Abstract:
An electronic circuit for converting trinary level input signals on a first line into binary level signals on two or more output lines using contemporary CMOS field effect transistor integrated circuits. According to one embodiment, conversion is accomplished using two CMOS inverters each asymmetrically configured to exhibit transconductances which differ by a factor in excess of 5. In another form, the circuit provides hysteresis through positive feedback to limit binary output state perturbations attributable to trinary signal level input noise. The invention also encompasses the use of decode logic and logically combined delay elements to eliminate &#34;glitches&#34; and facilitate selective enablement of the decoded states representing the intermediate of the trinary input levels.

Description:
This is a continuation of application Ser. No. 07/353,284, filed May 17, 1989, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an electronic circuit for communicating in trinary logic. More particularly, the invention involves, in its various facets, configurations of electronic devices suitable to receive a trinary level signal on a single input and provide a noise and transition &#34;glitch&#34; insensitive binary state output, or a fully decoded output on three lines. 
     The use of trinary logic for electronic circuits is relatively well-known. For example, U.S. Pat. Nos. 4,202,044 and 4,488,065 relate to the storage and retrieval of memory array data stored in trinary format. U.S Pat. No. 4,449,065 involves a circuit for converting a single trinary input signal into a pair of binary output signals using a clocked precharging. U.S. Pat. No. 3,832,576 introduces various circuit techniques for decoding a pair of binary outputs with logic gates. Although similar concepts exist in these representative prior teachings, the implementations are relatively constrained to particular circuit embodiments and provide no specific feedback compensation to offset for either input signal noise effects or slow input signal transition induced output state glitches. 
     SUMMARY OF THE INVENTION 
     The present invention has a number of features which facilitate the common use of trinary logic for contemporary electronic devices. The invention is particularly inclined to integrated circuit applications fabricated from complementary field effect transistors. In such integrated circuit applications, trinary logic is particularly useful for integrated circuit chips which are pad limited, in that the number of pads available is less than the number of input lines needed to operate the chip for both its routine and test modes of operation. 
     The present invention effectively extends the functionality of each input line or pad by permitting the trinary logic, with associated voltage levels of 0 volts, 2.5 volts, and 5 volts, to increase the information content transferrable through a single line or pad by a factor of 3:2. The present invention extends the fundamental concept to an efficient implementation in a contemporary CMOS field effect integrated circuit product. The invention also provides, in various forms of the circuit, noise suppression using feedback introduced hysteresis, and logic with delay to eliminate glitches in the binary output during slow transitions of the trinary input between voltage extremes. 
     The conversion from trinary logic to binary logic using contemporary CMOS integrated circuit field effect transistor devices is accomplished through a judicious selection of transconductance in each of the inverter configured pairs of complementary field effect transistors. The circuit receives a trinary input state as a voltage on one line and provides the equivalent binary state output voltages on a pair of lines. 
     As embodied, the transconductance ratio of p and n-channel transistors in the first complementary inverter is nominally 15:1. The second inverter exhibits a nominal ratio of 1:15. The transistor width and length dimensions take into account p-channel and n-channel transistor mobility characteristics. When the p-channel and n-channel transistors in such inverters exhibit nominal threshold voltages of -0.8 volts and +0.8 volts, respectively, such pair of inverters will serve to receive a trinary level signal on a single input line and provide the corresponding binary state values on two output lines. 
     Given that the voltage range assigned to a trinary mode state is significantly less than for binary state operation, uncompensated noise on the trinary input is more likely to produce erroneous outputs. One embodiment of the present invention provides a positive feedback with hysteresis to reduce noise sensitivity. The hysteresis is produced by shunting transistors enabled by the binary output state in each of the two trinary-to-binary inverter conversion paths. 
     Another facet of the invention includes circuit elements to eliminate &#34;glitches&#34; in the binary or decoded output stages. Glitches routinely occur during transitions of the trinary input, for example when the trinary input transitions through the intermediate 2.5 volt level during a switch from a 0 volt level to a 5 volts level. The invention provides decode logic which combines direct and delayed path signals to detect and ignore monetary occurrences of the midrange trinary level. 
