Patent Publication Number: US-8988959-B2

Title: Circuit and method for dynamically changing a trip point in a sensing inverter

Description:
TECHNICAL FIELD 
     This application is directed, in general, to sensing inverters such as sense amplifiers and, more specifically, to a circuit and method for dynamically changing a trip voltage in a sensing inverter. 
     BACKGROUND 
     Many digital logic circuits are implemented in complementary metal-oxide-semiconductors (CMOS). CMOS is a design style that uses both p-type and n-type (PMOS and NMOS) metal-oxide-semiconductor field effect transistors (MOSFETs) to create logic circuits used in electronic devices world-wide. CMOS technology is found in microprocessors, microcontrollers, semiconductor memory and many other digital logic integrated circuits (ICs). CMOS integrated circuits may be manufactured in great density and offer high operating speed, energy efficiency and a high degree of noise immunity relative to ICs based on bipolar transistors. For these reasons CMOS is a widely adopted technology in the field of electronic design. 
     As CMOS technology tends to become smaller, more dense and faster over time, improvements in high-speed processor design have outpaced those of certain semiconductor component integrated circuits, creating a bottleneck with respect to overall integrated circuit speed. One example of the bottleneck is a sense amplifier circuit where voltage transitions create long read and write cycle times, constraining the overall speed of a semiconductor memory integrated circuit. Memory circuits often use differential amplifiers in sense amplifier circuits because they offer certain advantages over sensing inverter circuits. Sensing inverter circuits are intrinsically slower when employing both a rising and falling transition in reading data. Differential amplifiers offer more flexibility, allowing integrated circuit designers to finely tune voltage differentials and power consumption to achieve desirable timing characteristics. 
     The same is true for many amplifier circuits where time is of the essence, which it almost always is. Single ended sensing amplifiers rely on detectable voltage transitions on an input line to trigger an output voltage. The voltage transitions constitute time delays, and therefore a bottleneck in the overall integrated circuit speed. Differential amplifiers overcome this by employing very small differential voltages to trigger an output voltage. The smaller voltage transitions take less time and speed up the sensing circuit. Improvements in differential amplifier application continue to develop in an effort to relieve the bottleneck in digital logic integrated circuits. 
     SUMMARY 
     One aspect provides a sensing inverter circuit. In one embodiment, the circuit includes: (1) a base inverter circuit couplable to logic-high and logic-low voltage sources at respective inputs thereof and configured to transition an output thereof from a previous logic-level voltage to a present logic-level voltage based on a logic value of an input voltage received by the base inverter circuit, and (2) a feedback circuit associated with the base inverter circuit and configured to employ the previous logic-level voltage to decouple one of the logic-high and logic-low voltage sources from one of the inputs and thereby shift a trip voltage of the base inverter circuit toward the input voltage. 
     Another aspect provides a method for dynamically adjusting a trip point in a sensing inverter circuit. In one embodiment, the method includes: (1) latching a first logic value from the sensing inverter circuit, (2) employing the first logic value in respectively coupling and decoupling first and second rail voltage sources to the sensing inverter circuit, thereby adjusting the trip voltage prior to sensing a second logic value, and (3) latching the second logic value. 
     Another aspect provides a memory system configured to employ a sensing inverter circuit. In one embodiment, the system includes: (1) an array of memory cells, (2) an addressing circuit configured to activate a memory cell of the array of memory cells and associated with a specified memory address, and (3) a sensing inverter circuit, having: (3a) a base inverter circuit couplable to logic-high and logic-low voltage sources at respective inputs thereof and configured to transition an output thereof from a previous logic-level voltage to a present logic-level voltage based on a logic value of an input voltage received by the base inverter circuit, and (3b) a feedback circuit associated with the base inverter circuit and configured to employ the previous logic-level voltage to decouple one of the logic-high and logic-low voltage sources from one of the inputs and thereby shift a trip voltage of the base inverter circuit toward the input voltage. 
    
    
     
       BRIEF DESCRIPTION 
       Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram of one embodiment of a semiconductor memory integrated circuit; 
         FIG. 2  is a diagram of one embodiment of an improved sensing inverter circuit contained in a sense amplifier circuit; 
         FIG. 3  is a diagram of another embodiment of an improved sensing inverter circuit contained in a sense amplifier circuit; and 
         FIG. 4  is a flow diagram of a method of dynamically changing a trip voltage in a sensing inverter circuit. 
     
