered from the bit lines that couple the sense amplifier to a memory cell. Novel applications of the sense amplifier in memory circuits also are described.

Description:
TECHNICAL FIELD 
     This invention relates to memory circuits and, more particularly, sense amplifier circuits for use within memory circuits. 
     BACKGROUND OF THE INVENTION 
     Sense amplifiers are used to detect differences between two voltages. Sense amplifiers typically are used, for example, to read the contents of memory cells in memory arrays. A memory cell whose contents are to be read are coupled to a sense amplifier via two bit lines. The sense amplifier detects the relative voltages on the two bit lines and, in response to those voltages, provides an output signal representative of a data value stored within the memory cell. 
     Conventional sense amplifiers are most sensitive to voltages around half of the source voltage (e.g., V dd  /2). However, this voltage typically is not available in the circuit and thus must be generated by additional circuitry. Conventional sense amplifiers include one or more additional stages, following the sensing stage, to restore output voltages to CMOS logic levels. These additional stages increase manufacturing costs, reduce operating speed, and require additional wafer space. Furthermore, conventional sense amplifiers draw current from supply lines (e.g., V dd ), and thus consume power even when the sense amplifier is not reading data from a memory cell. 
     SUMMARY OF THE INVENTION 
     Reduced manufacturing costs and wafer size, lower power consumption, and increased operating speed are achieved in memory circuits by providing a novel sense amplifier design that is most sensitive to voltages variations around the source voltage (V dd ). The sense amplifier includes two inverters that are regeneratively cross-coupled through a circuit that is controlled by a system clock. The inverters are powered from the bit lines that couple the sense amplifier to a memory cell. 
     Operating the sense amplifier at V dd  provides many advantages over conventional amplifiers. For example, because V dd  is readily available in the circuit, precharging of the bit lines is simplified. Moreover, the sense amplifier of the present invention outputs a true CMOS voltage (zero or V dd ) and thus does not require circuits found in conventional sense amplifiers for restoring output voltages to CMOS logic levels. Because the sense amplifier discharges the bit lines only slightly during a read operation, the bit lines remain at a voltage near V dd . This facilitates fast precharging of the bit lines alter the sensing operation is completed. Moreover, because the sense amplifier operates responsive to a system clock signal, the sense amplifier draws negligible current once the contents of a memory cell have been read. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a simplified block diagram of an array of memory cells and a control circuit constructed in accordance with the principles of the present invention; 
     FIG. 2 is a partial schematic diagram of the control circuit of FIG. 1; 
     FIG. 3 is a simplified schematic diagram of an illustrative embodiment of the sense amplifier of FIG. 2; 
     FIG. 4 is a schematic diagram of the write circuit of FIG. 2; and 
     FIG. 5 is a timing diagram of the memory cycle of the memory circuit of FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows an array of memory cells arranged in a conventional manner. The array of memory cells is arranged in a matrix having n horizontal rows and n vertical columns. The memory cells 10 in a given column are coupled by a word select line. During each memory cycle, a single word select line is energized to specify the group of memory cells active during that memory cycle. The memory cells within each row are coupled to each other by two bit lines, BIT and BIT. 
     Each memory cell 10 has two nodes. One of the two nodes stores an actual data value and is coupled to the bit line &#34;BIT.&#34; The other of the two nodes stores the complement of the data value stored by the first node. This second node is coupled to the bit line &#34;BIT.&#34; 
     Each row of memory cells is coupled via its respective bit lines to a control circuit 12. Control circuit 12 reads data from and writes data to a selected memory cell, as specified by the word select line (e.g., lines 24 and 26), within the row. Control circuit 12 outputs data read from the memory cells to a read bus 14 and receives data to be written to the memory cells from a write bus 16. A &#34;select&#34; control line 18 is provided to control circuit 12 to enable read and write operations to be performed on groups or subsets of the memory cells that are coupled to the energized word select line. 