     These and other unique features of the invention will be more clearly understood and more fully appreciated upon considering the detailed embodiments set forth hereinafter. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustrating a trinary-to-binary converter circuit. 
     FIG. 2 is a schematic illustrating a trinary-to-binary converter circuit having hysteresis. 
     FIG. 3 is a schematic illustrating a circuit with logic suitable to decode converted binary states. 
     FIG. 4 is a schematic illustrating a circuit suitable to decode the binary states and eliminate glitches. 
     FIG. 5 illustrates a circuit suitable to generate a trinary output in response to decoded state information. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The application of trinary logic to CMOS type field effect transistor integrated circuits is motivated in part by the increased functionality capable on an integrated circuit chip in relation to the number of access pins/pads available for conveying electrical signals to and from the integrated circuit chip. An additional, and a somewhat distinct motivation, is attributable to the accentuated reliability objectives being demanded by chip customers, requiring comprehensively testing of the numerous functions on an integrated chip without massively increasing the pins/pad count for the singular purpose of acceptance testing. 
     In the context of satisfying the latter objective, the present invention provides the capability to have test equipment introduce non-standard signal voltages to enable or control test functions. Thereby, the test signals share pin/pads with input-output lines having distinctly different operationally defined functions. It is in this context that the present invention finds particular great use and value. The invention provides the ability to receive trinary level test control signals and to fully decode such signals based upon the level of the trinary input and its duration. 
     FIG. 1 illustrates one embodiment of the invention. The CMOS field effect transistor circuit converts trinary input data on a first line, input pad 1, into binary data on the pair of output lines B 1  and B 2 , using a pair of CMOS inverters 2 and 3. The diode connected transistors and resistor within the dashed block 4 represent relatively conventional electrostatic discharge (ESD) protection devices connected to the input pads of integrated circuit chips. 
     What makes the present conversion circuit appealing is the efficacy with which trinary input data on node 6 of CMOS inverters 2 and 3 is converted into binary state output data on lines B 1  and B 2 . When configured as shown in FIG. 1, having the width/length ratios specified therein for the p-channel and n-channel transistors 7, 8, 9 and 11, the ratio of transconductance is approximately 15:1 for one CMOS inverter and 1:15 for the other CMOS inverter. These dimensions take into account the relative differences in device mobility. The p-channel transistors are fabricated to have a -0.8 volt threshold and n-channel transistors exhibit a +0.8 volt threshold. 
     During operation of the circuit in FIG. 1, trinary input signals on node 6 in the range of the 0-1.25 volts cause both B 1  and B 2  to remain near V DD , representing binary 1 values. With the trinary input signal on node 6 in the range of 1.25-3.75 volts, B 1  remains at the high, the binary 1 level, while B 2  is pulled low to the binary 0 level. The concluding trinary state input operating range requires that node 6 be between 3.75 volts and 5 volts (V DD ) This input causes both B 1  and B 2  to become binary 0 in value. A pair of CMOS inverters having complementary transconductance ratios thereby form the nucleus of a converter for receiving trinary state input signals on a single line and converting such into binary state output signals on two lines. 
     Reasonable care must be exercised in selecting the threshold voltages of the CMOS inverter field effect transistors in order to match the trinary input ranges with the transconductance ratios of the paired transistors. With the threshold of n-channel field effect transistor 11 at +0.8 volts, a trinary input at the low range of 0-1.25 volts does not enable transistor 11 sufficiently to offset the current furnished through fully enabled p-channel transistor 9. Thereby, line B 2  continues to be pulled to the level of V DD . In the midrange of the trinary state input signal, when node 6 lies between 1.25 and 3.75 volts, and is nominally 2.5 volts, high transconductance n-channel transistor 11 is sufficiently enabled to pull node B 2  to ground notwithstanding the continued enablement of materially lower transconductance p-channel transistor 9. Given the relative size of p-channel field effect transistor 9 in relation to n-channel field effect transistor 11, the threshold voltage of transistor 9 is not critical. On the other hand, the opposite situation exists for p-channel transistor 7 of CMOS inverter 2. Preferably, the threshold of p-channel transistor 7 is in the range of -0.8 volts to ensure that output line B 1  is pulled to ground by relatively smaller transconductance n-channel transistor for input node 6 voltages in the range  3.75 to 5 volts (V DD ). 