    
    
     DETAILED DESCRIPTION 
     Before describing various embodiments of the circuit or method introduced herein, sensing inversion will be generally described. 
     One of many CMOS digital logic devices is a sensing inverter. On its face, the function of a sensing inverter in digital logic is to detect a voltage level on an input line that represents a logic value, either a one or zero, logic-high or logic-low. The sensing inverter then transmits an output voltage on an output line that represents the logical opposite of the input logic value. According to the CMOS design style, this is accomplished by some combination of PMOS and NMOS devices. A consequence of the CMOS design style is the inherent time delay in transitioning PMOS and NMOS devices, revealing another function of the sensing inverter: time delay or “buffering.” A series of sensing inverters may be employed to create delay circuits within a digital logic integrated circuit. 
     PMOS and NMOS devices are complimentary. Each is a field-effect transistor (FET) having a gate, a source and a drain terminal. In a PMOS transistor, a low-impedance path exists between the source and drain when a low voltage is applied to the gate. Otherwise, the path is high-impedance. In a NMOS transistor, a low-impedance path exists between the source and drain when a high voltage is applied to the gate. Similar to the PMOS, the path is otherwise high-impedance. 
     A sensing inverter is implemented when, during a sensing cycle, a logic-low voltage is applied to the input line, a PMOS transistor coupled to a logic-high voltage source conducts from the logic-high voltage source to the output. An NMOS device coupled to a logic-low voltage source does not conduct, as its source-to-drain path is high-impedance. Likewise, when a logic-high voltage is applied to the input line, the NMOS transistor conducts, the PMOS transistor does not, and the output line is driven with a logic-low voltage. 
     When the sensing inverter input transitions from low-to-high or high-to-low, a time delay occurs as the PMOS and NMOS transistors change states. The length of the time delay is essentially the time required for the gate-to-source voltages to reach an inherent threshold voltage of the PMOS and NMOS transistors. 
     It is fundamentally realized herein that a previous logic output signal of a previous sensing cycle may be employed to bias the sensing inverter and decrease its response time. More specifically, it is realized herein that a previous logic output signal of a previous sensing cycle may be employed selectively to couple and decouple the logic-high and logic-low voltage sources to reduce the time delay on a present sensing cycle and speeding up the sensing cycle. It is also realized herein the benefit is apparent on consecutive sensing cycles where the previous logic output is logically opposite a present logic input signal to be sensed. In those scenarios, decoupling either the logic-high or logic low voltage source according to the previous logic output value yields a gate-to-source voltage that either rises or falls to the threshold voltage of the sensing inverter device faster than had both voltage sources been coupled. It is further realized herein the time delay of the sensing inverter is less critical when the sensing inverter is not transitioning logic states from one sensing cycle to the next. 
     It is still further realized herein that decoupling the logic-low voltage source effectively strengthens the pull-up strength of the logic-high voltage source and that decoupling the logic-high voltage source strengthens the pull-down strength of the logic-low voltage source. It is yet further realized herein that these strengthened voltage sources work in tandem with a transitioning input signal to achieve the threshold voltage earlier in the rising or falling of the input signal. 
     It is also realized herein that implementing the described sensing inverter in semiconductor memory as a sense amplifier yields improved access times. It is further realized herein that the implementation as the sense amplifier in memory integrated circuits reduces the size of the integrated circuit over a differential sense amplifier. 
     Before describing various embodiments of the circuit or method introduced herein, a memory system within which the circuit may be embodied or the method carried out will be described. 
       FIG. 1  is a block diagram of one embodiment of a memory system  100  in which one or more aspects of the invention may be implemented. The memory system  100  includes a memory cell array  102  and an addressing circuit  110 . The memory cell array  102  is arranged in rows and columns of memory cells  104 . The embodiment of  FIG. 1  illustrates a three-by-three array of memory cells. Alternative embodiments may use any number of rows and columns of memory cells. The size of an array is dependent on its application and may range from hundreds of memory cells to many millions. For instance, a 64 kilobyte array has 524,288 memory cells. A one gigabyte array has over eight billion memory cells. In the embodiment of  FIG. 1 , each memory cell  104  in a row is electrically connected to a word-line  106  for the row. For example, in the first row of memory cells  104 - 1 , each memory cell  104 - 1 , 1 ,  104 - 1 , 2  and  104 - 1 , 3 , is electrically connected to word-line  106 - 1 . Furthermore, each memory cell  104  in a column is electrically connected to a bit-line  108  for the column. For example, in the first column of memory cells,  108 - 1 , each memory cell  104 - 1 , 1 ,  104 - 2 , 1  and  104 - 3 , 1  is electrically connected to bit-line  108 - 1 . In certain embodiments, the bit-line  108  is actually a complimentary pair of bit-lines that provides for use of differential voltages. 
