Abstract:
A buffer circuit for converting logic signals generated by apparatus implemented in a TTL technology to logic signals processed by apparatus implemented by the CMOS technology includes an input stage (10, 11, 12, 13, 17), a voltage-control (14, 15) stage for causing the buffer circuit to vary the input voltage level required to switch the state of the buffer circuit output signal, and a hysteresis stage (16) for causing the switching of the output signal level to be different for the rising and falling edges of the input signal. The voltage-control stage (14, 15) provides a improvement in the noise margin of both the VTTL(High) switching level and the VTTL(Low) switching level.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to integrated circuit random access memory (RAM) systems and, more particularly, to the interface buffer circuits which convert signals generated by circuits implemented using transistor-to-transistor logic (TTL) technology to signals compatible with circuits implemented in complementary metal oxides semiconductor (CMOS) technology. The CMOS technology is typically used to implement RAM and dynamic RAM (DRAM) memory arrays. 
     2. Description of the Related Art 
     The buffer circuits, which convert the signals generated by the data processing system apparatus implemented in TTL technology into signals compatible with the memory apparatus implemented in CMOS technology, are critical to successful operation of the system interface for the following reasons. The TTL buffer circuits provide the interface for all signals introduced into the memory component. A incorrect detection of a voltage (signal) level can result in a malfunction. One source of malfunction has, in the past, been the incorrect level detection resulting from variation in the power supply voltage. Similarly, noise on the supply and/or input signals can result in an incorrect interpretation of the logic level of the input signal. Being an interface, the speed with which the signals are converted from TTL-compatible to CMOS-compatible signals can provide a limitation in the system performance. In low power applications, the power dissipation, whether in a static mode or in a switching mode, can be a critical parameter. And finally, &#34;ringing&#34; in the input signal should be minimized in order to prevent oscillation of the internal signals. 
     In addition, when a large number of switching circuits and a full chip activation are required, more noise is generated than the buffer circuits disclosed by the related art are capable of handling. This is particularly true when a high noise margin is required for the input buffer circuit. Differential-type input buffer circuits provide the best noise margins, but have the disadvantage that these circuit require a higher die-area and result in a greater power dissipation than conventional buffer circuits. 
     A need has therefore been felt for a buffer circuit for providing an interface between the circuits implemented in TTL technology and circuits implemented in CMOS technology which have high noise margins, are relatively insensitive to power supply variations, operate at high speed, can be implemented on a relatively low die-area, do not require a stable reference level, and are relatively intolerant to ringing noise imposed on the input signals. 
     SUMMARY OF THE INVENTION 
     The aforementioned and other features are provided, according to the present invention, by a buffer circuit having an input stage, a voltage-control stage, and an output feedback stage. An input voltage applied to the input stage, in turn, causes a signal to be applied to a network node, the network node being the input terminal for the output feedback stage. The voltage level of the network node is determined by the voltage-control stage, the signal from the input stage, and the supply voltage. The output signal of the output feedback stage is determined by the voltage level of network node and the output signal of the buffer circuit is a logic level signal. The presence of the voltage-control stage results in an improvement in the noise margins of the buffer circuit. 
     These and other features of the present invention will be understood upon the reading of the following description in conjunction with the Figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is schematic circuit diagram of the TTL-to-CMOS buffer circuit responsive to a negative ENABLE signal according to the present invention. 
     FIG. 2 is schematic circuit diagram of the TTL-to-CMOS buffer circuit responsive to a positive ENABLE signal according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     1. Detailed Description of the Drawings 
     Referring to FIG. 1, a schematic diagram of the TTL-to-CMOS buffer circuit responsive to a negative ENABLE (EN) signal, according to the present invention, is shown. An input terminal for receiving TTL-technology generated input signals is coupled to an gate terminal of p-channel transistor 10, to a gate terminal of n-channel transistor 11, and a gate terminal of n-channel transistor 15. A source terminal of transistor 11 is coupled to the VSS supply terminal, while a drain terminal of transistor 11 is coupled to a drain terminal of transistor 10, to a drain terminal of n-channel transistor 12, to a gate and a drain terminal of n-channel transistor 14, to a drain terminal of p-channel transistor 16, and to an input terminal of inverter amplifier 17. A source terminal of transistor 10 is coupled to a drain terminal of p-channel transistor 13. A gate terminal of transistor 13 is coupled to a gate terminal of transistor 12 and has the EN --  signal applied thereto. (The SIGNAL --  indicates the logic signal complement of the SIGNAL.) The source terminal of transistor 13 is coupled to the VDD power supply terminal. The source terminal of transistor 12 is coupled to the VSS power supply terminal. The source terminal of transistor 15 is coupled to VSS while the drain terminal of transistor 15 is coupled to the source terminal of transistor 14o The source terminal of transistor 16 is coupled to the VDD power supply terminal, while the gate terminal of the transistor 16 is coupled to the output terminal of inverting amplifier 17. The output terminal of inverting amplifier 17 is the output terminal of the buffer circuit. 