     Power for control circuit 12 is provided from V dd . Control circuit 12, in turn, powers the memory cells via the bit lines. The memory array operates synchronously, in response to system clock signals input to control circuit 12 via line 22. 
     FIG. 2 shows control circuit 12 in greater detail. Control circuit 12 includes a precharge circuit 28, a sense amplifier 30, a latch generator circuit 32, a read latch circuit 34, and a write circuit 36, each of which is described in greater detail below. Generally, however, control circuit 12 operates as follows. Precharge circuit 28 precharges the bit lines to V dd  in the first half of the memory cycle. In the second half of the memory cycle, sense amplifier 30 detects changes in the voltages on the bit lines, so as to read the data value from the memory cell. When the read is complete, latch generator circuit 32 generates a latch signal and outputs the latch signal simultaneously to read latch circuit 34 to latch the data and to write circuit 36 to initiate a write operation. 
     More particularly, in accordance with the invention, sense amplifier 30 is coupled to the bit lines to detect changes in the voltages on those bit lines. In response to changes in the bit line voltages, sense amplifier 30 determines which of the bit lines is coupled to a logic one and which is coupled to a logic zero. Upon reaching a decision, sense amplifier 30 outputs the data simultaneously to latch generator 32 and read latch 34. Latch generator 32 generates a latch signal in response to the output signal from amplifier 30 upon receipt of a system clock signal. The latch signal from latch generator 32 is provided simultaneously to read latch 34 and to write circuit 36. Read latch 34 latches the signal from sense amplifier 30 upon receiving a latch signal from latch generator 32. Read latch 34 outputs the latched data to read bus 14. Write circuit 36 receives incoming data to be written to a memory cell from write bus 16. Write circuit 36 initiates the write operation upon receiving the latch signal from latch generator 32 if the write enable line is energized. As will be described below, it is the provision of a common latch signal to read latch 34 and write circuit 36 that enables control circuit 12 to perform both a read and write operation on a memory cell within a single clock cycle. 
     Precharge circuit 28 includes enhancement-type P-channel transistors 38, 40, and 42. The gate terminal of transistors 38, 40, and 42 are connected to the clock signal. The drain of transistors 38 and 42 are connected to V dd . The source of transistors 38 and 42 are coupled together through transistor 40. The source terminal of transistor 38 is coupled to one bit line (e.g. BIT) and the source terminal of transistor 42 is coupled to the other bit line (e.g. BIT). Of course, it will be apparent to one skilled in the art that precharge circuit 28 could be any circuit which quickly charges the bit lines to V dd  upon receipt of a clock signal. 
     As described above, sense amplifier 30 detects relative changes in the voltages on the bit lines to determine which of the bit lines is connected to a logic one and which is connected to a logic zero, so as to read data from a memory cell 10. Sense amplifier 30, shown in simplified form in FIG. 3, includes two inverters 45 and 47 that are regeneratively cross-coupled through a balance circuit 52 (shown in greater detail in FIG. 2). As discussed further below, when balance circuit 52 is closed, inverters 45 and 47 remain in an metastable state. Once balance circuit 52 opens, the inverters drive to a stable state in a direction that is determined by the relative voltages on the bit lines. The direction of current flow between the inverters, as determined by the relative voltages on the bit lines, establishes the value of the data to be latched by read latch 34. 
     Inverters 45 and 47 are formed by complementary-symmetry MOS (CMOS) transistors 44 and 46, and transistors 48 and 50, respectively. The gates of transistors 44 and 46 are regeneratively coupled to the gates of transistors 48 and 50 through balance circuit 52, comprising transistors 54 and 56. The gate terminals of transistors 54 and 56 are controlled by the system clock signal. Balance circuit 52 appears to inverters 45 and 47 as a short circuit when the clock signal is low. Conversely, balance circuit 52 appears to inverters 45 and 47 as a open circuit when the clock signal is high. 