     In the embodiment of  FIG. 1 , each bit-line drives a sense amplifier circuit  112 . The sense amplifier circuit  112  serves the entire column of memory cells  104  by discriminating low level voltages stored in the memory cells  104  and outputting a CMOS logic level voltage representing a bit to be read from the memory cell array  102 . 
     Access is had to a memory cell  104 - n,m  via the addressing circuit  110 . A read cycle begins by the addressing circuit  110  pre-charging the appropriate word-line  106 - n  and bit-line  108 - m  corresponding to the memory cell  104 - n,m  to be read, thus activating the memory cell  104 - n,m . The memory cell  104 - n,m  then charges the bit-line  108 - m  to reflect a stored logic value, which is then amplified by the sense amplifier circuit  112 - m . The amplified logic value is output from the memory system  100  on an output line  114 . 
     Having described a memory system within which the circuit may be embodied or the method carried out, various embodiments of the circuit or method will be described. 
       FIG. 2  is one embodiment of the sense amplifier circuit  112  of  FIG. 1  in which one or more aspects of the invention may be implemented. The sense amplifier circuit  112  includes a base inverter circuit  202 . The base inverter circuit  202  has an input stage configured to receive an input signal  210  representing a bit stored in a memory cell. The base inverter circuit  202  also includes an output stage configured to transmit an output signal through a first and second inverter  208 - 1  and  208 - 2 . The output stage is operable to generate an amplified output  212  and a feedback signal  214 . 
     A PMOS  206  and a NMOS  204  respectively couple the base inverter circuit  202  to a logic-high rail  216  and a logic-low rail  218 . The PMOS  206  and NMOS  204  are operable to couple and decouple the base inverter circuit  202  according to the feedback signal  214 . In other embodiments, the logic-high rail  216  and logic-low rail  218  are couplable when the memory cell is activated by a word-line or a column select signal. 
     The sense amplifier circuit  112  is configured such that when the feedback signal  214  is high (indicating a previously read out bit being a zero) and the input signal  210  is high (indicating a present bit to be read being a one), the sense amplifier circuit  112  needs to transition from a logic-low state to a logic-high state. The logic-high rail  216  is decoupled by the PMOS  206 , the PMOS  206  being held open by the feedback signal  214 . The PMOS of the base inverter circuit  202  is thereby disabled. The logic-low rail  218  is coupled by the NMOS  204 , the NMOS  204  being held closed by the feedback signal  214 . The NMOS of the base inverter circuit  202  is thereby enabled. The sense amplifier circuit  112  is then operable to sense the input signal  210  and transmit an output logic value of one as the amplified output  212 . 
     Conversely, when the feedback signal  214  is low (indicating a previously read out bit being a one) and the input signal  210  is low (indicating a present bit to be read being a zero), the sense amplifier circuit  112  needs to transition from a logic-high state to a logic-low state. The sense amplifier circuit  112  is further operable to couple the logic-high rail  216 , decouple the logic-low rail  218 , and transmit an amplified output  212  representing a logic value of zero. 
       FIG. 3  is a second embodiment of the sense amplifier circuit  112  of  FIG. 1  in which one or more aspects of the invention may be implemented. The circuit of  FIG. 3  is functionally equivalent to that of  FIG. 2 . However, the circuit of  FIG. 3  employs a base inverter circuit  302  having a redundant inverter stage in parallel with a primary stage also present in the circuit of  FIG. 2 . The redundant parallel stage of the base inverter circuit  302  is directly couplable to the logic-high rail  216  and the logic-low rail  218 . 
       FIG. 4  is a method for dynamically changing a trip voltage in a sensing inverter circuit. The method begins at a step  410 . A first logic value is latched from the sensing inverter circuit at step  420 . The first logic value is then employed at step  430  to couple a first rail voltage source to the sensing inverter circuit, partially enabling the circuit, and to decouple a second rail voltage source, partially disabling the circuit. The trip voltage of the sensing inverter circuit is thereby adjusted, ready for a second logic value to be read. The second logic value is then latched at step  440 . The method ends at step  450 . In certain embodiments, the second logic value is logically opposite the first. 
     In alternate embodiments, the first rail voltage source and second rail voltage source are embodied as a pair of voltage sources: logic-high and logic-low voltage sources. In other embodiments the first rail voltage source and second rail voltage source are coupled and decoupled by PMOS and NMOS transistors. In certain embodiments, the logic-high voltage source is coupled to the sensing inverter circuit when the first logic value is high, and the logic-low voltage source is decoupled. A pull-up effect of the logic-high voltage source is strengthened by the decoupling, thereby shifting the trip voltage of the sensing inverter circuit up toward the logic-high voltage source level. Likewise, the logic-low voltage source is coupled when the first logic value is low, the logic-high voltage source is decoupled, and the trip voltage shifted down toward the logic-low voltage source level. 
     In certain embodiments, the first logic value and second logic value represent bits of data stored in a memory cell array and read out in consecutive read cycles from the memory. In some of those embodiments, the sensing inverter circuit is only operable while an associated column of the memory cell array is active, as indicated by a word-line or column select signal. 
     Those skilled in the art to which this application relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments.