Abstract:
A semiconductor integrated circuit equipped with an input buffer operation error prevention circuit is disclosed which comprises a data output signal level transition detector circuit for detecting at least one of a variation from a low level from a high level and a variation from a high level to a low level of a signal of a circuit of a stage preceding an output buffer and for generating a clock pulse and an input buffer signal terminal control circuit for controlling the terminal level of a first stage gate in an input buffer through the use of a clock pulse so as to cancel a fall in an input level detection margin of the input buffer which is caused when the output data of the output buffer varies from a &#34;0&#34; level to a &#34;1&#34; level and vice versa.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor integrated circuit having an input buffer operation error preventing circuit for preventing an input signal level error detection operation at an input buffer which is caused by an output noise upon a variation in the output data of an output buffer. 
     2. Description of the Related Art 
     In a semiconductor integrated circuit, if in order to obtain a high-speed access time the drive power of a data output buffer is increased to allow a high-speed rise and fall in an output data signal, then a noise signal is generated in a power supply line (including a ground line) due to a temporary large current through the output buffer. In this case, the output noise induces an input level detection error at, for example, a signal input buffer, causing a problem as will be set forth below. 
     FIGS. 1 and 2 show an output buffer and input buffer, respectively, and FIGS. 3A-3E shows the state of a typical error detection operation of the input buffer at the time when the output data of the output buffer varies. That is, at a &#34;0&#34; level output of the output buffer, a noise signal is induced on a V SS  line (a ground line) due to a drive peak current of an N-channel transistor TN in the output buffer, resulting in a potential variation. At this time, if in the input buffer an input signal of a TTL (transistor-transistor logic) level is a high level and there is a small margin in the input signal level, an input buffer of a first stage temporarily assumes the same state as upon receipt of a TTL input signal of a low level, due to the influence of a noise signal of a &#34;V SS  potential&#34; level, causing an output node A of a first stage in the input buffer to go high temporarily. On the other hand, at a &#34;1&#34; output level of the output buffer, a noise signal is induced on the V DD  power supply by a drive peak current of a P-channel transistor TP in the output buffer. If, at this time, the TTL input signal is at a low level in the input buffer and there is a small margin in that input signal level, the input buffer of the first stage temporarily assumes the same state as upon receipt of a TTL input signal of a high level, due to the influence of a noise signal of a &#34;V DD  &#34; potential level, causing the output node A of the first stage in the input buffer to go low temporarily. 
     In order to prevent the aforementioned problem, that is, prevent an operation error of the input buffer resulting from the output noise upon a variation in the output data, the usual practice is to reduce the drive power of the output buffer and hence to reduce an amount of output noise generated or, in a memory of a multi-bit structure, to reduce an amount of output noise generated, by displacing each bit output a corresponding time little by little. These methods present a problem because they are used at the sacrifice of data read-out speed. Another method is, prior to the varying of an input at an output buffer, shorting input and output terminals of a final stage of the output buffer so that, with the output waveform made less sharp, an output noise component may be reduced. For this method, reference is made to Wada, T., et al., &#34;A 34ns 1Mb CMOS SRAM using Triple Poly&#34;, ISSCC DIGEST OF TECHNICAL PAPERS, pp 262 to 263; Feb., 1987. According to this method, the input and output terminals of the output buffer are forced into conduction, offering a risk of inducing a large current therethrough or rather inducing a power supply potential variation. Furthermore, there is also a risk that the aforementioned conduction operation will be performed at the sacrifice of data read-out speed. 
     SUMMARY OF THE INVENTION 
     It is accordingly the object of the present invention to provide a semiconductor integrated circuit having an input buffer operation error preventing circuit, which, without the sacrifice of any data read-out speed, can prevent any input buffer error caused upon an output noise due to a variation in output data which would otherwise be involved in a conventional semiconductor integrated circuit. 
     According to the present invention, a semiconductor integrated circuit equipped with an input buffer operation error preventing circuit comprises a data signal level transition detector for detecting at least one of a variation from a low level to a high level and that from a high level to a low level of a signal of a circuit of a stage preceding an output buffer and for generating a clock pulse and an input buffer terminal control circuit for controllably adding a capacitor to an input terminal or output terminal of a first stage gate in an input buffer through the use of the clock pulse generated by the data output signal level transition detector circuit so as to cancel a fall in an input level detection margin of the input buffer which is caused when the output data of the output buffer varies from a &#34;0&#34; level to a 37 1&#34; level and vice versa. 
     According to the semiconductor integrated circuit having an input buffer&#39;s error operation preventing circuit, when an input level detection margin of the input buffer is decreased by a variation of the power supply potential caused by output noise variation in the output data of the output buffer, a capacitor is added to the input or output terminal of the first stage gate of the input buffer, thereby preventing the logical level of the input signal from being erroneously detected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an output buffer and output control circuit for explaining an input signal level detection error at an input buffer upon a variation in the output data of an output buffer in a semiconductor integrated circuit; 
     FIG. 2 is a circuit diagram showing an input buffer referred to in the description of the semiconductor integrated circuit of FIG. 1; 
     FIGS. 3A-3E are timing diagrams showing an input level detection error operation of a TTL input signal to an input buffer shown in FIG. 2 which is caused upon a variation in the output data of the output buffer in the semiconductor integrated circuit; 
     FIG. 4 is a circuit diagram showing a semiconductor integrated circuit according to an embodiment of the present invention which includes an input buffer operation error preventing circuit; 
     FIG. 5A is a circuit diagram showing a second data signal level transition detector in the semiconductor integrated circuit and FIG. 5B shows another second data signal level transition detector in the semiconductor integrated circuit of FIG. 4; 
     FIG. 6A shows a first data signal level transition detector in the semiconductor integrated circuit of FIG. 4 and FIG. 6B shows another variant of the second data signal level transition detector of FIG. 4; 
     FIG. 7A shows a delay circuit as shown in FIGS. 5A, 5B, 6A and 6B and FIG. 7B shows another variant of the delay circuit as shown in FIGS. 5A, 5B, 6A and 6B; 
     FIGS. 8A-8K are timing diagrams of the semiconductor integrated circuit having an input buffer operation error preventing circuit of FIG. 4; 
     FIG. 9 shows a semiconductor integrated circuit according to another embodiment of the present invention, including an input buffer operation error preventing circuit; 
     FIGS. 10A-10K are timing diagrams showing the operation of the semiconductor integrated circuit of FIG. 9; 
     FIG. 11 shows a semiconductor integrated circuit according to another embodiment of the present invention, including an input buffer operation error preventing circuit; 
     FIGS. 12Aa-12K are timing diagrams showing the operation of the semiconductor integrated circuit of FIG. 11; and 
     FIG. 13 shows a semiconductor device according to another embodiment of the present invention, including an input buffer operation error preventing circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The semiconductor integrated circuit according to the embodiments of the present invention will be explained below with reference to the accompanying drawings. 
     In the semiconductor memory integrated circuit shown in FIG. 4, reference numeral 1 shows an address input pad; 2, an input buffer; 3, an output control circuit, 4, an output buffer; 5, a data output pad and 6, an operation error presenting circuit, noting that DG shows a data signal generator and that V DD  and V SS  represent a power supply potential and reference potential (ground potential), respectively. The input buffer 2 comprises CMOS type two-input NOR gates with one input supplied with a TTL level input signal coming from an external source via an input pad 1 and with the other input supplied with a chip-enable &#34;0&#34; level input control signal. The output control circuit 3 comprises a NAND gate 7 and NOR gate 8 supplied at one terminal with data output signal D O  coming from the data signal generator DG, series circuit of CMOS inverters 9 and 10 connected to the output of the NAND gate 7, series circuit of CMOS inverters 11 and 12 connected to the output of the NOR gate 8, and CMOS inverter 13 for supplying a &#34;0&#34; level output control signal at a read time and &#34;1&#34; level output control signal at a write time to the other input to the NAND gate 7 in an inverted fashion, the output control signal being supplied to the other terminal of the NOR gate 8. A series circuit of a p channel transistor TP and N channel transistor TN is connected between the power supply line V DD  and the ground terminal V SS , and common junction of the drains of the transistors TP and TN is connected to a data output pad 5. 
     An operation error preventing circuit 6 comprises first data signal level transition detector F and second data signal level transition detector R for detecting signal level transition on output nodes B and C of a preceding circuit (for example, the NAND gate 7 and NOR gate 8 in the output control circuit 3) of the output buffer 4, delay circuit DL1 connected in series to the output of the first detector F, a series circuit of a delay circuit DL2 and CMOS inverter IV connected to the output of the second detector R. An input buffer signal terminal control circuit 17 comprises a series circuit of a capacitor CP1 and a P channel transistor P1 connected across the power supply terminal V DD  and the output terminal A of the input buffer 2 and series circuit of an N channel transistor N1 and a capacitor CN1 connected across the output terminal A and ground terminal V SS . An output φ p  of the delay circuit DL1 and output φ N  of the inverter IV are supplied to the gates of the P channel transistor P1 and the N channel transistor N1 in the control circuit 17 respectively. 
     The detector R detects the time of a transition in the data output signal of the data signal generator DG, that is, a time immediately preceding a variation of the N channel transistor TN from OFF to ON (immediately preceding a variation of the ground potential V SS ) and generates a &#34;0&#34; level clock pulse φ Rout . The second data signal level transition detector R is of such a type as shown, for example, in FIG. 5A or 5B. That is, in the circuit shown in FIG. 5A, a data output signal to the second detector R is supplied to one input of a NOR gate 22 via a delay circuit 21 and to the other input of the NOR gate 22 via an inverter 23 and output of the NOR gate 22 is inverted by an inverter 24 to generate a clock pulse φ ROUT . In the circuit shown in FIG. 5B, an input signal is supplied to one input of a NAND gate 27 via a series circuit of an inverter 25 and delay circuit 26 and directly to the other input of the NAND gate 27. The output of the NAND gate 27 is output as a clock pulse φ ROUT . 
     The first data signal level transition detector F detects the time of a transition in the data output signal of the data signal generator DG, that is, a time immediately preceding a variation of a p channel transistor TP of the output buffer 4 from OFF to ON in this case (immediately preceding a variation of the power supply potential V DD ) and generates a &#34;0&#34; level clock pulse φ FOUT . The circuit F is of such a type as shown, for example, in FIG. 6A or 6B. That is, the circuit of FIG. 6A is the same as that of FIG. 5B except that a series circuit of a NOR gate 31 and inverter 32 is connected in place of the aforementioned NAND gate 27. The circuit of FIG. 6B is the same as that of FIG. 5A except that a NAND gate 33 is used in place of the aforementioned series circuit of the NOR gate 22 and inverter 24. 
     The delay circuits 21 and 26 as shown in FIGS. 5A, 5B, 6A and 6B comprise an even number of series-connected inverters IV1, . . . , IVn and capacitors C1, . . . , Cn with each output connected relative to the ground terminal, as shown in FIGS. 7A or 7B, as required. 
     The delay circuits DL1 and DL2 are of such a type as shown, for example, in FIGS. 7A and 7B. The delay circuits DL1 and DL2 delay the aforementioned clock pulses φ FOUT  and φ ROUT  by a predetermined time and adjust the switching operation timing of the P channel transistor P1 and N channel transistor N1. 
     The series circuit of the capacitor CP1 and P channel transistor P1 and N channel transistor N1 and capacitor CN1 controllably add the capacitor CN1 or the capacitor CP1 to the output terminal A of the input buffer 2 in a direction toward cancelling a fall in the input level detection margin of the input buffer 2 which is caused by an output noise (power supply potential variation) upon a variation in the output data of the output buffer 4 from &#34;0&#34; to &#34;1&#34; and from &#34;1&#34; to &#34;0&#34;. 
     The output signal of the input buffer 2 is input to an address decoder via an inverter 14. 
     Referring to the voltage waveform of FIGS. 8A-8K, an explanation of the operation of the semiconductor integrated circuit will be set forth below in connection with preventing an input buffer operation error at the time of a variation in the data output of the output buffer. That is, upon a &#34;0&#34; read operation of the output buffer 4, for example, the node C of the output control circuit 3 varies from a low level to a high level and hence the second data signal level transition detector R generates a &#34;0&#34; level clock pulse φ ROUT . The clock pulse φ ROUT  has its timing adjusted past the delay circuit DL2 and inverter IV, generating a &#34;1&#34; level clock pulse φ N . The clock pulse φ N  is supplied to the gate of the N channel transistor N1. If, at this time, the TTL level input signal of the input buffer 2 is at a low level, no problem arises. If, on the other hand, the TTL level input signal is at a high level VIH and there is less detection margin, there is a risk that a detection error operation will occur. At this time, the N channel transistor N1 is temporarily turned ON by a clock pulse φ N  and, due to the capacitor CN1 added between the output terminal A of the input buffer 2 and the ground terminal, a potential on the output terminal A is lowered to the ground potential side, an operation which tends to cause the output terminal A to vary to a high level side is made less active, failing to cause a potential on the output terminal A of the input buffer 2 to go high temporarily. It is thus possible to improve a detection margin against a high level VIH of the TTL level input signal. 
     If, on the other hand, a &#34;1&#34; data is read out of the output buffer 4, a potential on the node B of the output control circuit 3 varies from the high level to the low level and hence a &#34;0&#34; level clock pulse φ FOUT  is produced from the first data signal level transition detector F. The clock pulse φ FOUT  has its timing adjusted past the delay circuit DL1 to generate a clock pulse φ P . The clock pulse φ P  is supplied to the gate of the P channel transistor P1. If, at this time, the TTL level input signal of the input buffer 2 is at a high level, no problem is encountered because there is an adequate detection margin. If, however, a TTL level input signal is at a low level VIL and there is less detection margin, there is a risk that the aforementioned detection error will occur. In this embodiment, the P channel transistor P1 is rendered ON temporarily and, due to a capacitance CP1 between the output terminal A of the input buffer 2 and the power supply terminal, the potential level on the output terminal A is raised, causing an operation which tends to cause the output terminal A to vary to a low level side. As a result, the situation under which the potential on the output terminal A of the input buffer 2 goes low temporarily is avoided, increasing a detection margin against the low level VIL of the TTL level input signal. 
     The present invention is not restricted to the aforementioned embodiment and can be changed or modified in various ways without departing from the spirit and scope of the present invention. For example, the p channel transistor P1 and capacitor CP1, as well as the N channel transistor N1 and capacitor CN1, may be rearranged in a proper fashion. 
     As shown in FIG. 9, in place of an output terminal A of an input buffer 2, a capacitor CP2 and a P channel transistor P2 are connected between an input terminal D of an input buffer 2 and a power supply terminal V DD  and a capacitor CN2 and an N channel transistor N2 are connected between the input terminal D and a power supply terminal V SS , a clock pulse φ P  being supplied to the gate of the P channel transistor P2 and clock pulse φ N  being supplied to the gate of the N channel transistor N2. 
     The operation error preventing operation is performed s in a manner similar to that as set forth above, the waveforms of the operation being shown in FIGS. 10A-10K. If the N channel transistor N2 is turned ON with the presence of the capacitor CN2, an input signal level on the input terminal D is raised together with a level on the power supply terminal V SS  and hence a VIH detection margin is improved. If the P channel transistor P2 is turned ON with the presence of the capacitor CP2, an input signal level on the input terminal D is lowered together with a level on the power supply terminal V DD , increasing a VIL detection margin. 
     An embodiment of FIG. 11, a combination of the embodiments shown in FIGS. 4 and 9, is of such a type that, as shown in FIG. 11, transistors P1, N1 and capacitors CP1, CN1 are provided on the output terminal A side of an input buffer 2 and transistor P2, N2 and capacitors CP2, CN2 are provided on the input terminal D side of the input buffer 2. This embodiment can obtain the same advantage as those of the aforementioned embodiments, noting that the waveform of respective parts are shown in FIGS. 12A-12K. 
     FIG. 13 shows another embodiment of the present invention. In this embodiment, a transistor 22 is connected across a junction between a capacitor CP1 and a transistor P1 and a power supply terminal V DD  and a signal φ P  is coupled from a delay circuit DL1 via an inverter 20 to the gate of the transistor 22, and an N channel transistor 23 is connected across a junction between a capacitor CN1 and a transistor N1 and a ground potential V SS . A signal φ N  is coupled from an inverter IV to the gate of the transistor 23 via an inverter 21. 
     A signal variation detection node of an output control circuit 3 may be located at any proper places in place of points B and C. For example, a rise variation on the output node of an inverter 9 and fall variation on an inverter 11 may be detected. 
     In the respective embodiments, the rise variation and fall variation of the output data are detected, thereby preventing a fall on the detection margin of the corresponding input signal. As the case may be, however, the capacitor CP1 and/or CP2 or capacitors CN1 and/or CN2 may be added only on the power supply terminal side or the ground terminal side. In this case, only the rise variation of the output data or only the fall variation may be detected, thereby preventing a fall in the detection margin of the input signal. 
     Furthermore, the semiconductor integrated circuit including an input buffer operation error preventing circuit can generally be applied to a semiconductor integrated circuit including input and output buffers.