     Referring to FIG. 2, a schematic circuit diagram for a TTL-to-CMOS buffer circuit responsive to a positive ENABLE signal is shown. The input terminal is coupled to a gate of p-channel transistor 20, to a gate of n-channel transistor 21, and to a gate of n-channel transistor 25. An EN signal is coupled to a gate of n-channel transistor 28, to a gate terminal of n-channel transistor 29, and to a gate of p-channel transistor 30. The VDD supply terminal is coupled to a source terminal of transistor 20 and to a source terminal of transistor 30. A drain terminal of transistor 20 is coupled to a drain terminal of transistor 21, to a drain terminal of p-channel 26, to a drain terminal of transistor 30, to a gate and drain terminal of n-channel transistor 24, and to an input terminal of inverting amplifier 27. A source terminal of transistor 21 is coupled to a drain terminal of transistor 28. A source terminal of transistor 28 is coupled to the VSS supply terminal and to a source terminal of transistor 29. A drain terminal of transistor 29 is coupled to a source terminal of transistor 25. A source terminal of transistor 24 is couple to a drain terminal of transistor 25. A source terminal of transistor 26 is coupled to the VDD supply terminal, while the gate terminal of transistor 26 is coupled to the output terminal of inverting amplifier 27. The output terminal of inverting amplifier 27 is the output terminal of the buffer circuit. 
     2. Operation of the Preferred Embodiment(s) 
     Referring once again to FIG. 1, the buffer circuit responds to the identified TTL voltage levels wherein VTTL(High) =2.4 volts and VTTL(LOW)=0.8 volts. The buffer circuit is on, i.e., enabled, when EN --  is low. Transistors 13, 10, 11, 12, and 16 along with inverting amplifier 17 form an input buffer. In the default condition, when the applied TTL signal is greater than VTTL(High), transistor 11 causes the input terminal of the inverting amplifier 17 to be lowered, resulting in a high logic signal being applied to the output terminal of the buffer circuit. When the buffer circuit input voltage is less than VTTL(Low) (minus a hysteresis factor), transistor 10 becomes more conducting and the input terminal to the inverting amplifier 17 has a high level applied thereto. As a result, the output terminal of the buffer circuit has a low signal level applied thereto. This output signal level is fed-back through transistor 16 to enhance the high signal applied to the input terminal of inverting amplifier 17. This feedback path is used to create a hysteresis factor resulting in a switching of the signals applied to the output terminal at different points on the rising and falling edges of the buffer circuit input signal. This feature is essential in minimizing the effect of noise considerations. Without this hysteresis factor, the buffer circuit output signal would switch levels in response to ringing signals superimposed on the buffer circuit input signal. 
     The (diode-coupled) transistor 14 brings the level of VTTL(High) from -2.4 volts to -1.6 volts, the diode conduction being higher when the supply voltage (and hence the input terminal to inverting amplifier 17) is at a higher voltage. At lower supply voltages (where obtaining a better VTTL(Low) is a problem), the diode (transistor 14) path is less conducting resulting in a more sensitive transition. The diode (transistor 14) is coupled in series with transistor 15o Transistor 15 is controlled by the buffer circuit input signal so as to reduce current dissipation when the input signal is at a low level, i.e., transistor 10 is conducting. 
     The current path from the input terminal of inverting amplifier 17 to the VSS voltage supply terminal is completely cut-off when VTTL(Low)≦the threshold voltage of transistor 15. The degree of conduction of transistor 14 and transistor 15 is determined jointly by the voltage at the input terminal of inverting amplifier 17 and the level of the input signal controlling transistor 15. 
     The degree of conduction of transistor 14 dynamically increases the effective pull-down strength when the power supply voltage is high (because a higher supply voltage results in a higher voltage at the input terminal of inverting amplifier 17), and reduces the effective pull-down strength when the power supply voltage is low. The capability of dynamic variation in the strength of the pull-down is the primary reason for the excellent input noise margin over a wide range of power supply variations. 
     The (diode-coupled) transistor 14 and transistor 15 combination brings the VTTL(High) to a lower value without degrading the VTTL(Low), a distinct advantage over the buffer circuits of the related art. By controlling the voltage level at the input terminal of the inverting amplifier 17, the voltage at which the switching occurs can be controlled and the noise margin can be increased. For VTTL(High), the present invention provides a 500 mv improvement in the noise margin, while for VTTL(Low), the present invention provides an improvement in the noise margin of 100 mv. 
     Referring to FIG. 2, a buffer circuit is shown which is enabled by a positive enable signal. The operation is generally consistent with the operation of the buffer circuit shown in FIG. 1. 
     While the invention has been described with particular reference to the preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents substituted for elements of the preferred embodiment without departing from the invention. For example, the present invention has been described with particular reference to the particular embodiments shown in the Figures. However, additional driver components can be cascaded to in the output to suit the application. In addition, many modifications may be made to adapt a particular situation and material to a teaching of the present invention without departing from the essential teachings of the present invention. 
     As is evident from the foregoing discussion, certain aspects of the invention are not limited to the particular details of the examples illustrated, and it is accordingly intended that the claims shall cover all modifications and applications as do not depart from the spirit and scope of the invention.