     Inverters 45 and 47 draw power for circuit operation from the bit lines. This provides an advantage over conventional sense amplifiers, which drew current from supply lines (e.g., V dd ), because the sense amplifier of the present invention draws negligible current once it has read the contents of a memory cell. Moreover, because the sense amplifier draws power from the bit lines, the sense amplifier is not susceptible to power supply noise as in conventional circuits. 
     The embodiment of sense amplifier 30 described above offers several other advantages over conventional sense amplifier designs. For example, sense amplifier 30 discharges the bit lines only slightly during a read operation, leaving the bit lines at a voltage near V dd . This facilitates fast precharging of the bit lines after the sensing operation is completed. Also, sense amplifier 30 is most sensitive around V dd  /2. Thus simplifies precharging of the bit lines, because V dd  is readily available. In contrast, conventional sense amplifier circuits are most sensitive to voltages around V dd  /2. Thus, conventional sense amplifiers required additional circuitry, and thus a more complex circuit design, to generate V dd  /2. Furthermore, the sense amplifier of the present invention is comprised of only a single stage, the output of which is a true CMOS voltage. Conventional sense amplifier designs require one or more additional stages, following the sensing stage, to restore output voltages to true CMOS voltages. These additional stages increase manufacturing costs, reduce operating speed, and require additional wafer space. 
     Sense amplifier 30 operates as follows. The amplifier is first selected by applying an appropriate voltage to select line 18 to establish a current path between inverters 45 and 47 and V ss  through transistor 58. The voltage on the bit lines causes cross-coupled inverters 45 and 47 to remain in an metastable state as long as the cross-connect formed by balance circuit 52 remains in a short circuit state. Upon receipt of a clock signal, balance circuit 52 appears to inverters 45 and 47 as an open circuit, causing the inverters to drive to a stable state in response to the difference in voltage between the bit lines. When the voltage on line BIT is higher than the voltage on line BIT, inverters 45 and 47 will drive the output of inverter 45 high, causing a logic one to be output to the &#34;Data In&#34; terminal of read latch 34. However, when the voltage on BIT is higher than the voltage on BIT, inverters 45 and 47 will drive the output of inverter 45 low, causing a logic zero to be output to the Data In terminal of read latch 34. 
     Latch generator circuit 32 includes transistors 60, 62, 64, and 66. Latch generator circuit causes the control circuit to operate in a synchronous mode by generating a clocked latch signal from the system clock. Latch generator circuit 32 receives the data signals output from sense amplifier 30, together with a system clock signal, and uses these three signals to generate a latch signal. The output of sense amplifier 30, comprising the signal read from the memory cell and its inverse, are input to transistors 64 and 66. The drain terminals of 64 and 66 are coupled to V dd  through the source terminal of transistor 60. The source terminals of transistors 64 and 66 are connected to V ss  through transistor 62. Transistors 60 and 62 are controlled by the clock signal. Upon receipt of a clock signal, transistors 60 and 62 cause current to flow from V dd  to V ss  when data are presented to latch generator circuit 32, from sense amplifier 30, on transistors 64 and 66. This generates a latch signal at the source terminal of transistor 60. The latch signal is simultaneously provided to read latch 34 and write circuit 36. 
     Read latch 34 receives at its Data In terminal the value of the existing contents of the memory cell (i.e., the value on the line BIT). This value is latched upon receipt by the read latch of the latch signal from latch generator circuit 32. Read latch 34 outputs the latched value to read bus 14. Read latch 34 can be any suitable circuit, such as a D-type flip-flop. 
     Write circuit 36 receives new data to be written to the memory cell via write bus 16. The data on write bus 16 is presented to write circuit 36 prior to the write circuit receiving the latch signal from latch generator 32. Upon receipt of the latch signal, write circuit 36 causes one of the bit lines to be drawn to ground, so as to write the new data to the memory cell. Write circuit 36 operates only when write enable line 20 is energized. It will be apparent to one skilled in the art in view of this disclosure that write circuit 36 can be constructed using any suitable circuitry. 
     FIG. 4 shows an exemplary embodiment of a circuit 67 within write circuit 36 for writing to one of the pair of bit lines (i.e., BIT or BIT). A corresponding duplicate circuit is provided within write circuit 36 for the other bit line (the signal on the data line is inverted). Circuit 67 includes transistors 68, 70, 72, 74, and 76. Transistors 68, 70, 72, and 74 provide a current path between V dd  and V ss . Transistors 68 and 74 are controlled by the latch signal from latch generator 32, while transistors 70 and 72 are controlled by write enable line 20 and data received from write bus 16, respectively. Transistor 76 is turned on, driving the bit line to V ss , when the data on write bus 16 is set high and a write is selected simultaneously on write enable line 20. Under these circumstances, the latch signal clocks circuit 67, and a write is performed. 
     Having described the structure of the synchronous static random access memory and the sense amplifier of the present invention, the operation of the memory will now be described with reference to FIG. 5. 
     FIG. 5 illustrates the timing of various signals within the memory of FIG. 1. During a first time interval, t 0  -t 1 , the clock signal goes low. The low clock signal actuates precharge circuit 28 and closes balance switch 52 by turning on transistors 54 and 56 of sense amplifier 30. Precharge circuit 28 charges the bit lines to a voltage of V dd , providing power to sense amplifier circuit 30. (For simplicity, the bit lines are depicted in interval t 0  -t 1  as already being in their precharged state.) The latch signal is low. Select line 18 is driven high to select the control circuits 12 required for this memory cycle. 
     During the interval t 1  -t 2 , the clock signal goes high, terminating the precharge of the bit lines. At the same time, a word select line (shown in FIG. 1) is driven high, causing one memory cell from each row of the array to be connected to a different one of control circuits 12 by a respective pair of bit lines. Because the bit lines provide power to sense amplifier 30, the voltage of both bit lines will drop slightly from V dd . However, the memory cell will cause the voltage of one of the bit lines to drop faster than the other. This occurs because one node of the memory cell stores a zero, and thus is connected to ground. 
     During the interval t 2  -t 3 , balance switch 52 is opened, causing inverters 45 and 47 of sense amplifier 30 to drive to a stable state. A short delay is provided prior to opening balance switch 52 so as to permit the voltages on the bit lines to sufficiently separate from each other (during interval t 1  -t 2 ) to facilitate the sensing operation. At time t 3 , the sensing operation is completed and the latch signal is generated. The data value output by sense amplifier 30 is latched in read latch 34 and the write operation is initiated. 
     During the interval t 3  -t 4 , when a write operation is desired, one of the bit lines is pulled to ground, causing a zero to be written to the memory cell node associated with that bit line. The other bit line remains charged to a voltage near V dd , causing a one to be written to the memory cell node associated with the second bit line. The write operation is complete at t 4 . If a write operation is not required (not shown), both bit lines remain charged to a voltage near V dd , though one bit line will be at a slightly higher voltage than the other. 
     During the interval t 4  -t 5 , a new memory cycle begins. The clock signal again goes low to initiate the precharge phase. Both bit lines are charged to V dd . The latch signal returns to zero. Sense amplifier 30 is again ready to read the contents of the memory cell. 
     Although the sense amplifier of the present invention has been described in the context of a static random access memory, the sense amplifier can also be used in a variety of other applications. For example, the sense amplifier of the present invention could be used to detect variations in voltages on differential wire pairs to communicate over long distances within an integrated circuit. Alternatively the sense amplifier could be used in a cache memory to quickly read the contents of the memory. Cache memories typically are accessed to perform read operations. Because the sense amplifier of the present invention requires that the bit lines discharge only slightly from V dd , the bit lines can be quickly charged to V dd  for subsequent read operations. 
     It will be understood that the foregoing is merely illustrative of the principles of the invention, and that various modifications can be made by those skilled in the an without departing from the scope and spirit of the invention.