     Referring now to FIG. 2, the binary levels B 1  and B 2  are made more distinctive and less susceptible to noise on the trinary input mode 6 by providing for each path a nominal increment of hysteresis through negative feedback from added conventional inverter stages 14 and 16, thereby providing biasing of the transconductance ratios associated with CMOS inverters 2 and 3. Converter 12 is responsible for generating the binary output signal B 1/ , while converter 13 is responsible for generating the binary output signal B 2/ , which two signals are the complements of the aforementioned binary signals B 1  and B 2  as further refined by hysteresis effects. The additional CMOS inverters 14 and 16 also eliminate any offset voltages produced as a consequence of analog effects in inverters 2 and 3, in pursuit of ideal binary state output signals. Shunting p-channel field effect transistors 17 and 18 are enabled whenever B 1/  or B 2/ , respectively, are pulled low. This action introduces shunting path hysteresis into the actions of inverters 2 and 3. 
     Nominally, p-channel transistor 17 is capable of conducting approximately 50% or less of the peak current through n-channel transistor 8, while p-channel transistor 18 is capable of conducting approximately 50% of the peak current through p-channel field 9. The hysteresis is thus preferably about 300 millivolts for a conventional noise voltage tolerance of +150 millivolts. 
     FIG. 3 illustrates a further varient of the invention, now including logic, within dashed block 19, to effectuate a full decode of the trinary input signal into responses on one of three output lines. The decoded output is generated by a set of logic elements, which logic elements include AND gate 21, XOR gate 22 and NOR gate 23. For the present application, XOR gate 22 could be replaced with the selectively configured AND gate, depicted generally as block 24. Operation of the decode on the binary values of lines B 1/  and B 2/  can be readily understood and related to the trinary state inputs by comparing the effects of trinary levels with the individual decoded output states. 
     FIG. 4 illustrates an embodiment which provides the further refinement of &#34;glitch&#34; elimination, through the inclusion of delay paths within the decode logic. The delays ensure that the decoded states are not generated when the intermediate trinary input signal level is present for a period less than the interval of the delay. Such excursion could be attributable to noise effects or slow transitions of the trinary input signal through the intermediate level. The combination of delay and logic elements depicted in FIG. 4 ensures that such abnormal input conditions do not generate decoded output states. 
     Although the delay functions are shown integrated into the decode logic, it should be apparent that the invention fully contemplates any delay arrangement which defers the propagation of the binary state output sufficiently to allow resolution of the long term validity by comparisons between the delayed and the undelayed signals. One should also understand and appreciate that the delay function could be used to time-selectively enable test modes. The trinary input pad would be designed to operate with conventional binary level signals, namely 0 volt or 5 volts, specified to exhibit minimum transition rates. The test mode enabling delay would defer the onset of the &#34;1&#34; state, as generated in FIG. 4, well beyond any normal operating mode transistion time interval. 
     FIG. 5 illustrates that the reverse direction of conversion, from decoded to trinary states, can also be implemented using conventional field effect transistor devices. 
     Consequently, the present invention provides circuits for translating trinary, binary and decoded signals with minimum circuit complexity, and as such provides a practical solution to the common pad limition problem for integrated circuit chips. 
     It will be understood by those skilled in the art that the embodiments set forth hereinbefore are merely exemplary of the various elements and usages essential to the present invention, and as such may be replaced by equivalents without departing from the invention thereof, which now will be defined by appended claims.